CRISPR-associated transposons (CASTs) are RNA-guided mobile genetic elements that are widespread in bacterial genomes. Here, we describe the UltraCAST, a suicide vector with the Vibrio cholerae Type I-F CAST system and Golden Gate assembly sites with fluorescent protein gene dropouts for guide RNA and a mini-transposon cargo cloning. We show an example of UltraCAST genome editing by disrupting a gene in the chromosome of Serratia symbiotica CWBI-2.3 T , a culturable relative of aphid endosymbionts. The UltraCAST can be used to flexibly insert DNA into specific genomic sites and facilitates testing this genome editing platform in non-model bacterial species that lack genetic tools.

The continuously high incidence of some endosymbionts in arthropods despite potential conflicts with their hosts is often explained by obligatory relationships, in which the host is fully dependent on its endosymbiont, fitness advantages conferred on hosts by facultative endosymbionts, or reproductive manipulation of hosts by endosymbionts (typically facultative). Yet continuously endosymbiont high incidence is sometimes observed without clear evidence supporting any of these mechanisms. This situation could potentially be explained by the presence of several coinfecting strains of the same endosymbiont species, each affecting the host differently such that their effects counteract one another when studied collectively. Here, we investigated Wolbachia endosymbionts of fleas, which stably persist in high loads in all females, with no indication that any of the above mechanisms explain their continuously high incidence. We sequenced fleas and identified two Wolbachia strains, designated as wSc1 and wSc2. We then correlated the strain composition in fleas with measures of their reproductive success. We found that fleas with high wSc1 and low wSc2 loads had a higher reproductive success than fleas that had high loads of both strains, low loads of both strains, or no Wolbachia, suggesting that wSc1 may provide a direct fitness advantage to their hosts. Conversely, the number of males and total offspring was negatively correlated with wSc2 levels, supporting male killing. Our research demonstrates that endosymbionts' continuously high incidence may persist through intricate relationships in nature.

This protocol describes recipes to prepare growth media and reagents used in the E. coli long-term evolution experiment (LTEE). Section 1: DM-glucose, Davis-Mingioli liquid medium supplemented with glucose Section 2: Sterile Saline Section 3: TA agar, Tetrazolium Arabinose agar Section 4: MG agar, Minimal Glucose agar (equivalent to DM agar) Section 5: MA agar, Minimal Arabinose agar Section 6: MC agar, Minimal Citrate agar Section 7: CC agar, Christensen Citrate agar Section 8: Stock Solutions, detailed instructions for stock solutions needed for media preparation DM-glucose: Davis-Mingioli medium (or sometimes called Davis Minimal medium) supplemented with glucose is used for propagating the LTEE populations and for performing related experiments. For propagating the LTEE, glucose is added to a concentration of 25 mg/L, which we refer to as "DM25". DM25 supports a stationary-phase density of about 5×107 cells/mL for E. coli REL606 and REL607, the founding strains of the LTEE. (The stationary-phase density of evolved LTEE clones varies, but tends to be approximately half that of the ancestral strains.) DM with higher concentrations of glucose is used for reviving cells from freezer stocks or for growing many cells to harvest for certain experiments. These other DM formulations are named in an analogous fashion of DMX, where X is the concentration of glucose in mg/L (e.g., 1000 mg/L glucose in DM1000). Sterile Saline: Used to dilute E. coli cultures, for instance when plating on agar to isolate colonies or to count CFUs to determine cell titers. TA agar: Tetrazolium Arabinose agar plates are used for distinguishing E. coli cells that can grow on the sugar arabinose (Ara+) from those that cannot (Ara–). Plating dilutions that give 150-250 colonies are used for monitoring the LTEE for contamination and also for co-culture competition assays that measure the relative fitness of two strains. Colonies grown from Ara– cells appear red on TA agar, while those of Ara+ strains appear pinkish-white. These phenotypes are very clear after 24 hours of incubation at 37°C for the REL606 (Ara–) and REL607 (Ara+) ancestors of the LTEE. Colonies of evolved clones can exhibit a wide variation of these color phenotypes. Some evolved clones may take longer than 24 hours to form visible colonies on TA. MG agar: Minimal Glucose agar has the same base composition as DM-glucose liquid medium, except agar is added as a solidifying agent, and the glucose concentration is increased to 4 g/L to support the growth of colonies. Plating dilutions that give 150-250 colonies or streaking out on MG-agar is used to isolate colonies from LTEE populations. Dilutions of the LTEE populations can also be plated on MG agar to monitor for unexpected growth, colony appearance, or CFU numbers that could indicate contamination. Ancestral clones form colonies within 24 hours on MG agar. Evolved clones typically also form colonies within 24 hours on MG agar, but some may take longer. Evolved clones also generally produce larger colonies than the ancestors on MG. MA agar: Minimal Arabinose agar is the same as MG agar except that the sugar arabinose is used instead of glucose. Ara– cells like those of strain REL606 will not form colonies on MA agar. Only Ara+ cells like those of strain REL607 will. Plating dilutions from the LTEE populations that give 150-250 CFUs on MA agar can be used to monitor the Ara– populations for contamination from Ara+ populations. The Ara+ ancestor and evolved strains generally form colonies within 24 hours on MA agar. However, some Ara+ populations have lost the ability to form colonies at later generations. Plating a large number of Ara– cells (>109) on an MA plate can also be used to select spontaneous mutants that have reverted from the Ara– marker state to the Ara+ marker state. Reversion to Ara+ among LTEE clones is usually via a single nucleotide substitution mutation in the araC gene. This mutation occurs at a rate of ~1010 cells/generation among non-mutator clones, and much higher among clones with mutator phenotypes. MC agar: Minimal Citrate agar is the same as MG/MA agar except that citrate is used as the carbon source. Strains that have evolved citrate utilization (Cit+) can form colonies on MC agar. CC agar: Christensen Citrate agar is an indicator medium that can be used to detect weak citrate utilization in colonies on the basis of a color change even, for strains that may not be able to form colonies on MC agar.

Serratia symbiotica bacteria exhibit a range of relationships with aphids. They may be co-obligate mutualists, commensals, or even pathogens depending on the strain, aphid host species, and environment. Serratia symbiotica CWBI-2.3T (CWBI), a culturable member of this group, is transmitted to embryos transovarially when it is injected into pea aphids (Acyrthosiphon pisum), the same route used by S. symbiotica strains that are vertically inherited endosymbionts. Yet, aphids colonized with CWBI die before they give birth to infected offspring. We evolved laboratory populations of CWBI through 15-30 serial passages at two different temperatures in rich media. These nutrient-replete conditions mimic aspects of environments within aphid hosts that lead to the evolution of reduced endosymbiont genomes. Unexpectedly, all S. symbiotica populations propagated at one temperature appeared to evolve slower growth after only a few days due to reactivation of a lytic prophage from the CWBI genome. Though these populations continued to reach saturating cell densities slower than cultures of the ancestor throughout the experiment, representative clones isolated from them had mutations affecting lipopolysaccharide biosynthesis and were resistant to the phage. Some evolved strains exhibited less virulence when injected into aphids, and we observed instances of gene inactivation and loss mediated by insertion elements. Our results illustrate how transposons and prophages can dominate laboratory evolution of newly cultured bacteria, particularly those that are host-associated in nature and have genomes rife with selfish DNA elements. They also suggest that bacteria-phage coevolution can catalyze evolutionary paths that contribute to converting pathogens into stably inherited endosymbionts.IMPORTANCELaboratory experiments can be used to explore evolutionary innovations in how microbes associate with animal hosts. Serratia symbiotica bacteria exhibit a variety of interactions with aphids. Some strains are obligate endosymbionts. Others have facultative associations with benefits or costs depending on the environmental context. S. symbiotica CWBI-2.3T (CWBI) resembles aphid endosymbionts in how it can be transovarially transmitted to aphid embryos. However, adults injected with CWBI do not survive long enough to give birth to infected offspring. We evolved this aphid protosymbiont in rich media to see if this would attenuate its virulence and recapitulate genome reduction observed in endosymbionts. We observed large deletions and gene inactivation, but reactivation of a prophage from the CWBI genome and then evolution of phage resistance dominated. Some evolved strains became less virulent to aphids, suggesting that evolution driven by selfish DNA elements can contribute to the emergence of new endosymbionts from pathogen ancestors.

Honey bees are indispensable insects. Their pollination services support modern agriculture and natural ecosystems. Managed honey bee colonies face increasing threats to their survival, ranging from environmental stressors that include agrochemicals to infestations of arthropod pests and infections with microbial pathogens. Like humans, honey bees have a native gut microbiome that supports their health. However, the bee gut microbiome has a simpler composition than the gut microbiome of mammals, and its main constituent bacterial species can be easily cultured outside of the host. This experimental tractability and the need for new methods for protecting hive health have made honey bees a testbed for synthetic microbiomes augmented with probiotic bacteria and engineered DNA. Here, we discuss the natural benefits of bee gut bacteria, recent progress in genetically modifying these bacteria, and how symbiont-mediated RNA interference and other microbiome engineering approaches can boost bee immunity and suppress bee pathogens and parasites. Finally, we discuss how emerging methods for microbiome engineering and biocontainment could be applied to honey bees and used to address challenges in translating these proof-of-principle achievements into safe and effective technologies for field applications at scale.

The bacterium Acinetobacter baylyi is a model organism known for its extreme natural competence and metabolic versatility. It is capable of taking up environmental DNA at a high rate across all growth phases. The type strain ADP1 was created by random mutagenesis of a precursor strain, BD4, to prevent it from forming cell chains in culture. ADP1 has since been distributed between research groups over several decades and acquired subsequent mutations during this time. In this study, we compare the genome sequences of A. baylyi BD4 and its modern descendants to identify and understand the effects of mutations acquired and engineered during its domestication. We demonstrate that the ADP1 variants in use today differ in their competence, growth on different carbon sources, and autoaggregation. In addition, we link the global carbon storage regulator CsrA and a transposon insertion that removes its C-terminal domain specifically to changes in both overall competence and an almost complete loss of competence during the stationary phase. Reconstructing the history of ADP1 and the diversity that has evolved in the variants currently in use improves our understanding of the desirable properties of this experimentally and industrially important bacterium and suggests ways that its reliability can be improved through further genome engineering.IMPORTANCEAcinetobacter baylyi ADP1 is a bacterial chassis of interest to microbiologists in academia and industry due to its extreme natural competence and wide metabolic range. Its ability to take up DNA from its environment makes it straightforward to efficiently edit its chromosome. We identify and characterize mutations that have been passed down to modern strains of ADP1 from the initial work in the 1960s, as well as subsequent mutations and genome edits separating strains in use by different research groups today. These mutations, including one in a global regulator (CsrA), have significant phenotypic consequences that have affected the reproducibility and consistency of experiments reported in the literature. We link a mutation in this global regulator to unexpected changes in natural competence. We also show that domesticated A. baylyi strains have impaired growth on a variety of carbon sources.

Excess utilization of translational resources is a critical source of burden on cells engineered to overexpress exogenous proteins. To improve translational efficiency, researchers often modify codon usage in an exogenous gene to more closely match the composition of a host organism’s highly expressed genes. Despite empirical data showing the benefits of codon optimization, little is known about the quantitative relationships between codon usage, protein yield, and the burden imposed on a host cell by protein overexpression. We develop and experimentally evaluate a stochastic gene expression model that considers the impact of codon usage bias on the availability of ribosomes and different tRNAs in a cell. In agreement with other studies, our model shows that increasing exogenous protein expression decreases production of native cellular proteins in a linear fashion. We also find that the slope of this relationship is modulated by how well the codon usage bias of the exogenous gene and the host’s genes match. Lastly, our model predicts that an overoptimization domain exists where further increasing usage of optimal codons worsens yield and burden. We test our model by expressing sfGFP and mCherry2 from constructs that have a wide range of codon optimization levels in Escherichia coli. The results agree with our model, including for an mCherry2 gene sequence that appears to less efficiently express this gene due to codon overoptimization. Our model reproduces experimentally observed relationships between codon usage bias, gene expression, and burden for overexpressed proteins. Furthermore, it suggests that more nuanced recoding strategies that seek to match a host’s overall codon usage bias are less burdensome and will lead to greater protein yields compared to strategies that simply maximize usage of optimal codons. Increasing the level of mechanistic detail in gene expression models can lead to insights that allow researchers to engineer more optimal cellular systems.

Naturally competent bacteria can be engineered into platforms for detecting environmental DNA. This capability could be used to monitor the spread of pathogens, invasive species, and resistance genes, among other applications. Here, we create Acinetobacter baylyi ADP1-ISx biosensors that detect specific target DNA sequences through natural transformation. We tested strains with DNA sensors that consisted of either a mutated antibiotic resistance gene (TEM-1 bla or nptII) or a counterselectable gene flanked by sequences from the fungus Pseudogymnoascus destructans, which causes white-nose syndrome in bats. Upon uptake of homologous DNA, recombination restored antibiotic resistance gene function or removed the counterselectable gene, enabling selection of cells that sensed the target DNA. The antibiotic resistance gene and P. destructans biosensors could detect as few as 3,000 or 5,000,000 molecules of their DNA targets, respectively, and their sensitivity was not affected by excess off-target DNA. These results demonstrate how A. baylyi can be reprogrammed into a modular platform for monitoring environmental DNA.

Brochosomes are proteinaceous nanostructures produced by leafhopper insects with superhydrophobic and antireflective properties. Unfortunately, the production and study of brochosome-based materials has been limited by poor understanding of their major constituent subunit proteins, known as brochosomins, as well as their sensitivity to redox conditions due to essential disulfide bonds. Here, we used cell-free gene expression (CFE) to achieve recombinant production and analysis of brochosomin proteins. Through the optimization of redox environment, reaction temperature, and disulfide bond isomerase concentration, we achieved soluble brochosomin yields of up to 341 ± 30 μg/mL. Analysis using dynamic light scattering and transmission electron microscopy revealed distinct aggregation patterns among cell-free mixtures with different expressed brochosomins. We anticipate that the CFE methods developed here will accelerate the ability to change the geometries and properties of natural and modified brochosomes, as well as facilitate the expression and structural analysis of other poorly understood protein complexes.

Adenosine-to-inosine (A-to-I) messenger RNA (mRNA) editing can affect the sequence and function of translated proteins and has been extensively investigated in eukaryotes. However, the prevalence of A-to-I mRNA editing in bacteria, its governing regulatory principles, and its biological significance are poorly understood. Here, we show that A-to-I mRNA editing occurs in hundreds of transcripts across dozens of gammaproteobacterial species, with most edits predicted to recode protein sequences. Furthermore, we reveal conserved regulatory determinants controlling editing across gammaproteobacterial species. Using Acinetobacter baylyi as a model, we show that mutating TadA, the mediating enzyme, reduces editing across all sites. Conversely, overexpressing TadA resulted in the editing of >300 transcripts, attesting to the editing potential of TadA. Notably, we show for the first time, at the protein level, that normal levels of A-to-I mRNA editing lead to wild-type bacteria expressing two protein isoforms from a single gene. Finally, we show that a TadA mutant with deficient editing activity does not grow at high temperatures, suggesting that RNA editing has a functional role in bacteria. Our work reveals that A-to-I mRNA editing in bacteria is widespread and has the potential to reshape the bacterial transcriptome and proteome.

Antibiotic resistance is one of the most pressing threats to human health, yet recent work highlights how loss of resistance may also drive pathogenesis in some bacteria. In two recent studies, we found that β-lactam antibiotic and nutrient stresses faced during infection selected for the genetic inactivation of the Pseudomonas aeruginosa (Pa) antibiotic efflux pump mexEFoprN. Unexpectedly, efflux pump mutations increased Pa virulence during infection; however, neither the prevalence of efflux pump inactivating mutations in real human infections, nor the mechanisms driving increased virulence of efflux pump mutants are known. We hypothesized that human infection would select for efflux pump mutations that drive increased virulence in Pa clinical isolates. Using genome sequencing of hundreds of Pa clinical isolates, we show that mexEFoprN efflux pump inactivating mutations are enriched in Pa cystic fibrosis isolates relative to Pa intensive care unit clinical isolates. Combining RNA-seq, metabolomics, genetic approaches, and infection models we show that efflux pump mutants have elevated expression of two key Pa virulence factors, elastase and rhamnolipids, which increased Pa virulence and lung damage during both acute and chronic infections. Increased virulence factor production was driven by higher Pseudomonas quinolone signal levels in the efflux pump mutants. Finally, genetic restoration of the efflux pump in a representative ICU clinical isolate and the notorious CF Pa Liverpool epidemic strain reduced their virulence. Together, our findings suggest that mutations inactivating antibiotic resistance mechanisms could lead to greater patient mortality and morbidity.

Excess utilization of translational resources is a critical source of burden on cells engineered to overexpress exogenous proteins. To improve protein yields and genetic stability, researchers often use codon optimization strategies that improve translational efficiency by matching an exogenous gene's codon usage with that of the host organism's highly expressed genes. Despite empirical data that shows the benefits of codon optimization, little is known quantitatively about the relationship between codon usage bias and the burden imposed by protein overexpression. Here, we develop and experimentally evaluate a stochastic gene expression model that considers the impact of codon usage bias on the availability of ribosomes and different tRNAs in a cell. In agreement with other studies, our model shows that increasing exogenous protein expression decreases production of native cellular proteins in a linear fashion. We also find that the slope of this relationship is modulated by how well the codon usage bias of the exogenous gene and the host's genes match. Strikingly, we predict that an overoptimization domain exists where further increasing usage of optimal codons worsens yield and burden. We test our model by expressing sfGFP and mCherry2 from constructs that have a wide range of codon optimization levels in Escherichia coli. The results agree with our model, including for an mCherry2 gene sequence that appears to lose expression and genetic stability from codon overoptimization. Our findings can be leveraged by researchers to predict and design more optimal cellular systems through the use of more nuanced codon optimization strategies.

Organelles and endosymbionts have naturally evolved dramatically reduced genome sizes compared to their free-living ancestors. Synthetic biologists have purposefully engineered streamlined microbial genomes to create more efficient cellular chassis and define the minimal components of cellular life. During natural or engineered genome streamlining, deletion of many non-essential genes in combination often reduces bacterial fitness for idiosyncratic or unknown reasons. We investigated how and to what extent laboratory evolution could overcome these defects in six variants of the transposon-free Acinetobacter baylyi strain ADP1-ISx that each had a deletion of a different 22- to 42-kilobase region and two strains with larger deletions of 70 and 293 kilobases. We evolved replicate populations of ADP1-ISx and each deletion strain for ~300 generations in a chemically defined minimal medium or a complex medium and sequenced the genomes of endpoint clonal isolates. Fitness increased in all cases that were examined except for two ancestors that each failed to improve in one of the two environments. Mutations affecting nine protein-coding genes and two small RNAs were significantly associated with one of the two environments or with certain deletion ancestors. The global post-transcriptional regulators rnd (ribonuclease D), csrA (RNA-binding carbon storage regulator), and hfq (RNA-binding protein and chaperone) were frequently mutated across all strains, though the incidence and effects of these mutations on gene function and bacterial fitness varied with the ancestral deletion and evolution environment. Mutations in this regulatory network likely compensate for how an earlier deletion of a transposon in the ADP1-ISx ancestor of all the deletion strains restored csrA function. More generally, our results demonstrate that fitness lost during genome streamlining can usually be regained rapidly through laboratory evolution and that recovery tends to occur through a combination of deletion-specific compensation and global regulatory adjustments.

Bacterial biofilms are stable multicellular structures that can enable long term host association. Yet, the role of biofilms in supporting gut mutualism is still not fully understood. Here, we investigate Snodgrassella alvi, a beneficial bacterial symbiont of honey bees, and find that biofilm formation is required for its colonization of the bee gut. We constructed fifteen S. alvi mutants containing knockouts of genes known to promote colonization with putative roles in biofilm formation. Genes required for colonization included staA and staB, encoding trimeric autotransporter adhesins (TAAs) and mltA, encoding a lytic transglycosylase. Intriguingly, TAAs are considered virulence factors in pathogens but support mutualism by the symbiont S. alvi. In vitro, biofilm formation was reduced in ΔstaB cells and abolished in the other two mutants. Loss of staA also reduced auto-aggregation and cell-cell connections. Based on structural predictions, StaA/B are massive (>300 nm) TAAs with many repeats in their stalk regions. Further, we find that StaA/B are conserved across Snodgrassella species, suggesting that StaA/B-dependent colonization is characteristic of this symbiont lineage. Finally, staA deletion increases sensitivity to bactericidal antimicrobials, suggesting that the biofilm indirectly buffers against antibiotic stress. In all, the inability of two biofilm-deficient strains (ΔstaA and ΔmltA) to effectively mono-colonize bees indicates that S. alvi biofilm formation is required for colonization of the bee gut. We envision the bee gut system as a genetically tractable model for studying the physical basis of biofilm-mutualist-gut interactions.

Mechanistic understanding of interactions in many host-microbe systems, including the honey bee microbiome, is limited by a lack of easy-to-use genome engineering approaches. To this end, we demonstrate a one-step genome engineering approach for making gene deletions and insertions in the chromosomes of honey bee gut bacterial symbionts. Electroporation of linear or non-replicating plasmid DNA containing an antibiotic resistance cassette flanked by regions with homology to a symbiont genome reliably results in chromosomal integration. This lightweight approach does not require expressing any exogenous recombination machinery. The high concentrations of large DNAs with long homology regions needed to make the process efficient can be readily produced using modern DNA synthesis and assembly methods. We use this approach to knock out genes, including genes involved in biofilm formation, and insert fluorescent protein genes into the chromosome of the betaproteobacterial bee gut symbiont Snodgrassella alvi. We are also able to engineer the genomes of multiple strains of S. alvi and another species, Snodgrassella communis, which is found in the bumble bee gut microbiome. Finally, we use the same method to engineer the chromosome of another bee symbiont, Bartonella apis, which is an alphaproteobacterium. As expected, gene knockout in S. alvi using this approach is recA-dependent, suggesting that this straightforward procedure can be applied to other microbes that lack convenient genome engineering methods. IMPORTANCE:Honey bees are ecologically and economically important crop pollinators with bacterial gut symbionts that influence their health. Microbiome-based strategies for studying or improving bee health have utilized wild-type or plasmid-engineered bacteria. We demonstrate that a straightforward, single-step method can be used to insert cassettes and replace genes in the chromosomes of multiple bee gut bacteria. This method can be used for investigating the mechanisms of host-microbe interactions in the bee gut community and stably engineering symbionts that benefit pollinator health.

ABSTRACTSerratia symbioticabacteria exhibit a range of relationships with aphids. They may be co-obligate mutualists, commensals, or even pathogens depending on the strain, aphid host species, and environment.Serratia symbioticaCWBI-2.3T(CWBI), a culturable member of this group, is transmitted to embryos transovarially when it is injected into pea aphids (Acyrthosiphon pisum), the same route used byS. symbioticastrains that are vertically inherited endosymbionts. Yet, aphids colonized with CWBI die before they give birth to infected offspring. We evolved laboratory populations of CWBI through 15-30 serial passages at two different temperatures in rich media. These conditions mimic aspects of the nutritional environment in aphid hosts that lead to the evolution of reduced endosymbiont genomes. Unexpectedly, allS. symbioticapopulations propagated at one temperature appeared to evolve slower growth after only a few days due to reactivation of a lytic prophage from the CWBI genome. Though these populations continued to reach saturating cell densities slower than cultures of the ancestor throughout the experiment, most bacteria in them had mutations affecting lipopolysaccharide biosynthesis and were resistant to the phage. Some evolved strains exhibited less virulence when injected into aphids, and we observed instances of gene inactivation and loss mediated by insertion elements. Our results illustrate how transposons and prophages can dominate laboratory evolution of newly cultured bacteria, particularly those that are host-associated in nature and have genomes rife with selfish DNA elements. They also suggest that bacteria-phage coevolution can catalyze evolutionary paths that contribute to converting pathogens into stably inherited endosymbionts.IMPORTANCELaboratory experiments can be used to explore evolutionary innovations in how microbes associate with animal hosts.Serratia symbioticabacteria exhibit a variety of interactions with aphids. Some strains are obligate endosymbionts. Others have facultative associations with benefits or costs depending on the environmental context.S. symbioticaCWBI-2.3T(CWBI) resembles aphid endosymbionts in how it can be transovarially transmitted to aphid embryos. However, adults injected with CWBI do not survive long enough to give birth to infected offspring. We evolved this aphid protosymbiont in rich media to see if this would attenuate its virulence and recapitulate genome reduction observed in endosymbionts. We observed large deletions and gene inactivation, but reactivation of a prophage from the CWBI genome and then evolution of phage resistance dominated. Some evolved strains became less virulent to aphids, suggesting that evolution driven by selfish DNA elements can contribute to the emergence of new endosymbionts from pathogen ancestors.

Foundational techniques in molecular biology-such as cloning genes, tagging biomolecules for purification or identification, and overexpressing recombinant proteins-rely on introducing non-native or synthetic DNA sequences into organisms. These sequences may be recognized by the transcription and translation machinery in their new context in unintended ways. The cryptic gene expression that sometimes results has been shown to produce genetic instability and mask experimental signals. Computational tools have been developed to predict individual types of gene expression elements, but it can be difficult for researchers to contextualize their collective output. Here, we introduce CryptKeeper, a software pipeline that visualizes predictions of Escherichia coli gene expression signals and estimates the translational burden possible from a DNA sequence. We investigate several published examples where cryptic gene expression in E. coli interfered with experiments. CryptKeeper accurately postdicts unwanted gene expression from both eukaryotic virus infectious clones and individual proteins that led to genetic instability. It also identifies off-target gene expression elements that resulted in truncations that confounded protein purification. Incorporating negative design using CryptKeeper into reverse genetics and synthetic biology workflows can help to mitigate cloning challenges and avoid unexplained failures and complications that arise from unintentional gene expression.

Engineered DNA will slow the growth of a host cell if it redirects limiting resources or otherwise interferes with homeostasis. Escape mutants that alleviate this burden can rapidly evolve and take over cell populations, making genetic engineering less reliable and predictable. Synthetic biologists often use genetic parts encoded on plasmids, but their burden is rarely characterized. We measured how 301 BioBrick plasmids affected Escherichia coli growth and found that 59 (19.6%) were burdensome, primarily because they depleted the limited gene expression resources of host cells. Overall, no BioBricks reduced the growth rate of E. coli by >45%, which agreed with a population genetic model that predicts such plasmids should be unclonable. We made this model available online for education (https://barricklab.org/burden-model) and added our burden measurements to the iGEM Registry. Our results establish a fundamental limit on what DNA constructs and genetic modifications can be successfully engineered into cells.

Parasites, including pathogens, can adapt to better exploit their hosts on many scales, ranging from within an infection of a single individual to series of infections spanning multiple host species. However, little is known about how the genomes of parasites in natural communities evolve when they face diverse hosts. We investigated how Bartonella bacteria that circulate in rodent communities in the dunes of the Negev Desert in Israel adapt to different species of rodent hosts. We propagated 15 Bartonella populations through infections of either a single host species (Gerbillus andersoni or Gerbillus pyramidum) or alternating between the two. After 20 rodent passages, strains with de novo mutations replaced the ancestor in most populations. Mutations in two mononucleotide simple sequence repeats (SSRs) that caused frameshifts in the same adhesin gene dominated the evolutionary dynamics. They appeared exclusively in populations that encountered G. andersoni and altered the dynamics of infections of this host. Similar SSRs in other genes are conserved and exhibit ON/OFF variation in Bartonella isolates from the Negev Desert dunes. Our results suggest that SSR-based contingency loci could be important not only for rapidly and reversibly generating antigenic variation to escape immune responses but that they may also mediate the evolution of host specificity.