Genetic evidence for the actin homolog gene mreBH and the bacitracin resistance gene bcrC as targets of the alternative sigma factor SigI of Bacillus subtilis

Bacterial SEAL domains undergo autoproteolysis and function in regulated intramembrane proteolysis

Gram-positive bacteria use SigI/RsgI-family sigma factor/anti-sigma factor pairs to sense and respond to cell wall defects and plant polysaccharides. In Bacillus subtilis, this signal transduction pathway involves regulated intramembrane proteolysis (RIP) of the membrane-anchored anti-sigma factor RsgI. However, unlike most RIP signaling pathways, site-1 cleavage of RsgI on the extracytoplasmic side of the membrane is constitutive and the cleavage products remain stably associated, preventing intramembrane proteolysis. The regulated step in this pathway is their dissociation, which is hypothesized to involve mechanical force. Release of the ectodomain enables intramembrane cleavage by the RasP site-2 protease and activation of SigI. The constitutive site-1 protease has not been identified for any RsgI homolog. Here, we report that RsgI's extracytoplasmic domain has structural and functional similarities to eukaryotic SEA domains that undergo autoproteolysis and have been implicated in mechanotransduction. We show that site-1 proteolysis in B. subtilis and Clostridial RsgI family members is mediated by enzyme-independent autoproteolysis of these SEA-like domains. Importantly, the site of proteolysis enables retention of the ectodomain through an undisrupted β-sheet that spans the two cleavage products. Autoproteolysis can be abrogated by relief of conformational strain in the scissile loop, in a mechanism analogous to eukaryotic SEA domains. Collectively, our data support the model that RsgI-SigI signaling is mediated by mechanotransduction in a manner that has striking parallels with eukaryotic mechanotransducive signaling pathways.

Bacterial SEAL domains undergo autoproteolysis and function in regulated intramembrane proteolysis

Gram-positive bacteria use SigI/RsgI-family sigma factor/anti-sigma factor pairs to sense and respond to cell wall defects and plant polysaccharides. In Bacillus subtilis this signal transduction pathway involves regulated intramembrane proteolysis (RIP) of the membrane-anchored anti-sigma factor RsgI. However, unlike most RIP signaling pathways, site-1 cleavage of RsgI on the extracytoplasmic side of the membrane is constitutive and the cleavage products remain stably associated, preventing intramembrane proteolysis. The regulated step in this pathway is their dissociation, which is hypothesized to involve mechanical force. Release of the ectodomain enables intramembrane cleavage by the RasP site-2 protease and activation of SigI. The constitutive site-1 protease has not been identified for any RsgI homolog. Here, we report that RsgI's extracytoplasmic domain has structural and functional similarities to eukaryotic SEA domains that undergo autoproteolysis and have been implicated in mechanotransduction. We show that site-1 proteolysis in B. subtilis and Clostridial RsgI family members is mediated by enzyme-independent autoproteolysis of these SEA-like (SEAL) domains. Importantly, the site of proteolysis enables retention of the ectodomain through an undisrupted ß-sheet that spans the two cleavage products. Autoproteolysis can be abrogated by relief of conformational strain in the scissile loop, in a mechanism analogous to eukaryotic SEA domains. Collectively, our data support the model that RsgI-SigI signaling is mediated by mechanotransduction in a manner that has striking parallels with eukaryotic mechanotransducive signaling pathways.

SIGNIFICANCE

SEA domains are broadly conserved among eukaryotes but absent in bacteria. They are present on diverse membrane-anchored proteins some of which have been implicated in mechanotransducive signaling pathways. Many of these domains have been found to undergo autoproteolysis and remain noncovalently associated following cleavage. Their dissociation requires mechanical force. Here, we identify a family of bacterial SEA-like (SEAL) domains that arose independently from their eukaryotic counterparts but have structural and functional similarities. We show these SEAL domains autocleave and the cleavage products remain stably associated. Importantly, these domains are present on membrane-anchored anti-sigma factors that have been implicated in mechanotransduction pathways analogous to those in eukaryotes. Our findings suggest that bacterial and eukaryotic signaling systems have evolved a similar mechanism to transduce mechanical stimuli across the lipid bilayer.

Peptidoglycan NlpC/P60 peptidases in bacterial physiology and host interactions

The bacterial cell wall is composed of a highly crosslinked matrix of glycopeptide polymers known as peptidoglycan that dictates bacterial cell morphology and protects against environmental stresses. Regulation of peptidoglycan turnover is therefore crucial for bacterial survival and growth and is mediated by key protein complexes and enzyme families. Here, we review the prevalence, structure, and activity of NlpC/P60 peptidases, a family of peptidoglycan hydrolases that are crucial for cell wall turnover and division as well as interactions with antibiotics and different hosts. Understanding the molecular functions of NlpC/P60 peptidases should provide important insight into bacterial physiology, their interactions with different kingdoms of life, and the development of new therapeutic approaches.

Intrinsically disordered protein regions are required for cell wall homeostasis in Bacillus subtilis

Intrinsically disordered protein regions (IDRs) have been implicated in diverse nuclear and cytoplasmic functions in eukaryotes, but their roles in bacteria are less clear. Here, we report that extracytoplasmic IDRs in Bacillus subtilis are required for cell wall homeostasis. The B. subtilis σ^I transcription factor is activated in response to envelope stress through regulated intramembrane proteolysis (RIP) of its membrane-anchored anti-σ factor, RsgI. Unlike canonical RIP pathways, we show that ectodomain (site-1) cleavage of RsgI is constitutive, but the two cleavage products remain stably associated, preventing intramembrane (site-2) proteolysis. The regulated step in this pathway is their dissociation, which is triggered by impaired cell wall synthesis and requires RsgI's extracytoplasmic IDR. Intriguingly, the major peptidoglycan polymerase PBP1 also contains an extracytoplasmic IDR, and we show that this region is important for its function. Disparate IDRs can replace the native IDRs on both RsgI and PBP1, arguing that these unstructured regions function similarly. Our data support a model in which the RsgI-σ^I signaling system and PBP1 represent complementary pathways to repair gaps in the PG meshwork. The IDR on RsgI senses these gaps and activates σ^I, while the IDR on PBP1 directs the synthase to these sites to fortify them.

σI from Bacillus subtilis: Impact on Gene Expression and Characterization of σI-Dependent Transcription That Requires New Types of Promoters with Extended -35 and -10 Elements

The σI sigma factor from Bacillus subtilis is a σ factor associated with RNA polymerase (RNAP) that was previously implicated in adaptation of the cell to elevated temperature. Here, we provide a comprehensive characterization of this transcriptional regulator. By transcriptome sequencing (RNA-seq) of wild-type (wt) and σI-null strains at 37°C and 52°C, we identified ∼130 genes affected by the absence of σI Further analysis revealed that the majority of these genes were affected indirectly by σI The σI regulon, i.e., the genes directly regulated by σI, consists of 16 genes, of which eight (the dhb and yku operons) are involved in iron metabolism. The involvement of σI in iron metabolism was confirmed phenotypically. Next, we set up an in vitro transcription system and defined and experimentally validated the promoter sequence logo that, in addition to -35 and -10 regions, also contains extended -35 and -10 motifs. Thus, σI-dependent promoters are relatively information rich in comparison with most other promoters. In summary, this study supplies information about the least-explored σ factor from the industrially important model organism B. subtilisIMPORTANCE In bacteria, σ factors are essential for transcription initiation. Knowledge about their regulons (i.e., genes transcribed from promoters dependent on these σ factors) is the key for understanding how bacteria cope with the changing environment and could be instrumental for biotechnologically motivated rewiring of gene expression. Here, we characterize the σI regulon from the industrially important model Gram-positive bacterium Bacillus subtilis We reveal that σI affects expression of ∼130 genes, of which 16 are directly regulated by σI, including genes encoding proteins involved in iron homeostasis. Detailed analysis of promoter elements then identifies unique sequences important for σI-dependent transcription. This study thus provides a comprehensive view on this underexplored component of the B. subtilis transcription machinery.

Regulation of biomass degradation by alternative σ factors in cellulolytic clostridia

Bacteria can adjust their genetic programs via alternative σ factors to face new environmental pressures. Here, we analyzed a unique set of paralogous alternative σ factors, termed σ^Is, which fine-tune the regulation of one of the most intricate cellulolytic systems in nature, the bacterial cellulosome, that is involved in degradation of environmental polysaccharides. We combined bioinformatics with experiments to decipher the regulatory networks of five σ^Is in Clostridium thermocellum, the epitome of cellulolytic microorganisms, and one σ^I in Pseudobacteroides cellulosolvens which produces the cellulosomal system with the greatest known complexity. Despite high homology between different σ^Is, our data suggest limited cross-talk among them. Remarkably, the major cross-talk occurs within the main cellulosomal genes which harbor the same σ^I-dependent promoter elements, suggesting a promoter-based mechanism to guarantee the expression of relevant genes. Our findings provide insights into the mechanisms used by σ^Is to differentiate among their corresponding regulons, representing a comprehensive overview of the regulation of the cellulosome to date. Finally, we show the advantage of using a heterologous host system for analysis of multiple σ^Is, since information generated by their analysis in their natural host can be misinterpreted owing to a cascade of interactions among the different σ^Is.

The Essential UPP Phosphatase Pair BcrC and UppP Connects Cell Wall Homeostasis during Growth and Sporulation with Cell Envelope Stress Response in Bacillus subtilis

The bacterial cell wall separates the cell from its surrounding and protects it from environmental stressors. Its integrity is maintained by a highly regulated process of cell wall biosynthesis. The membrane-located lipid II cycle provides cell wall building blocks that are assembled inside the cytoplasm to the outside for incorporation. Its carrier molecule, undecaprenyl phosphate (UP), is then recycled by dephosphorylation from undecaprenyl pyrophosphate (UPP). In Bacillus subtilis, this indispensable reaction is catalyzed by the UPP phosphatases BcrC and UppP. Here, we study the physiological function of both phosphatases with respect to morphology, cell wall homeostasis and the resulting cell envelope stress response (CESR). We demonstrate that uppP and bcrC represent a synthetic lethal gene pair, which encodes an essential physiological function. Accordingly, cell growth and morphology were severely impaired during exponential growth if the overall UPP phosphatase level was limiting. UppP, but not BcrC, was crucial for normal sporulation. Expression of bcrC, but not uppP, was upregulated in the presence of cell envelope stress conditions caused by bacitracin if UPP phosphatase levels were limited. This homeostatic feedback renders BcrC more important during growth than UppP, particularly in defense against cell envelope stress.

Genetic evidence that multiple proteases are involved in modulation of heat-induced activation of the sigma factor SigI in Bacillus subtilis

The Bacillus subtilis sigI-rsgI operon encodes the heat-inducible sigma factor SigI and its cognate anti-sigma factor RsgI. The heat-activated SigI positively regulates expression of sigI itself and genes involved in cell wall homeostasis and heat resistance. It remains unknown which protease(s) may contribute to degradation of RsgI and heat-induced activation of SigI. In this study, we found that transcription of sigI from its σI-dependent promoter under heat stress was downregulated in a strain lacking the heat-inducible sigma factor SigB. Deletion of protease-relevant clpP, clpC or rasP severely impaired sigI expression during heat stress, whereas deletion of clpE partially impaired sigI expression. Complementation of mutations with corresponding intact genes restored sigI expression. In a null mutant of rsgI, SigI was activated and sigI expression was strongly upregulated during normal growth and under heat stress. In this rsgI mutant, further inactivation of rasP or clpE did not affect sigI expression, whereas further inactivation of clpP or clpC severely or partially impaired sigI expression. Spx negatively influenced sigI expression during heat stress. Possible implications are discussed. Given that clpC, clpP and spx are directly regulated by SigB, SigB appears to control sigI expression under heat stress via ClpC, ClpP and Spx.

Revisiting the Regulation of the Primary Scaffoldin Gene in Clostridium thermocellum

Cellulosomes are considered to be one of the most efficient systems for the degradation of plant cell wall polysaccharides. The central cellulosome component comprises a large, noncatalytic protein subunit called scaffoldin. Multiple saccharolytic enzymes are incorporated into the scaffoldins via specific high-affinity cohesin-dockerin interactions. Recently, the regulation of genes encoding certain cellulosomal components by multiple RNA polymerase alternative σ^I factors has been demonstrated in Clostridium (Ruminiclostridium) thermocellum In the present report, we provide experimental evidence demonstrating that the C. thermocellum cipA gene, which encodes the primary cellulosomal scaffoldin, is regulated by several alternative σ^I factors and by the vegetative σ^A factor. Furthermore, we show that previously suggested transcriptional start sites (TSSs) of C. thermocellum cipA are actually posttranscriptional processed sites. By using comparative bioinformatic analysis, we have also identified highly conserved σ^I- and σ^A-dependent promoters upstream of the primary scaffoldin-encoding genes of other clostridia, namely, Clostridium straminisolvens, Clostridium clariflavum, Acetivibrio cellulolyticus, and Clostridium sp. strain Bc-iso-3. Interestingly, a previously identified TSS of the primary scaffoldin CbpA gene of Clostridium cellulovorans matches the predicted σ^I-dependent promoter identified in the present work rather than the previously proposed σ^A promoter. With the exception of C. cellulovorans, both σ^I and σ^A promoters of primary scaffoldin genes are located more than 600 nucleotides upstream of the start codon, yielding long 5'-untranslated regions (5'-UTRs). Furthermore, these 5'-UTRs have highly conserved stem-loop structures located near the start codon. We propose that these large 5'-UTRs may be involved in the regulation of both the primary scaffoldin and other cellulosomal components.IMPORTANCE Cellulosome-producing bacteria are among the most effective cellulolytic microorganisms known. This group of bacteria has biotechnological potential for the production of second-generation biofuels and other biocommodities from cellulosic wastes. The efficiency of cellulose hydrolysis is due to their cellulosomes, which arrange enzymes in close proximity on the cellulosic substrate, thereby increasing synergism among the catalytic domains. The backbone of these multienzyme nanomachines is the scaffoldin subunit, which has been the subject of study for many years. However, its genetic regulation is poorly understood. Hence, from basic and applied points of view, it is imperative to unravel the regulatory mechanisms of the scaffoldin genes. The understanding of these regulatory mechanisms can help to improve the performance of the industrially relevant strains of C. thermocellum and related cellulosome-producing bacteria en route to the consolidated bioprocessing of biomass.

YodL and YisK Possess Shape-Modifying Activities That Are Suppressed by Mutations in Bacillus subtilis mreB and mbl

Many bacteria utilize actin-like proteins to direct peptidoglycan (PG) synthesis. MreB and MreB-like proteins are thought to act as scaffolds, guiding the localization and activity of key PG-synthesizing proteins during cell elongation. Despite their critical role in viability and cell shape maintenance, very little is known about how the activity of MreB family proteins is regulated. Using a Bacillus subtilis misexpression screen, we identified two genes, yodL and yisK, that when misexpressed lead to loss of cell width control and cell lysis. Expression analysis suggested that yodL and yisK are previously uncharacterized Spo0A-regulated genes, and consistent with these observations, a ΔyodL ΔyisK mutant exhibited reduced sporulation efficiency. Suppressors resistant to YodL's killing activity occurred primarily in mreB mutants and resulted in amino acid substitutions at the interface between MreB and the highly conserved morphogenic protein RodZ, whereas suppressors resistant to YisK occurred primarily in mbl mutants and mapped to Mbl's predicted ATP-binding pocket. YodL's shape-altering activity appears to require MreB, as a ΔmreB mutant was resistant to the effects of YodL but not YisK. Similarly, YisK appears to require Mbl, as a Δmbl mutant was resistant to the cell-widening effects of YisK but not of YodL. Collectively, our results suggest that YodL and YisK likely modulate MreB and Mbl activity, possibly during the early stages of sporulation.

IMPORTANCE

The peptidoglycan (PG) component of the cell envelope confers structural rigidity to bacteria and protects them from osmotic pressure. MreB and MreB-like proteins are thought to act as scaffolds for PG synthesis and are essential in bacteria exhibiting nonpolar growth. Despite the critical role of MreB-like proteins, we lack mechanistic insight into how their activities are regulated. Here, we describe the discovery of two B. subtilis proteins, YodL and YisK, which modulate MreB and Mbl activities. Our data suggest that YodL specifically targets MreB, whereas YisK targets Mbl. The apparent specificities with which YodL and YisK are able to differentially target MreB and Mbl make them potentially powerful tools for probing the mechanics of cytoskeletal function in bacteria.

Teichoic Acid Polymers Affect Expression and Localization of dl-Endopeptidase LytE Required for Lateral Cell Wall Hydrolysis in Bacillus subtilis

In Bacillus subtilis, the dl-endopeptidase LytE is responsible for lateral peptidoglycan hydrolysis during cell elongation. We found that σ(I)-dependent transcription of lytE is considerably enhanced in a strain with a mutation in ltaS, which encodes a major lipoteichoic acid (LTA) synthase. Similar enhancements were observed in mutants that affect the glycolipid anchor and wall teichoic acid (WTA) synthetic pathways. Immunofluorescence microscopy revealed that the LytE foci were considerably increased in these mutants. The localization patterns of LytE on the sidewalls appeared to be helix-like in LTA-defective or WTA-reduced cells and evenly distributed on WTA-depleted or -defective cell surfaces. These results strongly suggested that LTA and WTA affect both σ(I)-dependent expression and localization of LytE. Interestingly, increased LytE localization along the sidewall in the ltaS mutant largely occurred in an MreBH-independent manner. Moreover, we found that cell surface decorations with LTA and WTA are gradually reduced at increased culture temperatures and that LTA rather than WTA on the cell surface is reduced at high temperatures. In contrast, the amount of LytE on the cell surface gradually increased under heat stress conditions. Taken together, these results indicated that reductions in these anionic polymers at high temperatures might give rise to increases in SigI-dependent expression and cell surface localization of LytE at high temperatures.

IMPORTANCE

The bacterial cell wall is required for maintaining cell shape and bearing environmental stresses. The Gram-positive cell wall consists of mesh-like peptidoglycan and covalently linked wall teichoic acid and lipoteichoic acid polymers. It is important to determine if these anionic polymers are required for proliferation and environmental adaptation. Here, we demonstrated that these polymers affect the expression and localization of a peptidoglycan hydrolase LytE required for lateral cell wall elongation. Moreover, we found that cell surface decorations with teichoic acid polymers are substantially decreased at high temperatures and that the peptidoglycan hydrolase is consequently increased. These findings suggest that teichoic acid polymers control lateral peptidoglycan hydrolysis by LytE, and bacteria drastically change their cell wall content to adapt to their environment.

Loss of σI affects heat-shock response and virulence gene expression in Bacillus anthracis

The pathogenesis of Bacillus anthracis depends on several virulence factors, including the anthrax toxin. Loss of the alternative sigma factor σI results in a coordinate decrease in expression of all three toxin subunits. Our observations suggest that loss of σI alters the activity of the master virulence regulator AtxA, but atxA transcription is unaffected by loss of σI. σI-containing RNA polymerase does not appear to directly transcribe either atxA or the toxin gene pagA. As in Bacillus subtilis, loss of σI in B. anthracis results in increased sensitivity to heat shock and transcription of sigI, encoding σI, is induced by elevated temperature. Encoded immediately downstream of and part of a bicistronic message with sigI is an anti-sigma factor, RsgI, which controls σI activity. Loss of RsgI has no direct effect on virulence gene expression. sigI appears to be expressed from both the σI and σA promoters, and transcription from the σA promoter is likely more significant to virulence regulation. We propose a model in which σI can be induced in response to heat shock, whilst, independently, σI is produced under non-heat-shock, toxin-inducing conditions to indirectly regulate virulence gene expression.

Decoding Biomass-Sensing Regulons of Clostridium thermocellum Alternative Sigma-I Factors in a Heterologous Bacillus subtilis Host System

The Gram-positive, anaerobic, cellulolytic, thermophile Clostridium (Ruminiclostridium) thermocellum secretes a multi-enzyme system called the cellulosome to solubilize plant cell wall polysaccharides. During the saccharolytic process, the enzymatic composition of the cellulosome is modulated according to the type of polysaccharide(s) present in the environment. C. thermocellum has a set of eight alternative RNA polymerase sigma (σ) factors that are activated in response to extracellular polysaccharides and share sequence similarity to the Bacillus subtilis σ^I factor. The aim of the present work was to demonstrate whether individual C. thermocellum σ^I-like factors regulate specific cellulosomal genes, focusing on C. thermocellum σ^I6 and σ^I3 factors. To search for putative σ^I6- and σ^I3-dependent promoters, bioinformatic analysis of the upstream regions of the cellulosomal genes was performed. Because of the limited genetic tools available for C. thermocellum, the functionality of the predicted σ^I6- and σ^I3-dependent promoters was studied in B. subtilis as a heterologous host. This system enabled observation of the activation of 10 predicted σ^I6-dependent promoters associated with the C. thermocellum genes: sigI6 (itself, Clo1313_2778), xyn11B (Clo1313_0522), xyn10D (Clo1313_0177), xyn10Z (Clo1313_2635), xyn10Y (Clo1313_1305), cel9V (Clo1313_0349), cseP (Clo1313_2188), sigI1 (Clo1313_2174), cipA (Clo1313_0627), and rsgI5 (Clo1313_0985). Additionally, we observed the activation of 4 predicted σ^I3-dependent promoters associated with the C. thermocellum genes: sigI3 (itself, Clo1313_1911), pl11 (Clo1313_1983), ce12 (Clo1313_0693) and cipA. Our results suggest possible regulons of σ^I6 and σ^I3 in C. thermocellum, as well as the σ^I6 and σ^I3 promoter consensus sequences. The proposed -35 and -10 promoter consensus elements of σ^I6 are CNNAAA and CGAA, respectively. Additionally, a less conserved CGA sequence next to the C in the -35 element and a highly conserved AT sequence three bases downstream of the -10 element were also identified as important nucleotides for promoter recognition. Regarding σ^I3, the proposed -35 and -10 promoter consensus elements are CCCYYAAA and CGWA, respectively. The present study provides new clues for understanding these recently discovered alternative σ^I factors.

A membrane transporter required for 3-hydroxybutyrate uptake during the early sporulation stage in Bacillus subtilis

Exogenous 3-hydroxybutyrate can be utilized by a variety of soil bacteria as a carbon and energy source. However, the membrane transporter responsible for 3-hydroxybutyrate uptake remains unidentified. The Bacillus subtilis strain 168 gene yxjC (herein renamed hbuT) encodes a putative gluconate transporter GntT-type membrane transporter with a previously unknown function. hbuT is organized within the same operon with genes that are used for metabolism of 3-hydroxybutyrate. Here we report that a null mutation of hbuT reduced uptake of 3-hydroxybutyrate by B. subtilis cells grown in nutrient sporulation medium. The SigE-controlled HbuT transporter apparently plays a major role in the uptake of 3-hydroxybutyrate. Uptake of 3-hydroxybutyrate by the HbuT transporter occurred in a specific manner at the early sporulation stage. SigE-controlled hbuT expression and 3-hydroxybutyrate uptake were also subject to CcpA-mediated glucose repression. hbuT expression was not induced by exogenous 3-hydroxybutyrate and B. subtilis cells could not utilize 3-hydroxybutyrate as a sole carbon source for growth. HbuT homologs are present in a wide variety of Gram-positive Bacillus species, some Gram-negative Acinetobacter species and a small group of other bacteria. This is the first tentative identification of a membrane transporter responsible for the uptake of 3-hydroxybutyrate in bacteria.

MreB-Dependent Inhibition of Cell Elongation during the Escape from Competence in Bacillus subtilis

During bacterial exponential growth, the morphogenetic actin-like MreB proteins form membrane-associated assemblies that move processively following trajectories perpendicular to the long axis of the cell. Such MreB structures are thought to scaffold and restrict the movement of peptidoglycan synthesizing machineries, thereby coordinating sidewall elongation. In Bacillus subtilis, this function is performed by the redundant action of three MreB isoforms, namely MreB, Mbl and MreBH. mreB and mbl are highly transcribed from vegetative promoters. We have found that their expression is maximal at the end of exponential phase, and rapidly decreases to a low basal level upon entering stationary phase. However, in cells developing genetic competence, a stationary phase physiological adaptation, expression of mreB was specifically reactivated by the central competence regulator ComK. In competent cells, MreB was found in complex with several competence proteins by in vitro pull-down assays. In addition, it co-localized with the polar clusters formed by the late competence peripheral protein ComGA, in a ComGA-dependent manner. ComGA has been shown to be essential for the inhibition of cell elongation characteristic of cells escaping the competence state. We show here that the pathway controlling this elongation inhibition also involves MreB. Our findings suggest that ComGA sequesters MreB to prevent cell elongation and therefore the escape from competence.

In bacterial cells, like in their eukaryotic counterparts, precise spatiotemporal localization of proteins is critical for their cellular function. This study shows that the expression and the localization of the bacterial actin-like MreB protein are growth phase-dependent. During exponential growth, we previously showed that MreB, together with other morphogenetic factors, forms discrete assemblies that move in a directed manner along peripheral tracks. Here, we demonstrate that in cells that develop genetic competence during stationary phase, transcription of mreB is specifically activated and MreB relocalizes to the cell poles. Our findings suggest a model in which MreB sequestration by the late competence protein ComGA prevents cell elongation during the escape from competence.

Two enzymes, TilS and HprT, can form a complex to function as a transcriptional activator for the cell division protease gene ftsH in Bacillus subtilis

The FtsH protein is an ATP-dependent cytoplasmic membrane protease involved in the control of membrane protein quality, cell division and heat shock response in Bacillus subtilis and many other bacteria. TilS, the tRNA(Ile2) lysidine synthetase, is a tRNA-binding protein that can modify pre-tRNA(Ile2). HprT, the hypoxanthine-guanine phosphoribosyltransferase, is implicated in purine salvage. Both tilS and hprT are essential for cell viability of B. subtilis. In this report, by co-purification experiments and gel filtration analyses, we show that there is complex formation between co-expressed TilS and HprT. Electrophoretic mobility shift assays and in vitro transcription analyses demonstrated that the TilS/HprT complex functions as a specific DNA-binding protein that can stimulate ftsH transcription in vitro. Two regions located upstream of the ftsH promoter have been identified as the TilS/HprT-binding sites and shown to be required for TilS/HprT-dependent ftsH transcription in vitro and in vivo. Results from gel supershift assays support the notion that the TilS/HprT complex likely employs its distinct segments for interaction with these two distinct TilS/HprT-binding sites, respectively. In conclusion, we present the first evidence that bi-functional TilS and HprT can form a complex to function as a transcriptional activator to stimulate ftsH transcription.

Involvement of OpcR, a GbsR-type transcriptional regulator, in negative regulation of two evolutionarily closely related choline uptake genes in Bacillus subtilis

The osmoprotectant glycine betaine can be generated intracellularly from conversion of the exogenous precursor choline by enzymes encoded by the gbsAB operon in Bacillus subtilis. Uptake of choline from outside B. subtilis cells is mediated through two evolutionarily closely related ATP-binding cassette transporters, OpuB and OpuC. Expression of the opuB operon and of the opuC operon is known to be osmoinducible. Here, we show that choline exerts a suppressive effect on opuC expression during normal growth and under osmotic stress. In the absence of the choline-responsive repressor GbsR, opuB expression is also suppressed by choline. We also report that a gene (formerly yvbF, now designated opcR) located immediately upstream of the opuC operon negatively regulates transcription of the opuC operon and, in the absence of GbsR, also that of the opuB operon. An inverted repeat (TTGTAAA-N8-TTTACAA) that overlaps with the -35 hexamer of the promoters of both operons has been identified as the OpcR operator. OpcR belongs to the GbsR-type transcriptional regulators. Its orthologues with unknown function are present in some other Bacillus species. Moreover, deletion analyses revealed that a region located further upstream of the promoters of the opuB operon and the opuC operon is critical for expression of both operons during normal growth and under osmotic stress. Osmotic induction of these two operons appears not to be OpcR mediated. OpcR is not a choline-responsive repressor. The possible biological role of OpcR is discussed.

FtsEX is required for CwlO peptidoglycan hydrolase activity during cell wall elongation in Bacillus subtilis

The peptidoglycan (PG) sacculus, a meshwork of polysaccharide strands cross-linked by short peptides, protects bacterial cells against osmotic lysis. To enlarge this covalently closed macromolecule, PG hydrolases must break peptide cross-links in the meshwork to allow insertion of new glycan strands between the existing ones. In the rod-shaped bacterium Bacillus subtilis, cell wall elongation requires two redundant endopeptidases, CwlO and LytE. However, it is not known how these potentially autolytic enzymes are regulated to prevent lethal breaches in the cell wall. Here, we show that the ATP-binding cassette transporter-like FtsEX complex is required for CwlO activity. In Escherichia coli, FtsEX is thought to harness ATP hydrolysis to activate unrelated PG hydrolases during cell division. Consistent with this regulatory scheme, B. subtilis FtsE mutants that are unable to bind or hydrolyse ATP cannot activate CwlO. Finally, we show that in cells depleted of both CwlO and LytE, the PG synthetic machinery continues moving circumferentially until cell lysis, suggesting that cross-link cleavage is not required for glycan strand polymerization. Overall, our data support a model in which the FtsEX complex is a remarkably flexible regulatory module capable of controlling a diverse set of PG hydrolases during growth and division in different organisms.

The WalRK (YycFG) and σ(I) RsgI regulators cooperate to control CwlO and LytE expression in exponentially growing and stressed Bacillus subtilis cells

The WalRK (YycFG) two-component system co-ordinates cell wall metabolism with growth by regulating expression of autolysins and proteins that modulate autolysin activity. Here we extend its role in cell wall metabolism by showing that WalR binds to 22 chromosomal loci in vivo. Among the newly identified genes of the WalRK bindome are those that encode the wall-associated protein WapA, the penicillin binding proteins PbpH and Pbp5, the minor teichoic acid synthetic enzymes GgaAB and the regulators σ(I) RsgI. The putative WalR binding sequence at many newly identified binding loci deviates from the previously defined consensus. Moreover, expression of many newly identified operons is controlled by multiple regulators. An unusual feature is that WalR binds to an extended DNA region spanning multiple open reading frames at some loci. WalRK directly activates expression of the sigIrsgI operon from a newly identified σ(A) promoter and represses expression from the previously identified σ(I) promoter. We propose that this regulatory link between WalRK and σ(I) RsgI expression ensures that the endopeptidase requirement (CwlO or LytE) for cell viability is fulfilled during growth and under stress conditions. Thus the WalRK and σ(I) RsgI regulatory systems cooperate to control cell wall metabolism in growing and stressed cells.

Reexamining transcriptional regulation of the Bacillus subtilis htpX gene and the ykrK gene, encoding a novel type of transcriptional regulator, and redefining the YkrK operator

HtpX is an integral cytoplasmic membrane metalloprotease well conserved in numerous bacteria. A recent study showed that expression of the Bacillus subtilis htpX gene is under dual negative control by Rok and a novel type of transcriptional regulator, YkrK. Here we report that expression of the B. subtilis htpX gene is strongly heat inducible. Contrary to the previous prediction, ykrK expression has been found to be not subject to autoregulation. We have identified the htpX promoter and the authentic ykrK promoter, which is also distinct from the previously predicted one. We have redefined a conserved inverted repeat sequence to be the YkrK operator, which is somewhat different from the previously proposed one. We provide evidence that YkrK is not a substrate of HtpX and that heat induction of htpX is not YkrK mediated. We have also found that the absence of FtsH or HtpX alone did not impair B. subtilis cell viability on LB agar plates at high temperature, whereas the absence of both FtsH and HtpX caused a severe growth defect under heat stress. This finding supports the notion that FtsH and HtpX may have partially overlapping functions in heat resistance. Finally, we show that htpX expression is subject to transient negative control by sigB under heat stress in a Rok- and YkrK-independent manner. Triple negative control of htpX expression at high temperature by rok, sigB, and ykrK may help cells to prevent uncontrolled and detrimental oversynthesis of the HtpX protease.

Genetic evidence for involvement of the alternative sigma factor SigI in controlling expression of the cell wall hydrolase gene lytE and contribution of LytE to heat survival of Bacillus subtilis

The Bacillus subtilis cell wall hydrolase LytE is involved in cell wall turnover and cell separation during vegetative growth. lytE transcription is known to be driven by a YycF-activated SigA-dependent promoter. The cell wall regulator SigI is an alternative sigma factor that has been shown to be heat stress-inducible and to be essential for survival of B. subtilis at high temperature. However, none of the previously identified target genes of SigI contribute to heat resistance. We now demonstrate that lytE expression is heat-inducible and that heat induction of lytE expression is strongly dependent on SigI. We have also found that the lytE mutant shows the same growth defect at high temperature as the sigI mutant. Introducing an extra copy of lytE into the sigI mutant could rescue its growth defect. Our data strongly suggest that SigI-dependent lytE expression under heat stress is important for heat survival of B. subtilis.

Assessment of transcriptional responses of Bacillus subtilis cells to the antibiotic enduracidin, which interferes with cell wall synthesis, using a high-density tiling chip

The cell envelope is the target for many antibiotics. In Gram-positive bacteria, membrane alterations and dysfunction caused by antibiotics are sensed mainly by two classes of signal transduction systems: the ECF sigma factors and the two-component signal transduction systems (TCSs). Enduracidin is an antibiotic that inhibits the transglycosylation step of peptidoglycan biosynthesis, and is an attractive target for further antibiotic development studies. We assessed transcriptional responses to enduracidin in Bacillus subtilis cells using a high-density tiling chip, and compared the results with responses to bacitracin, which inhibits the lipid II cycle of peptidoglycan synthesis. We exploited the quantitative advantage of the tiling chip to introduce a new criterion, an increase in transcriptional level, in addition to the conventional induction ratio, in order to distinguish genes of biological significance from those with lower induction ratios. Our results indicate that introduction of the new criterion led to unambiguous identification of core transcriptional responses to antibiotics, with a reduction in the number of possible background genes, compared to previous results obtained using gene arrays. We identified 129 genes that were significantly upregulated by enduracidin and/or bacitracin. Notably, we found that inactivation of the LiaRS TCS, which was the system most strongly induced by the two antibiotics, resulted in increased sensitivity to enduracidin, probably through a failure to induce LiaIH proteins. We noted that 33 genes belonging to the SigM regulon were induced by both antibiotics. Consistent with stronger induction of the SigM regulon in enduracidin-treated cells, inactivation of sigM resulted in increased sensitivity to enduracidin. In addition, and for the first time, we found that the Spx regulon was induced in cells challenged by enduracidin and bacitracin, suggesting that thiol-oxidative stress occurred in cells treated with antibiotics. These findings contribute to further our understanding of the molecular nature of genetic systems involved in antibiotic resistance.

Time-resolved transcriptome analysis of Bacillus subtilis responding to valine, glutamate, and glutamine

Microorganisms can restructure their transcriptional output to adapt to environmental conditions by sensing endogenous metabolite pools. In this paper, an Agilent customized microarray representing 4,106 genes was used to study temporal transcript profiles of Bacillus subtilis in response to valine, glutamate and glutamine pulses over 24 h. A total of 673, 835, and 1135 amino-acid-regulated genes were identified having significantly changed expression at one or more time points in response to valine, glutamate, and glutamine, respectively, including genes involved in cell wall, cellular import, metabolism of amino-acids and nucleotides, transcriptional regulation, flagellar motility, chemotaxis, phage proteins, sporulation, and many genes of unknown function. Different amino acid treatments were compared in terms of both the global temporal profiles and the 5-minute quick regulations, and between-experiment differential genes were identified. The highlighted genes were analyzed based on diverse sources of gene functions using a variety of computational tools, including T-profiler analysis, and hierarchical clustering. The results revealed the common and distinct modes of action of these three amino acids, and should help to elucidate the specific signaling mechanism of each amino acid as an effector.

The cell wall regulator {sigma}I specifically suppresses the lethal phenotype of mbl mutants in Bacillus subtilis

Bacterial actin homologues are thought to have a role in cell shape determination by positioning the cell wall synthetic machinery. They are also thought to control other functions, including cell polarity and chromosome segregation in various organisms. Bacillus subtilis and many other gram-positive bacteria have three actin isoforms, MreB, Mbl, and MreBH, which colocalize in helical structures that span the length of the cell, close to the inner surface of the cytoplasmic membrane. Deletion of the mbl gene has previously been reported to produce viable, although poorly growing, mutant cells. We now show that under normal conditions Deltambl cells are nonviable but suppressors allowing growth readily accumulate. In the presence of high concentrations of Mg(2+), viable, nonsuppressed mutants can be obtained. A screen for suppressor mutations revealed that deletion of rsgI restores Mg(2+)-independent growth of the mbl mutant. Recent work has shown that rsgI deletion leads to upregulation of the alternative sigma factor sigma(I). The basis of suppression is not yet clear, but it is independent of the Mg(2+) effect. We found that the construction of a triple mutant lacking all three actin homologues became possible in the rsgI background. Triple mutant cells are spherical, but no significant defect in chromosome segregation was detected.