Protein kinases are key regulators of the eukaryotic cell cycle and have consequently emerged as attractive targets for drug development. Their well-defined active sites make them particularly amenable to inhibition by small molecules, underscoring their druggability. The Leishmania kinome, shaped by diverse evolutionary processes, harbours a unique repertoire of potential drug targets. Here, we used the cysteine-directed protein kinase probe SM1-71 to identify four essential protein kinases MPK4, MPK5, MPK7 and AEK1 as candidates for covalent kinase inhibitor development, as well as CLK1/CLK2 for which covalent inhibitors have already been identified. We leveraged the absence of natural analog-sensitive (AS) kinases in L. mexicana to establish an in vivo chemical-genetic AS kinase platform for investigating essential functions of protein kinases. Using CRISPR-Cas9-mediated precision genome editing, we endogenously engineered two kinetochore-associated protein kinases, KKT2 and KKT3, and cyclin-dependent kinase CRK9, to generate AS kinases. We show that KKT2 and CRK9 kinase activities are essential for both promastigote and intracellular amastigote survival; KKT2 kinase activity being required for progression through mitosis at a stage preceding mitotic spindle assembly, while CRK9 kinase activity is required for S phase, consistent with its role in trans-splicing. This study demonstrates the utility of AS chemical genetics in Leishmania and identifies KKT2 and CRK9 as having critical roles in Leishmania cell cycle regulation and therefore being promising drug targets.

Abstract Leishmaniasis is caused by Leishmania parasites, which undergo cellular adaptation when transitioning from the insect stage (promastigote) to the mammalian stage (amastigote). While the ubiquitin-proteasome system (UPS) is vital for life cycle progression, global ubiquitination dynamics have remained unmapped. We established a quantitative ubiquitinomics workflow for Leishmania mexicana , identifying over 9,100 ubiquitination sites across 38% of the proteome, revealing thousands of stage-specific regulatory events. Promastigote-enriched sites associate with cell motility, while amastigote-enriched sites link to metabolism and glycosome organization. We identified extensive ubiquitination on UPS components, including the essential virulence factor deubiquitinase 2 (DUB2). Using inducible gene deletion and XL-BioID proximitomics, we identified 111 potential DUB2 substrates. High-confidence substrates include the E2 conjugating enzyme UBC2, which is required for differentiation, and SUMO, a critical regulator of ubiquitin crosstalk. The discovery of UBC2 as a substrate of DUB2 directly links ubiquitination with promastigote to amastigote differentiation. Our findings provide a comprehensive map of the Leishmania ubiquitinome and demonstrate that DUB2 acts as a pleiotropic regulator controlling post-translational modifications of essential proteins involved in life cycle progression.

Bacillus subtilis sporulation involves a fascinating phagocytic process in which the mother cell engulfs the forespore, internalizing the latter as a cell-within-a-cell. Peptidoglycan remodelling machinery, along with the SpoIIIAA-AH:SpoIIQ complex, are crucial to this process. The forespore protein SpoIIQ and the mother cell protein SpoIIIAH, which localize to opposite sides of the sporulation septum, are indispensable for sporulation. These proteins interact through their extracytoplasmic domains across the intermembrane space and are proposed to contribute to an intercellular zipper and/or a channel connecting the forespore and the mother cell. Here, we show using (1) site-directed mutagenesis of SpoIIQ, (2) in vivo and in vitro interaction and localization studies, and (3) σG activation and sporulation assays that spores are formed efficiently from cells in which direct interaction between SpoIIIAH and SpoIIQ (H-Q) is disrupted. We propose that the H-Q interaction is dispensable for sporulation and that the essential function of SpoIIQ is in recruitment of other components to the septum/engulfment complex such as SpoIIE, GerM and/or the other SpoIIIA proteins.

In Leishmania parasites, as for their hosts, the ubiquitin (Ub) proteasome system is important for cell viability. As part of a systematic gene deletion study, it was discovered that four cysteine protease-type deubiquitinases (DUBs) are essential for parasite survival in the promastigote stage, including DUB16. Here, we have purified and characterised recombinant DUB16 from Leishmania donovani, which belongs to the Ub C-terminal hydrolase (UCH) family. DUB16 efficiently hydrolyses C-terminal aminocoumarin and rhodamine conjugates of Ub consistent with proposed cellular roles of UCH-type DUBs in regenerating free monomeric Ub from small molecule Ub adducts arising from adventitious metabolic processes. The crystal structure of DUB16 reveals a typical UCH-type DUB fold, and a relatively short and disordered cross-over loop that appears to restrict access to the catalytic cysteine. At close to stoichiometric enzyme to substrate ratios, DUB16 exhibits DUB activity towards diubiquitins linked through isopeptide bonds between Lys11, Lys48 or Lys63 residues of the proximal Ub and the C-terminus of the distal Ub. With 100-1000-fold higher turnover rates, DUB16 cleaves the ubiquitin-ribosomal L40 fusion protein to give the mature products. A DUB-targeting cysteine-reactive cyanopyrrolidine compound, IMP-1710, inhibits DUB16 activity. IMP-1710 was shown in promastigote cell viability assays to have parasite killing activity with EC50 values of 1-2 μM, comparable with the anti-leishmanial drug, miltefosine. L. mexicana parasites engineered to overproduce DUB16 showed a modest increase in resistance to IMP-1710, providing evidence that IMP-1710 inhibits DUB16 in vivo. While it is highly likely that IMP-1710 has additional targets, these results suggest that DUB16 is a potential target for the development of new anti-leishmanial compounds.

Pseudomonas aeruginosa PA01 is one of the major causes of disease persistence and mortality in patients with lung pathologies, relying on various host metabolites as carbon and energy sources for growth. The ict-ich-ccl operon (pa0878, pa0882 and pa0883) in PAO1 is required for growth on the host molecule itaconate, a C5-dicarboxylate. However, it is not known how itaconate is taken up into P. aeruginosa. Here, we demonstrate that a genetically linked tripartite ATP-independent periplasmic (TRAP) transporter (pa0884-pa0886), which is homologous to the known C4-dicarboxylate-binding TRAP system, is essential for growth on itaconate, but not for the closely related C4-dicarboxylate succinate. Using tryptophan fluorescence spectroscopy, we demonstrate that the substrate-binding protein (SBP), IctP (PA0884), binds itaconate but still retains higher affinity for the related C4-dicarboxylates. The structures of IctP bound to itaconate (1.80 Å) and succinate (1.75 Å) revealed an enclosed ligand-binding pocket with ion pairing interactions with the ligand carboxylates. The C2 methylene group that is the distinguishing feature of itaconate compared with succinate is accommodated by a unique change in the IctP-binding site from a Leu to Val, which distinguishes it from closely related C4-dicarboxylate-binding SBPs. Together, these data suggest that this transporter, which we name IctPQM, has duplicated from a canonical C4-dicarboxylate transporter, and its evolution towards itaconate specificity enables this pathogen to now access a key metabolite for persistence in the host.

Our understanding of the skin microbiome has significantly advanced with the rise of sequencing technologies. While Corynebacterium species are a major component of the human skin microbiome, research has largely centered on other prevalent genera like Staphylococcus and Cutibacterium. Prior to this study, complete genomes for skin-associated Corynebacterium were limited. For instance, only nine were available for the commonly identified species Corynebacterium tuberculostearicum. In this study, we explored Corynebacterium diversity from a single skin site, the axilla, using selective media to enrich for these bacteria. Long-read whole-genome sequencing and bioinformatic analysis of the enriched isolates provided unprecedented insight into the diversity of this genus at a single body site. The study yielded 215 closed genomes, comprising 30 distinct representative genomes following dereplication. These genomes span seven distinct species, including two new species provisionally named Corynebacterium axilliensis and Corynebacterium jamesii, as well as species not previously linked to the skin. Pangenome analysis of these isolates uncovered potential metabolic differences, antimicrobial resistance genes, novel biosynthetic gene clusters, prophages, and phage defense systems. This study represents the most detailed analysis to date of Corynebacterium from a single skin niche and highlights extensive variation even within a single host. Our culture-based Nanopore sequencing approach has expanded the number of publicly available skin Corynebacterium genomes, providing a valuable resource for future studies investigating the diversity and function of this important skin genus. IMPORTANCE:This study uncovers the hidden diversity of Corynebacterium, an important yet often overlooked group of bacteria inhabiting human skin. Focusing on the underarm and using advanced sequencing techniques, we identified over 200 complete bacterial genomes. This collection includes species not previously known to reside on the skin, as well as two entirely new species, highlighting how little is currently known about this cutaneous genus. Most of these bacteria were isolated from a single individual, underscoring the vast microbial diversity that can exist within one person. By closely examining the corynebacterial community at a single site, we begin to uncover the complex relationships within a true microbial ecosystem. These findings deepen our understanding of the skin microbiome and provide a valuable resource for future research into how these microbes affect skin health, hygiene, and disease.

The skin microbiome is dominated by a few key genera, among which Staphylococcus is one of the most well characterized. Recent studies have examined the roles of various Staphylococcus species such as Staphylococcus epidermidis and Staphylococcus hominis within broader skin microbial communities. However, these investigations often rely on isolates from multiple individuals and hence limit their ability to capture intra-community interactions. In this study, we focused on the axillary micro-biome of a single healthy individual to characterize the genetic and functional diversity of resident Staphylococcus isolates. Using a low-cost, high-throughput DNA extraction and long-read whole-genome sequencing pipeline, we generated complete genomes for 93 isolates spanning 7 genetically distinct lineages across 3 major skin species. These comprised one dominant and three additional lineages of S. epidermidis, two of S. hominis and one of Staphylococcus capitis. Functional and metabolic analyses revealed species-and strain-specific features, suggesting potential metabolic cross-feeding and specialization within this community, including within strains of S. epidermidis. These findings highlight the metabolic complexity and potential interdependence of staphylococci inhabiting a single skin site and the need for strain-level resolution of the community. The strains form part of the York Skin Microbiome (YSM) collection, a growing open biobank of genetically diverse skin isolates from matched individuals.

Replication and segregation of the nucleus and kinetoplast, the mitochondrial DNA, are tightly coordinated in trypanosomatid parasites, but the signalling pathways that govern this process are unknown. Here, we characterise the mitotic spindle kinase (MSK), a key regulator of this coordination in Leishmania . Using chemical genetics, we engineered an analog-sensitive MSK to inhibit its activity. We show that inhibition of MSK impairs mitotic spindle elongation and blocks both nuclear and kinetoplast segregation, halting cell cycle progression and leading to cell death. We combined chemical genetics with proximity-based phosphoproteomics to identify four substrates: two GTPase-activating proteins, a nuclear segregation protein, and a hypothetical protein. We demonstrate that MSK co-localises with these four proteins in the nucleus, mitotic spindle, kinetoplast, and cytoplasm. Our findings establish MSK as a critical kinase that controls the co-ordinated segregation of the nucleus and kinetoplast, providing a new avenue for understanding cell cycle regulation in Leishmania . ### Competing Interest Statement The authors have declared no competing interest. Medical Research Council, MR/P027989/1 Wellcome Trust, https://ror.org/029chgv08, 223045/Z/21/Z

In Escherichia coli and Salmonella typhimurium, cysteine biosynthesis requires the products of 20 or more cys genes co-ordinately regulated by CysB. Under conditions of sulphur limitation and in the presence of the inducer, N-acetylserine, CysB binds to cys promoters and activates the transcription of the downstream coding sequences. CysB is a homotetramer, comprising an N-terminal DNA binding domain (DBD) and a C-terminal effector binding domain (EBD). The crystal structure of a dimeric EBD fragment of CysB from Klebsiella aerogenes revealed a protein fold similar to that seen in Lac repressor but with a different symmetry in the dimer so that the mode of DNA binding was not apparent. To elucidate the subunit arrangement in the tetramer, we determined the crystal structure of intact CysB in complex with N-acetylserine. The tetramer has two subunit types that differ in the juxtaposition of their winged helix-turn-helix DNA binding domains with respect to the effector binding domain. In the assembly, the four EBDs form a core with the DNA binding domains arranged in pairs on the surface. N-acetylserine makes extensive polar interactions in an enclosed binding site, and its binding is accompanied by substantial conformational rearrangements of surrounding residues that are propagated to the protein surface where they appear to alter the arrangement of the DNA binding domains. The results are (i) discussed in relation to the extensive mutational data available for CysB and (ii) used to propose a structural mechanism of N-acetylserine induced CysB activation.

YejABEF is an ATP- binding cassette transporter that is implicated in the sensitivity of Escherichia coli to anti- microbial peptides, the best- characterized example being microcin C, a peptide- nucleotide antibiotic that targets aspartyl- tRNA synthetase. Here the structure of the extracellular solute binding protein, YejA, has been determined, revealing an oligopeptide- binding protein fold enclosing a ligand- binding pocket larger than those of other peptide- binding proteins of known structure. Prominent electron density in this cavity defines an undecapeptide sequence LGEPRYAFNFN, an observation that is confirmed by mass spectrometry. In the structure, the peptide interactions with the protein are mediated by main chain hydrogen bonds with the exception of Arg5 whose guanidinium side chain makes a set of defining polar interactions with four YejA residues. More detailed characterization of purified recombinant YejA, by a combination of ESI and MALDI- mass spectrometry as well as thermal shift assays, reveals a set of YejA complexes containing overlapping peptides 10-19 residues in length. All contain the sequence LGEPRYAFN. Curiously, these peptides correspond to residues 8-26 of the mature YejA protein, which belong to a unique N- terminal extension that distinguishes YejA from other cluster C oligopeptide binding proteins of known structure. This 35- residue extension is well- ordered and packs across the surface of the protein. The undecapeptide ligand occupies only a fraction of the enclosed pocket volume suggesting the possibility that much larger peptides or peptide conjugates could be accommodated, though thermal shift assays of YejA binding to antimicrobial peptides and peptides unrelated to LGEPRYAFNFN have not provided evidence of binding. While the physiological significance of this 'auto- binding' is not clear, the experimental data suggest that it is not an artefact of the crystallization process and that it may have a function in the sensing of periplasmic or membrane stress.

The production of human body odor is the result of the action of commensal skin bacteria, including Staphylococcus hominis, acting to biotransform odorless apocrine gland secretions into volatile chemicals like thioalcohols such as 3-methyl-3-sulphanylhexan-1-ol (3M3SH). As the secreted odor precursor Cys-Gly-3M3SH contains a dipeptide, yet the final enzyme in the biotransformation pathway only functions on Cys-3M3SH, we sought to identify the remaining step in this human-adapted biochemical pathway using a novel coupled enzyme assay. Purification of this activity from S. hominis extracts led to the identification of the M20A-family PepV peptidase (ShPepV) as the primary Cys-Gly-3M3SH dipeptidase. To establish whether this was a primary substrate for PepV, the recombinant protein was purified and demonstrated broad activity against diverse dipeptides. The binding site for Cys-Gly-3M3SH was predicted using modeling, which suggested mutations that might accommodate this ligand more favorably. Indeed, a D437A resulted in an almost sixfold increase in the kcat/Km, whereas other introduced mutations reduced or abolished function. Together, these data identify an enzyme capable of catalyzing the missing step in an ancient human-specific biochemical transformation and suggest that the production of 3M3SH uses neither a dedicated transporter nor a peptidase for its breakdown, with only the final cleavage step, catalyzed by PatB cysteine-S-conjugate β-lyase, being a unique enzyme.

In eukaryotic cells, molecular fate and cellular responses are shaped by multicomponent enzyme systems which reversibly attach ubiquitin and ubiquitin-like modifiers to target proteins. The extent of the ubiquitin proteasome system in Leishmania mexicana and its importance for parasite survival has recently been established through deletion mutagenesis and life-cycle phenotyping studies. The ubiquitin conjugating E2 enzyme UBC2, and the E2 enzyme variant UEV1, with which it forms a stable complex in vitro, were shown to be essential for the differentiation of promastigote parasites to the infectious amastigote form. To investigate further, we used immunoprecipitation of Myc-UBC2 or Myc-UEV1 to identify interacting proteins in L. mexicana promastigotes. The interactome of UBC2 comprises multiple ubiquitin-proteasome components including UEV1 and four RING E3 ligases, as well as potential substrates predicted to have roles in carbohydrate metabolism and intracellular trafficking. The smaller UEV1 interactome comprises six proteins, including UBC2 and shared components of the UBC2 interactome consistent with the presence of intracellular UBC2-UEV1 complexes. Recombinant RING1, RING2 and RING4 E3 ligases were shown to support ubiquitin transfer reactions involving the E1, UBA1a, and UBC2 to available substrate proteins or to unanchored ubiquitin chains. These studies define additional components of a UBC2-dependent ubiquitination pathway shown previously to be essential for promastigote to amastigote differentiation.

Leishmaniases are a collection of neglected tropical diseases caused by kinetoplastid parasites in the genus Leishmania. Current chemotherapies are severely limited and the need for new antileishmanials is of pressing international importance. Bromodomains are epigenetic reader domains that have shown promising therapeutic potential for cancer therapy and may also present an attractive target to treat parasitic diseases. Here, we investigate Leishmania donovani bromodomain factor 5 (LdBDF5) as a target for antileishmanial drug discovery. LdBDF5 contains pair of bromodomains (BD5.1 and BD5.2) in an N-terminal tandem repeat. We purified recombinant bromodomains of L. donovani BDF5 and determined the structure of BD5.2 by X-ray crystallography. Using a histone peptide microarray and fluorescence polarisation assay, we identified binding interactions of LdBDF5 bromodomains with acetylated peptides derived from histones H2B and H4. In orthogonal biophysical assays including thermal shift assays, fluorescence polarisation and NMR, we showed that BDF5 bromodomains bind to human bromodomain inhibitors SGC-CBP30, bromosporine and I-BRD9, moreover, SGC-CBP30 exhibited activity against Leishmania promastigotes in cell viability assays. These findings exemplify the potential BDF5 holds as a drug target in Leishmania and provide a foundation for the future development of optimised antileishmanial compounds targeting this epigenetic reader protein.

Microbes that have evolved to live on lignocellulosic biomass face unique challenges in the effective and efficient use of this material as food. The bacterium Shewanella sp. ANA-3 has the potential to utilize arabinan and arabinoxylan, and uptake of the monosaccharide, l-arabinose, derived from these polymers, is known to be mediated by a single ABC transporter. We demonstrate that the substrate binding protein of this system, GafASw, binds specifically to l-arabinofuranose, which is the rare furanose form of l-arabinose found in lignocellulosic biomass. The structure of GafASw was resolved to 1.7 Å and comparison to Escherichia coli YtfQ (GafAEc) revealed binding site adaptations that confer specificity for furanose over pyranose forms of monosaccharides, while selecting arabinose over another related monosaccharide, galactose. The discovery of a bacterium with a natural predilection for a sugar found abundantly in certain lignocellulosic materials suggests an intimate connection in the enzymatic release and uptake of the sugar, perhaps to prevent other microbes scavenging this nutrient before it mutarotates to l-arabinopyranose. This biological discovery also provides a clear route to engineer more efficient utilization of plant biomass components in industrial biotechnology.

Rhodococcus equi is a Gram-positive facultative intracellular pathogen associated with life-threatening bronchopneumonial disease in foals. Key to R. equi’s intracellular survival in host macrophages is the production of virulence associated proteins (Vaps). Numerous vap genes are found on virulence plasmids isolated from different species, and the Vaps share a high degree of sequence identity. VapA has been extensively studied, and although vapK and vapN genes from other R. equi virulence plasmids have been shown to be essential for R. equi intracellular survival, their mode of action is less characterised. We, therefore, examined whether VapK and VapN worked mechanistically in the same way as VapA. Indeed, like VapA, VapK and VapN neutralised lysosomal pH and reduced lysosomal hydrolase activity. A loss of VapA and R. equi virulence could be regained by the presence of either VapK or VapN. The acid-neutralisation activity was also observed to a lesser extent with VapB. There was a differential activity across these virulence-promoting Vaps with the most “acid-neutralising” activity found with VapN, then VapA and K, and finally VapB. These data suggest that VapA production, which is often found in equine infections, can be substituted by VapK and B (produced by plasmids often found in porcine species) or VapN (produced by plasmids often isolated in bovine and human samples). These data imply that the molecular mechanism(s) that VapA uses to neutralise lysosomal acidity should also be seen in VapN and K which will help guide researchers in identifying their precise mode of action and aid the future development of targeted therapeutics.

In eukaryotic cells, reversible attachment of ubiquitin and ubiquitin-like modifiers (Ubls) to specific target proteins is conducted by multicomponent systems whose collective actions control protein fate and cell behaviour in precise but complex ways. In trypanosomatids, attachment of ubiquitin and Ubls to target proteins regulates the cell cycle, endocytosis, protein sorting and degradation, autophagy and various aspects of infection and stress responses. The extent of these systems in trypanosomatids has been surveyed in recent reports, while in Leishmania mexicana, essential roles have been defined for many ubiquitin-system genes in deletion mutagenesis and life-cycle phenotyping campaigns. The first steps to elucidate the pathways of ubiquitin transfer among the ubiquitination components and to define the acceptor substrates and the downstream deubiquitinases are now being taken.

Peptide transporters play important nutritional and cell signalling roles in Bacillus subtilis, which are pronounced during stationary phase adaptations and development. Three high-affinity ATP-binding cassette (ABC) family transporters are involved in peptide uptake - the oligopeptide permease (Opp), another peptide permease (App) and a less well-characterized dipeptide permease (Dpp). Here we report crystal structures of the extracellular substrate binding proteins, OppA and DppE, which serve the Opp and Dpp systems, respectively. The structure of OppA was determined in complex with endogenous peptides, modelled as Ser-Asn-Ser-Ser, and with the sporulation-promoting peptide Ser-Arg-Asn-Val-Thr, which bind with K d values of 0.4 and 2 µM, respectively, as measured by isothermal titration calorimetry. Differential scanning fluorescence experiments with a wider panel of ligands showed that OppA has highest affinity for tetra- and penta-peptides. The structure of DppE revealed the unexpected presence of a murein tripeptide (MTP) ligand, l-Ala-d-Glu-meso-DAP, in the peptide binding groove. The mode of MTP binding in DppE is different to that observed in the murein peptide binding protein, MppA, from Escherichia coli, suggesting independent evolution of these proteins from an OppA-like precursor. The presence of MTP in DppE points to a role for Dpp in the uptake and recycling of cell wall peptides, a conclusion that is supported by analysis of the genomic context of dpp, which revealed adjacent genes encoding enzymes involved in muropeptide catabolism in a gene organization that is widely conserved in Firmicutes.

Ubiquitination is a post-translational modification conserved across eukaryotic species. It contributes to a variety of regulatory pathways, including proteasomal degradation, DNA repair, and cellular differentiation. The ubiquitination of substrate proteins typically requires three ubiquitination enzymes: a ubiquitin-activating E1, a ubiquitin-conjugating E2, and an E3 ubiquitin ligase. Cooperation between E2s and E3s is required for substrate ubiquitination, but some ubiquitin-conjugating E2s are also able to catalyze by themselves the formation of free di-ubiquitin, independently or in cooperation with a ubiquitin E2 variant. Here, we describe a method for assessing (i) di-ubiquitin formation by an E1 together with an E2 and an E2 variant, and (ii) the cooperation of an E3 with an E1 and E2 (with or without the E2 variant). Reaction products are assessed using western blotting with one of two antibodies: the first detects all ubiquitin conjugates, while the second specifically recognizes K63-linked ubiquitin. This allows unambiguous identification of ubiquitinated species and assessment of whether K63 linkages are present. We have developed these methods for studying ubiquitination proteins of Leishmania mexicana , specifically the activities of the E2, UBC2, and the ubiquitin E2 variant UEV1, but we anticipate the assays to be applicable to other ubiquitination systems with UBC2/UEV1 orthologues.

Leishmania are unicellular parasites that cause human and animal diseases. Like other kinetoplastids, they possess large transcriptional start regions (TSRs) which are defined by histone variants and histone lysine acetylation. Cellular interpretation of these chromatin marks is not well understood. Eight bromodomain factors, the reader modules for acetyl-lysine, are found across Leishmania genomes. Using L. mexicana, Cas9-driven gene deletions indicate that BDF1-5 are essential for promastigotes. Dimerisable, split Cre recombinase (DiCre)-inducible gene deletion of BDF5 show it is essential for both promastigotes and murine infection. ChIP-seq identifies BDF5 as enriched at TSRs. XL-BioID proximity proteomics shows the BDF5 landscape is enriched for BDFs, HAT2, proteins involved in transcriptional activity, and RNA processing; revealing a Conserved Regulators of Kinetoplastid Transcription (CRKT) Complex. Inducible deletion of BDF5 causes global reduction in RNA polymerase II transcription. Our results indicate the requirement of Leishmania to interpret histone acetylation marks through the bromodomain-enriched CRKT complex for normal gene expression and cellular viability.

EARLY FLOWERING3 sub-nuclear localization responds to changes in ambient temperature