Genome editing tools,through the disruption of an organism'snative genetic material or the introduction of non-native DNA, facilitatefunctional investigations to link genotypes to phenotypes. Transposonshave been instrumental genetic tools in microbiology, enabling genome-wide,randomized disruption of genes and insertions of new genetic elements.Due to this randomness, identifying and isolating particular transposonmutants (i.e., those with modifications at a genetic locus of interest)can be laborious, often requiring one to sift through hundreds orthousands of mutants. Programmable, site-specific targeting of transposonsbecamepossible with recently described CRISPR-associated transposase (CASTs)systems, allowing the streamlined recovery of desired mutants in asingle step. Like other CRISPR-derived systems, CASTs can be programmedby guide-RNA that is transcribed from short DNA sequence(s). Here,we describe a CAST system and demonstrate its function in bacteriafrom three classes of Proteobacteria. A dual plasmid strategy is demonstrated:(i) CAST genes are expressed from a broad-host-range replicative plasmidand (ii) guide-RNA and transposon are encoded on a high-copy, suicidalpUC plasmid. Using our CAST system, single-gene disruptions were performedwith on-target efficiencies approaching 100% in Beta- and Gammaproteobacteria(Burkholderia thailandensis and Pseudomonas putida, respectively). We also reporta peak efficiency of 45% in the Alphaproteobacterium Agrobacterium fabrum. In B. thailandensis, we performed simultaneous co-integration of transposons at twodifferent target sites, demonstrating CAST's utility in multilocusstrategies. The CAST system is also capable of high-efficiency largetransposon insertion totaling over 11 kbp in all three bacteria tested.Lastly, the dual plasmid system allowed for iterative transposon mutagenesisin all three bacteria without loss of efficiency. Given these iterativecapabilities and large payload capacity, this system will be helpfulfor genome engineering experiments across several fields of research.

Metabolite exchange within marine microbial communities transfers carbon and other major elements through global cycles and forms the basis of microbial interactions. Yet lack of gene annotations and concern about the quality of existing ones remain major impediments to revealing currencies of carbon flux. We employed an arrayed mutant library of the marine bacterium Ruegeria pomeroyi DSS-3 to experimentally annotate substrates of organic compound transporter systems, using mutant growth and compound drawdown analyses to link transporters to their cognate substrates. Mutant experiments verified substrates for thirteen R. pomeroyi transporters. Four were previously hypothesized based on gene expression data (taurine, glucose/xylose, isethionate, and cadaverine/putrescine/spermidine); five were previously hypothesized based on homology to experimentally annotated transporters in other bacteria (citrate, glycerol, N-acetylglucosamine, fumarate/malate/succinate, and dimethylsulfoniopropionate); and four had no previous annotations (thymidine, carnitine, cysteate, and 3-hydroxybutyrate). These bring the total number of experimentally-verified organic carbon influx transporters to 18 of 126 in the R. pomeroyi genome. In a longitudinal study of a coastal phytoplankton bloom, expression patterns of the experimentally annotated transporters linked them to different stages of the bloom, and also led to the hypothesis that citrate and 3-hydroxybutyrate were among the most highly available bacterial substrates. Improved functional annotation of the gatekeepers of organic carbon uptake is critical for deciphering carbon flux and fate in microbial ecosystems.

Although research on promoters has spanned decades, the precise prediction of promoter activity from DNA sequence remains a challenge even in model organisms. Recent literature has identified important differences in the core sequence of σ 70 promoters across classes of Proteobacteria as well as a lack of transferability when promoters are moved from host to host. Currently, there is a need for synthetic constitutive promoters spanning a range of expression levels in species outside of Escherichia coli. Additionally, characterization data defining behavior of the same promoter across multiple species would be extremely valuable to the field. Here, we analyzed promoter activity in three classes of Proteobacteria, which enabled us to better understand the sequence elements correlated with a strong promoter in different hosts. In doing so, we identified and characterized constitutive promoters spanning a range of expression in these species for community use and described the portability of a subset of these promoters as they were moved between hosts. These promoter libraries have broad applications as predictable genetic tools to control gene expression in diverse species (1–3). This work adds to the toolkit for gene expression in non-model bacteria and is a step towards the larger goal of accurate promoter prediction in a given host from a de novo sequence.

Background CRISPR-Cas systems have expanded the possibilities for gene editing in bacteria and eukaryotes. There are many excellent tools for designing CRISPR-Cas guide RNAs (gRNAs) for model organisms with standard Cas enzymes. GuideMaker is intended as a fast and easy-to-use design tool for challenging projects with (i) non-standard Cas enzymes, (ii) non-model organisms, or (iii) projects that need to design a panel of gRNA for genome-wide screens. Findings GuideMaker can rapidly design gRNAs for gene targets across the genome using a degenerate protospacer-adjacent motif (PAM) and a genome. The tool applies hierarchical navigable small world graphs to speed up the comparison of guide RNAs and optionally provides on-target and off-target scoring. This allows the user to design effective gRNAs targeting all genes in a typical bacterial genome in similar to 1-2 minutes. Conclusions GuideMaker enables the rapid design of genome-wide gRNA for any CRISPR-Cas enzyme in non-model organisms. While GuideMaker is designed with prokaryotic genomes in mind, it can efficiently process eukaryotic genomes as well. GuideMaker is available as command-line software, a stand-alone web application, and a tool in the CyCverse Discovery Environment. All versions are available under a Creative Commons CC0 1.0 Universal Public Domain Dedication.

Transposons have been instrumental tools in microbiology enabling random mutagenesis, with transposons like Tn5 and Mariner, and site-specific DNA integrations with Tn7. However, programmable targeting of transposons was impossible until CRISPR-associated transposase (CasTn) systems were described. Like other CRISPR-derived systems, CasTn can be programmed with a short DNA encoded sequence that is transcribed into a guide-RNA. Here we describe a broad-host-range CasTn system and demonstrate its function in bacteria from three classes of the Proteobacteria. The CasTn genes are expressed from a broad-host-range replicative plasmid, while the guide-RNA and transposon are provided on a high-copy pUC plasmid that is suicidal in most bacteria outside of E. coli. Using our CasTn system, single-gene disruptions were performed with on-target efficiencies approaching 100% in the Beta- and Gammaproteobacteria, Burkholderia thailandensis, and Pseudomonas putida , respectively. The results were more modest in the Alphaproteobacterium Agrobacterium fabrum , with a peak efficiency of 45%, though for routine single-gene disruptions, this efficiency is adequate. In B. thailandensis, the system allowed simultaneous co-integration of transposons at two different target sites. The CasTn system is also capable of high-efficiency large transposon insertion totaling over 11 kbp in P. putida . Given the iterative capabilities and large payload size, this system will be helpful for genome engineering experiments across several fields of research. Significance The genetic modification of bacteria to disrupt native genes and integrate recombinant genes is necessary for basic and applied research. Traditional methods for targeted disruptions and insertions are often cumbersome and inefficient, limiting experiments' scale and throughput. This work developed a system for targeted transposon mutagenesis that is easy to use, iterative, and efficient. We demonstrate that the system functions across three different classes of the Proteobacteria in species widely used in research and biotechnology. Moreover, the framework of the system and accompanying plasmids that we developed will facilitate porting the system to other bacteria. Our system provides a fast and efficient protocol to genetically modify these bacteria by inserting desired genetic cargo into specific genomic targets.

The ability to construct defined genetic mutations in many bacteria is difficult and limited. Transposon mutagenesis is often highly efficient, but is not site specific, thus selections are often needed to identify mutants of interest. The construction of arrayed mutant libraries would help to fill this need, though these libraries are costly and time consuming. To enable easier construction of arrayed libraries we developed a workflow and methodology using a hierarchical barcoding scheme to identify mutants within a multiwell plate. We applied this method to the marine Alphaproteobacterium Ruegeria pomeroyi DSS-3 and created a library with over 2,800 disrupted genes.

Plant evolution has produced enzymes that may not be optimal for maximizing yield and quality in today’s agricultural environments and plant biotechnology applications. By improving enzyme performance, it should be possible to alleviate constraints on yield and quality currently imposed by kinetic properties or enzyme instability. Enzymes can be optimized more quickly than naturally possible by applying directed evolution, which entails mutating a target gene in vitro and screening or selecting the mutated gene products for the desired characteristics. Continuous directed evolution is a more efficient and scalable version that accomplishes the mutagenesis and selection steps simultaneously in vivo via error-prone replication of the target gene and coupling of the host cell’s growth rate to the target gene’s function. However, published continuous systems require custom plasmid assembly, and convenient multipurpose platforms are not available. We discuss two systems suitable for continuous directed evolution of enzymes, OrthoRep in Saccharomyces cerevisiae and EvolvR in Escherichia coli, and our pilot efforts to adapt each system for high-throughput plant enzyme engineering. To test our modified systems, we used the thiamin synthesis enzyme THI4, previously identified as a prime candidate for improvement. Our adapted OrthoRep system shows promise for efficient plant enzyme engineering.