YkdA and YvtA, HtrA-like serine proteases in Bacillus subtilis, engage in negative autoregulation and reciprocal cross-regulation of ykdA and yvtA gene expression

The CssRS two-component system of Bacillus subtilis contributes to teicoplanin and polymyxin B response

CssRS is a two-component system that plays a pivotal role in mediating the secretion stress response in Bacillus subtilis. This system upregulates the synthesis of membrane-bound HtrA family proteases that cope with misfolded proteins that accumulate within the cell envelope as a result of overexpression or heat shock. Recent studies have shown the induction of CssRS-regulated genes in response to cell envelope stress. We investigated the induction of the CssRS-regulated htrA promoter in the presence of different cell wall- and membrane-active substances and observed induction of the CssRS-controlled genes by glycopeptides (vancomycin and teicoplanin), polymyxins B and E, certain β-lactams, and detergents. Teicoplanin was shown to elicit remarkably stronger induction than vancomycin and polymyxin B. Teicoplanin and polymyxin B induced the spxO gene expression in a CssRS-dependent fashion, resulting in increased activity of Spx, a master regulator of disulfide stress in Bacillus subtilis. The CssRS signaling pathway and Spx activity were demonstrated to be involved in Bacillus subtilis resistance to teicoplanin and polymyxin B.

Enhancing bacterial fitness and recombinant enzyme yield by engineering the quality control protease HtrA of Bacillus subtilis

IMPORTANCE

In the expanding market of recombinant proteins, microbial cell factories such as Bacillus subtilis are key players. Microbial cell factories experience secretion stress during high-level production of secreted proteins, which can negatively impact product yield and cell viability. The CssRS two-component system and CssRS-regulated quality control proteases HtrA and HtrB play critical roles in the secretion stress response. HtrA has a presumptive dual function in protein quality control by exerting both chaperone-like and protease activities. However, its potential role as a chaperone has not been explored in B. subtilis. Here, we describe for the first time the beneficial effects of proteolytically inactive HtrA on α-amylase yields and overall bacterial fitness.

A linear and circular dual-conformation noncoding RNA involved in oxidative stress tolerance in Bacillus altitudinis

Circular RNAs have been extensively studied in eukaryotes, but their presence and/or biological functionality in bacteria are unclear. Here, we show that a regulatory noncoding RNA (DucS) exists in both linear and circular conformation in Bacillus altitudinis. The linear forms promote B. altitudinis tolerance to H2O2 stress, partly through increased translation of a stress-responsive gene, htrA. The 3' end sequences of the linear forms are crucial for RNA circularization, and formation of circular forms can decrease the levels of the regulatory linear cognates. Bioinformatic analysis of available RNA-seq datasets from 30 bacterial species revealed multiple circular RNA candidates, distinct from DucS, for all the examined species. Experiments testing for the presence of selected circular RNA candidates in four species successfully validated 7 out of 9 candidates from B. altitudinis and 4 out of 5 candidates from Bacillus paralicheniformis; However, none of the candidates tested for Bacillus subtilis and Escherichia coli were detected. Our work identifies a dual-conformation regulatory RNA in B. altitutidinis, and indicates that circular RNAs exist in diverse bacteria. However, circularization of specific RNAs does not seem to be conserved across species, and the circularization mechanisms and biological functionality of the circular forms remain unclear.

The ins and outs of Bacillus proteases: activities, functions and commercial significance

Because the majority of bacterial species divide by binary fission, and do not have distinguishable somatic and germline cells, they could be considered to be immortal. However, bacteria ‘age’ due to damage to vital cell components such as DNA and proteins. DNA damage can often be repaired using efficient DNA repair mechanisms. However, many proteins have a functional ‘shelf life’; some are short lived, while others are relatively stable. Specific degradation processes are built into the life span of proteins whose activities are required to fulfil a specific function during a prescribed period of time (e.g. cell cycle, differentiation process, stress response). In addition, proteins that are irreparably damaged or that have come to the end of their functional life span need to be removed by quality control proteases. Other proteases are involved in performing a variety of specific functions that can be broadly divided into three categories: processing, regulation and feeding. This review presents a systematic account of the proteases of Bacillus subtilis and their activities. It reviews the proteases found in, or associated with, the cytoplasm, the cell membrane, the cell wall and the external milieu. Where known, the impacts of the deletion of particular proteases are discussed, particularly in relation to industrial applications.

Relative contributions of non-essential Sec pathway components and cell envelope-associated proteases to high-level enzyme secretion by Bacillus subtilis

Background

Bacillus subtilis is an important industrial workhorse applied in the production of many different commercially relevant proteins, especially enzymes. Virtually all of these proteins are secreted via the general secretion (Sec) pathway. Studies from different laboratories have demonstrated essential or non-essential contributions of various Sec machinery components to protein secretion in B. subtilis. However, a systematic comparison of the impact of each individual Sec machinery component under conditions of high-level protein secretion was so far missing.

Results

In the present study, we have compared the contributions of non-essential Sec pathway components and cell envelope-associated proteases on the secretion efficiency of three proteins expressed at high level. This concerned the α-amylases AmyE from B. subtilis and AmyL from Bacillus licheniformis, and the serine protease BPN’ from Bacillus amyloliquefaciens. We compared the secretion capacity of mutant strains in shake flask cultures, and the respective secretion kinetics by pulse-chase labeling experiments. The results show that secDF, secG or rasP mutations severely affect AmyE, AmyL and BPN’ secretion, but the actual effect size depends on the investigated protein. Additionally, the chaperone DnaK is important for BPN’ secretion, while AmyE or AmyL secretion are not affected by a dnaK deletion. Further, we assessed the induction of secretion stress responses in mutant strains by examining AmyE- and AmyL-dependent induction of the quality control proteases HtrA and HtrB. Interestingly, the deletion of certain sip genes revealed a strong differential impact of particular signal peptidases on the magnitude of the secretion stress response.

Conclusions

The results of the present study highlight the importance of SecDF, SecG and RasP for protein secretion and reveal unexpected differences in the induction of the secretion stress response in different mutant strains.

Proteases HtrA and HtrB for α-amylase secreted from Bacillus subtilis in secretion stress

HtrA and HtrB are two important proteases across species. In biotechnological industries, they are related to degradation of secreted heterologous proteins from bacteria, especially in the case of overproduction of α-amylases in Bacillus subtilis. Induction of HtrA and HtrB synthesis follows the overproduction of α-amylases in B. subtilis. This is different from the order usually observed in B. subtilis, i.e., the production of proteases is prior to the secretion of proteins. This discrepancy suggests three possibilities: (i) HtrA and HtrB are constantly synthesized from the end of the exponential phase, and then are synthesized more abundantly due to secretion stress; (ii) There is a hysteresis mechanism that holds HtrA and HtrB back from their large amount of secretion before the overproduction of α-amylases; (iii) Heterologous amylases could be a stress to B. subtilis leading to a general response to stress. In this review, we analyze the literature to explore these three possibilities. The first possibility is attributed to the regulatory pathway of CssR-CssS. The second possibility is because sigma factor σD plays a role in the overproduction of α-amylases and is subpopulation dependent with the switch between "ON" and "OFF" states that is fundamental for a bistable system and a hysteresis mechanism. Thus, sigma factor σD helps to hold HtrA and HtrB back from massive secretion before the overproduction of α-amylases. The third possibility is that several sigma factors promote the secretion of proteases at the end of the exponential phase of growth under the condition that heterologous amylases are considered as a stress.

Distinct Contribution of the HtrA Protease and PDZ Domains to Its Function in Stress Resilience and Virulence of Bacillus anthracis

Anthrax is a lethal disease caused by the Gram-positive spore-producing bacterium Bacillus anthracis. We previously demonstrated that disruption of htrA gene, encoding the chaperone/protease HtrA_BA (High Temperature Requirement A of B. anthracis) results in significant virulence attenuation, despite unaffected ability of ΔhtrA strains (in which the htrA gene was deleted) to synthesize the key anthrax virulence factors: the exotoxins and capsule. B. anthracis ΔhtrA strains exhibited increased sensitivity to stress regimens as well as silencing of the secreted starvation-associated Neutral Protease A (NprA) and down-modulation of the bacterial S-layer. The virulence attenuation associated with disruption of the htrA gene was suggested to reflect the susceptibility of ΔhtrA mutated strains to stress insults encountered in the host indicating that HtrA_BA represents an important B. anthracis pathogenesis determinant. As all HtrA serine proteases, HtrA_BA exhibits a protease catalytic domain and a PDZ domain. In the present study we interrogated the relative impact of the proteolytic activity (mediated by the protease domain) and the PDZ domain (presumably necessary for the chaperone activity and/or interaction with substrates) on manifestation of phenotypic characteristics mediated by HtrA_BA. By inspecting the phenotype exhibited by ΔhtrA strains trans-complemented with either a wild-type, truncated (ΔPDZ), or non-proteolytic form (mutated in the catalytic serine residue) of HtrA_BA, as well as strains exhibiting modified chromosomal alleles, it is shown that (i) the proteolytic activity of HtrA_BA is essential for its N-terminal autolysis and subsequent release into the extracellular milieu, while the PDZ domain was dispensable for this process, (ii) the PDZ domain appeared to be dispensable for most of the functions related to stress resilience as well as involvement of HtrA_BA in assembly of the bacterial S-layer, (iii) conversely, the proteolytic activity but not the PDZ domain, appeared to be dispensable for the role of HtrA_BA in mediating up-regulation of the extracellular protease NprA under starvation stress, and finally (iv) in a murine model of anthrax, the HtrA_BA PDZ domain, was dispensable for manifestation of B. anthracis virulence. The unexpected dispensability of the PDZ domain may represent a unique characteristic of HtrA_BA amongst bacterial serine proteases of the HtrA family.

Stabilizing displayed proteins on vegetative Bacillus subtilis cells

Microbes engineered to display heterologous proteins could be useful biotechnological tools for protein engineering, lignocellulose degradation, biocatalysis, bioremediation, and biosensing. Bacillus subtilis is a promising host to display proteins, as this model Gram-positive bacterium is genetically tractable and already used industrially to produce enzymes. To gain insight into the factors that affect displayed protein stability and copy number, we systematically compared the ability of different protease-deficient B. subtilis strains (WB800, BRB07, BRB08, and BRB14) to display a Cel8A-LysM reporter protein in which the Clostridium thermocellum Cel8A endoglucanase is fused to LysM cell wall binding modules. Whole-cell cellulase measurements and fractionation experiments demonstrate that genetically eliminating extracytoplasmic bacterial proteases improves Cel8A-LysM display levels. However, upon entering stationary phase, for all protease-deficient strains, the amount of displayed reporter dramatically decreases, presumably as a result of cellular autolysis. This problem can be partially overcome by adding chemical protease inhibitors, which significantly increase protein display levels. We conclude that strain BRB08 is well suited for stably displaying our reporter protein, as genetic removal of its extracellular and cell wall-associated proteases leads to the highest levels of surface-accumulated Cel8A-LysM without causing secretion stress or impairing growth. A two-step procedure is presented that enables the construction of enzyme-coated vegetative B. subtilis cells that retain stable cell-associated enzyme activity for nearly 3 days. The results of this work could aid the development of whole-cell display systems that have useful biotechnological applications.

The Cellular Mechanisms that Ensure an Efficient Secretion in Streptomyces

Gram-positive soil bacteria included in the genus Streptomyces produce a large variety of secondary metabolites in addition to extracellular hydrolytic enzymes. From the industrial and commercial viewpoints, the S. lividans strain has generated greater interest as a host bacterium for the overproduction of homologous and heterologous hydrolytic enzymes as an industrial application, which has considerably increased scientific interest in the characterization of secretion routes in this bacterium. This review will focus on the secretion machinery in S. lividans.

Release of an HtrA-Like Protease from the Cell Surface of Thermophilic Brevibacillus sp. WF146 via Substrate-Induced Autoprocessing of the N-terminal Membrane Anchor

High-temperature requirement A (HtrA)-like proteases participate in protein quality control in prokaryotes and eukaryotes by degrading damaged proteins; however, little is known about HtrAs produced by thermophiles. HtrAw is an HtrA-like protease of thermophilic Brevibacillus sp. WF146. The intact form of HtrAw (iHtrAw) consisting of a transmembrane segment-containing N-terminal domain, a trypsin-like protease domain, and a C-terminal PDZ domain was produced in Escherichia coli. Purified iHtrAw itself is unable to cleave the N-terminal domain, but requires protein substrates to autoprocess the N-terminal domain intermolecularly, yielding a short form (sHtrAw). Mutation at the substrate-binding site in the PDZ domain affects the conversion of iHtrAw to sHtrAw. Deletion analysis revealed that the N-terminal domain is not necessary for enzyme folding, activity, and thermostability. Compared with other known HtrAs, HtrAw contains an additional Ca²⁺-binding Dx[DN]xDG motif important for enzyme stability and/or activity. When produced in an htrA/htrB double deletion mutant of Bacillus subtilis, iHtrAw localized predominantly to the cell pellet, and the amount of sHtrAw in the culture supernatant increased at elevated temperatures. Moreover, HtrAw increased the heat resistance of the B. subtilis mutant. In strain WF146, HtrAw exists in both a cell-associated intact form and a cell-free short form; an increase in growth temperature enhanced HtrAw production and the amount of cell-free short form. Release of the short form of HtrAw from the membrane may have the advantage of allowing the enzyme to freely access and degrade damaged proteins surrounding the bacterium living at high temperatures.

The Three Streptomyces lividans HtrA-Like Proteases Involved in the Secretion Stress Response Act in a Cooperative Manner

Overproduction of Sec-proteins in S. lividans accumulates misfolded proteins outside of the cytoplasmic membrane where the accumulated proteins interfere with the correct functioning of the secretion machinery and with the correct cell functionality, triggering the expression in S. lividans of a CssRS two-component system which regulates the degradation of the accumulated protein, the so-called secretion stress response. Optimization of secretory protein production via the Sec route requires the identification and characterisation of quality factors involved in this process. The phosphorylated regulator (CssR) interacts with the regulatory regions of three genes encoding three different HtrA-like proteases. Individual mutations in each of these genes render degradation of the misfolded protein inoperative, and propagation in high copy number of any of the three proteases encoding genes results on indiscriminate alpha-amylase degradation. None of the proteases could complement the other two deficiencies and only propagation of each single copy protease gene can restore its own deficiency. The obtained results strongly suggest that the synthesis of the three HtrA-like proteases needs to be properly balanced to ensure the effective degradation of misfolded overproduced secretory proteins and, at the same time, avoid negative effects in the secreted proteins and the secretion machinery. This is particularly relevant when considering the optimisation of Streptomyces strains for the overproduction of homologous or heterologous secretory proteins of industrial application.

Molecular engineering of secretory machinery components for high-level secretion of proteins in Bacillus species

Secretory expression of valuable enzymes by Bacillus subtilis and its related species has attracted intensive work over the past three decades. Although many proteins have been expressed and secreted, the titers of some recombinant enzymes are still low to meet the needs of practical applications. Signal peptides that located at the N-terminal of nascent peptide chains play crucial roles in the secretion process. In this mini-review, we summarize recent progress in secretory expression of recombinant proteins in Bacillus species. In particular, we highlighted and discussed the advances in molecular engineering of secretory machinery components, construction of signal sequence libraries and identification of functional signal peptides with high-throughput screening strategy. The prospects of future research are also proposed.

Degradation of extracytoplasmic catalysts for protein folding in Bacillus subtilis

The general protein secretion pathway of Bacillus subtilis has a high capacity for protein export from the cytoplasm, which is exploited in the biotechnological production of a wide range of enzymes. These exported proteins pass the membrane in an unfolded state, and accordingly, they have to fold into their active and protease-resistant conformations once membrane passage is completed. The lipoprotein PrsA and the membrane proteins HtrA and HtrB facilitate the extracytoplasmic folding and quality control of exported proteins. Among the native exported proteins of B. subtilis are at least 10 proteases that have previously been implicated in the degradation of heterologous secreted proteins. Recently, we have shown that these proteases also degrade many native membrane proteins, lipoproteins, and secreted proteins. The present studies were therefore aimed at assessing to what extent these proteases also degrade extracytoplasmic catalysts for protein folding. To this end, we employed a collection of markerless protease mutant strains that lack up to 10 different extracytoplasmic proteases. The results show that PrsA, HtrA, and HtrB are indeed substrates of multiple extracytoplasmic proteases. Thus, improved protein secretion by multiple-protease-mutant strains may be related to both reduced proteolysis and improved posttranslocational protein folding and quality control.

Extracytoplasmic proteases determining the cleavage and release of secreted proteins, lipoproteins, and membrane proteins in Bacillus subtilis

Gram-positive bacteria are known to export many proteins to the cell wall and growth medium, and accordingly, many studies have addressed the respective protein export mechanisms. In contrast, very little is known about the subsequent fate of these proteins. The present studies were therefore aimed at determining the fate of native exported proteins in the model organism Bacillus subtilis. Specifically, we employed a gel electrophoresis-based liquid chromatography-mass spectrometry approach to distinguish the roles of the membrane-associated quality control proteases HtrA and HtrB from those of eight other proteases that are present in the cell wall and/or growth medium of B. subtilis. Notably, HtrA and HtrB were previously shown to counteract potentially detrimental "protein export stresses" upon overproduction of membrane or secreted proteins. Our results show that many secreted proteins, lipoproteins, and membrane proteins of B. subtilis are potential substrates of extracytoplasmic proteases. Moreover, potentially important roles of HtrA and HtrB in the folding of native secreted proteins into a protease-resistant conformation, the liberation of lipoproteins from the membrane-cell wall interface, and the degradation of membrane proteins are uncovered. Altogether, our observations show that HtrA and HtrB are crucial for maintaining the integrity of the B. subtilis cell even under nonstress conditions.

Analysis of the proteolysis of bioactive peptides using a peptidomics approach

Identifying the peptidases that inactivate bioactive peptides (e.g., peptide hormones and neuropeptides) in mammals is an important unmet challenge. This protocol describes a recent approach that uses liquid chromatography-mass spectrometry (LC-MS) peptidomics to identify endogenous cleavage sites of a bioactive peptide; it also addresses the subsequent biochemical purification of a candidate peptidase on the basis of these cleavage sites and the validation of the candidate peptidase's role in the physiological regulation of the bioactive peptide by examining a peptidase-knockout mouse. We highlight the successful application of this protocol in the discovery that insulin-degrading enzyme (IDE) regulates physiological calcitonin gene-related peptide (CGRP) levels, and we detail the key stages and steps in this approach. This protocol requires 7 d of work; however, the total time for this protocol is highly variable because of its dependence on the availability of biological reagents such as purified enzymes and knockout mice. The protocol is valuable because it expedites the characterization of mammalian peptidases, such as IDE, which in certain instances can be used to develop novel therapeutics.

Integrated OMICS guided engineering of biofuel butanol-tolerance in photosynthetic Synechocystis sp. PCC 6803

BACKGROUND

Photosynthetic cyanobacteria have been recently proposed as a 'microbial factory' to produce butanol due to their capability to utilize solar energy and CO2 as the sole energy and carbon sources, respectively. However, to improve the productivity, one key issue needed to be addressed is the low tolerance of the photosynthetic hosts to butanol.

RESULTS

In this study, we first applied a quantitative transcriptomics approach with a next-generation RNA sequencing technology to identify gene targets relevant to butanol tolerance in a model cyanobacterium Synechocystis sp. PCC 6803. The results showed that 278 genes were induced by the butanol exposure at all three sampling points through the growth time course. Genes encoding heat-shock proteins, oxidative stress related proteins, transporters and proteins involved in common stress responses, were induced by butanol exposure. We then applied GC-MS based metabolomics analysis to determine the metabolic changes associated with the butanol exposure. The results showed that 46 out of 73 chemically classified metabolites were differentially regulated by butanol treatment. Notably, 3-phosphoglycerate, glycine, serine and urea related to general stress responses were elevated in butanol-treated cells. To validate the potential targets, we constructed gene knockout mutants for three selected gene targets. The comparative phenotypic analysis confirmed that these genes were involved in the butanol tolerance.

CONCLUSION

The integrated OMICS analysis provided a comprehensive view of the complicated molecular mechanisms employed by Synechocystis sp. PCC 6803 against butanol stress, and allowed identification of a series of potential gene candidates for tolerance engineering in cyanobacterium Synechocystis sp. PCC 6803.

A novel two-component system involved in secretion stress response in Streptomyces lividans

Background

Misfolded proteins accumulating outside the bacterial cytoplasmic membrane can interfere with the secretory machinery, hence the existence of quality factors to eliminate these misfolded proteins is of capital importance in bacteria that are efficient producers of secretory proteins. These bacteria normally use a specific two-component system to respond to the stress produced by the accumulation of the misfolded proteins, by activating the expression of HtrA-like proteases to specifically eliminate the incorrectly folded proteins.

Methodology/Principal Findings

Overproduction of alpha-amylase in S. lividans causing secretion stress permitted the identification of a two-component system (SCO4156-SCO4155) that regulates three HtrA-like proteases which appear to be involved in secretion stress response. Mutants in each of the genes forming part of the two-genes operon that encodes the sensor and regulator protein components accumulated misfolded proteins outside the cell, strongly suggesting the involvement of this two-component system in the S. lividans secretion stress response.

Conclusions/Significance

To our knowledge this is the first time that a specific secretion stress response two-component system is found to control the expression of three HtrA-like protease genes in S. lividans, a bacterium that has been repeatedly used as a host for the synthesis of homologous and heterologous secretory proteins of industrial application.

High external pH enables more efficient secretion of alkaline α-amylase AmyK38 by Bacillus subtilis

Background

Bacillus subtilis genome-reduced strain MGB874 exhibits enhanced production of exogenous extracellular alkaline cellulase Egl-237 and subtilisin-like alkaline protease M-protease. Here, we investigated the suitability of strain MGB874 for the production of α-amylase, which was anticipated to provoke secretion stress responses involving the CssRS (Control secretion stress Regulator and Sensor) system.

Results

Compared to wild-type strain 168, the production of a novel alkaline α-amylase, AmyK38, was severely decreased in strain MGB874 and higher secretion stress responses were also induced. Genetic analyses revealed that these phenomena were attributable to the decreased pH of growth medium as a result of the lowered expression of rocG, encoding glutamate dehydrogenase, whose activity leads to NH₃ production. Notably, in both the genome-reduced and wild-type strains, an up-shift of the external pH by the addition of an alkaline solution improved AmyK38 production, which was associated with alleviation of the secretion stress response. These results suggest that the optimal external pH for the secretion of AmyK38 is higher than the typical external pH of growth medium used to culture B. subtilis. Under controlled pH conditions, the highest production level (1.08 g l^-1) of AmyK38 was obtained using strain MGB874.

Conclusions

We demonstrated for the first time that RocG is an important factor for secretory enzyme production in B. subtilis through its role in preventing acidification of the growth medium. As expected, a higher external pH enabled a more efficient secretion of the alkaline α-amylase AmyK38 in B. subtilis. Under controlled pH conditions, the reduced-genome strain MGB874 was demonstrated to be a beneficial host for the production of AmyK38.

Signal perception by the secretion stress-responsive CssRS two-component system in Bacillus subtilis

The CssRS two-component system responds to heat and secretion stresses in Bacillus subtilis by controlling expression of HtrA and HtrB chaperone-type proteases and positively autoregulating its own expression. Here we report on the features of the CssS extracellular loop domain that are involved in signal perception and on CssS subcellular localization. Individual regions of the CssS extracellular loop domain contribute differently to signal perception and activation. The conserved hydrophilic 26-amino-acid segment juxtaposed to transmembrane helix 1 is involved in the switch between the deactivated and activated states, while the conserved 19-amino-acid hydrophobic segment juxtaposed to transmembrane 2 is required for signal perception and/or transduction. Perturbing the size of the extracellular loop domain increases CssS kinase activity and makes it unresponsive to secretion stress. CssS is localized primarily at the septum but is also found in a punctate pattern with lower intensity throughout the cell cylinder. Moreover, the CssRS-controlled HtrA and HtrB proteases are randomly distributed in foci throughout the cell surface, with more HtrB than HtrA foci in unstressed cells.

HtrA is a major virulence determinant of Bacillus anthracis

We demonstrate that disruption of the htrA (high temperature requirement A) gene in either the virulent Bacillus anthracis Vollum (pXO1(+) , pXO2(+) ), or in the ΔVollum (pXO1(-), pXO2(-), nontoxinogenic and noncapsular) strains, affect significantly the ability of the resulting mutants to withstand heat, oxidative, ethanol and osmotic stress. The ΔhtrA mutants manifest altered secretion of several proteins, as well as complete silencing of the abundant extracellular starvation-associated neutral protease A (NprA). VollumΔhtrA bacteria exhibit delayed proliferation in a macrophage infection assay, and despite their ability to synthesize the major B. anthracis toxins LT (lethal toxin) and ET (oedema toxin) as well as the capsule, show a decrease of over six orders of magnitude in virulence (lethal dose 50% = 3 × 10(8) spores, in the guinea pig model of anthrax), as compared with the parental wild-type strain. This unprecedented extent of loss of virulence in B. anthracis, as a consequence of deletion of a single gene, as well as all other phenotypic defects associated with htrA mutation, are restored in their corresponding trans-complemented strains. It is suggested that the loss of virulence is due to increased susceptibility of the ΔhtrA bacteria to stress insults encountered in the host. On a practical note, it is demonstrated that the attenuated Vollum ΔhtrA is highly efficacious in protecting guinea pigs against a lethal anthrax challenge.

Contributions of the pre- and pro-regions of a Staphylococcus hyicus lipase to secretion of a heterologous protein by Bacillus subtilis

Bacillus subtilis is a well-established cell factory for efficient secretion of many biotechnologically relevant enzymes that are naturally produced by it or related organisms. However, the use of B. subtilis as a host for production of heterologous secretory proteins can be complicated by problems related to inefficient translocation of the foreign proteins across the plasma membrane or to inefficient release of the exported proteins from the cell surface into the surrounding medium. Therefore, there is a clear need for tools that allow more efficient membrane targeting, translocation, and release during the production of these proteins. In the present study, we investigated the contributions of the pre (pre(lip)) and pro (pro(lip)) sequences of a Staphylococcus hyicus lipase to secretion of a heterologous protein, the alkaline phosphatase PhoA of Escherichia coli, by B. subtilis. The results indicate that the presence of the pro(lip)-peptide, in combination with the lipase signal peptide (pre(lip)), contributes significantly to the efficient secretion of PhoA by B. subtilis and that pre(lip) directs PhoA secretion more efficiently than the authentic signal peptide of PhoA. Genome-wide transcriptional analyses of the host cell responses indicate that, under the conditions tested, no known secretion or membrane-cell wall stress responses were provoked by the production of PhoA with any of the pre- and pro-region sequences used. Our data underscore the view that the pre-pro signals of the S. hyicus lipase are very useful tools for secretion of heterologous proteins in B. subtilis.

Stress-responsive systems set specific limits to the overproduction of membrane proteins in Bacillus subtilis

Essential membrane proteins are generally recognized as relevant potential drug targets due to their exposed localization in the cell envelope. Unfortunately, high-level production of membrane proteins for functional and structural analyses is often problematic. This is mainly due to their high overall hydrophobicity. To develop new concepts for membrane protein overproduction, we investigated whether the biogenesis of overproduced membrane proteins is affected by stress response-related proteolytic systems in the membrane. For this purpose, the well-established expression host Bacillus subtilis was used to overproduce eight essential membrane proteins from B. subtilis and Staphylococcus aureus. The results show that the sigma(W) regulon (responding to cell envelope perturbations) and the CssRS two-component regulatory system (responding to unfolded exported proteins) set critical limits to membrane protein production in large quantities. The identified sigW or cssRS mutant B. subtilis strains with significantly improved capacity for membrane protein production are interesting candidate expression hosts for fundamental research and biotechnological applications. Importantly, our results pinpoint the interdependent expression and function of membrane-associated proteases as key parameters in bacterial membrane protein production.

Sec-dependent protein translocation across biological membranes: evolutionary conservation of an essential protein transport pathway (Review)

All living organisms, no matter how simple or complex, possess the ability to translocate proteins across biological membranes and into different cellular compartments. Although a range of membrane transport processes exist, the major pathway used to translocate proteins across the bacterial cytoplasmic membrane or the eukaryotic endoplasmic reticulum membrane is conserved and is known as the Sec or Sec61 pathway, respectively. Over the past two decades the Sec and Sec61 pathways have been studied extensively and are well characterised at the genetic and biochemical levels. However, it is only now with the recent structural determination of a number of the key elements of the pathways that the translocation complex is beginning to give up its secrets in exquisite molecular detail. This article will focus on the routes of Sec- and Sec61-dependent membrane targeting and the nature of the translocation channel in bacteria and eukaryotes.

Towards the development of Bacillus subtilis as a cell factory for membrane proteins and protein complexes

Background

The Gram-positive bacterium Bacillus subtilis is an important producer of high quality industrial enzymes and a few eukaryotic proteins. Most of these proteins are secreted into the growth medium, but successful examples of cytoplasmic protein production are also known. Therefore, one may anticipate that the high protein production potential of B. subtilis can be exploited for protein complexes and membrane proteins to facilitate their functional and structural analysis. The high quality of proteins produced with B. subtilis results from the action of cellular quality control systems that efficiently remove misfolded or incompletely synthesized proteins. Paradoxically, cellular quality control systems also represent bottlenecks for the production of various heterologous proteins at significant concentrations.

Conclusion

While inactivation of quality control systems has the potential to improve protein production yields, this could be achieved at the expense of product quality. Mechanisms underlying degradation of secretory proteins are nowadays well understood and often controllable. It will therefore be a major challenge for future research to identify and modulate quality control systems of B. subtilis that limit the production of high quality protein complexes and membrane proteins, and to enhance those systems that facilitate assembly of these proteins.

Role of group A Streptococcus HtrA in the maturation of SpeB protease

The serine protease high-temperature requirement A (HtrA) (DegP) of the human pathogen Streptococcus pyogenes (group A Streptococcus; GAS) is localized to the ExPortal secretory microdomain and is reportedly essential for the maturation of cysteine protease streptococcal pyrogenic exotoxin B (SpeB). Here, we utilize HSC5 (M5 serotype) and the in-frame isogenic mutant HSC5DeltahtrA to determine whether HtrA contributes to the maturation of other GAS virulence determinants. Mutanolysin cell wall extracts and secreted proteins were arrayed by 2-DE and identified by MALDI-TOF PMF analysis. HSC5DeltahtrA had elevated levels of cell wall-associated M protein, whilst the supernatant had higher concentrations of M protein fragments and a reduced amount of mature SpeB protease, compared to wild-type (WT). Western blot analysis and protease assays revealed a delay in the maturation of SpeB in the HSC5DeltahtrA supernatant. HtrA was unable to directly process SpeB zymogen (proSpeB) to the active form in vitro. We therefore conclude that HtrA plays an indirect role in the maturation of cysteine protease SpeB.

Bacillus protein secretion: an unfolding story

Bacillus subtilis and its close relatives are widely used for the production of enzymes for the detergent, food and beverage industries. These organisms not only produce an appropriate range of enzymes but also have the capacity to secrete them into the culture medium at high concentrations. Purification from the culture medium rather than from the cytoplasm considerably reduces downstream processing costs. In recent years, considerable effort has been aimed at developing B. subtilis as a host for the production of heterologous proteins. The folded state of the target protein at various stages of the secretion pathway has proved to be important.

Comparative proteomic analysis of high cell density cultivations with two recombinant Bacillus megaterium strains for the production of a heterologous dextransucrase

High cell density cultivations were performed under identical conditions for two Bacillus megaterium strains (MS941 and WH320), both carrying a heterologous dextransucrase (dsrS) gene under the control of the xylA promoter. At characteristic points of the cultivations (end of batch, initial feeding, before and after induction) the proteome was analyzed based on two dimensional gel electrophoresis and mass spectrometric protein identification using the protein database "bmegMEC.v2" recently made available.

High expression but no secretion of DsrS was found for the chemical mutant WH320 whereas for MS 941, a defined protease deficient mutant of the same parent strain (DSM319), not even expression of DsrS could be detected. The proteomic analysis resulted in the identification of proteins involved in different cellular pathways such as in central carbon and overflow metabolism, in protein synthesis, protein secretion and degradation, in cell wall metabolism, in cell division and sporulation, in membrane transport and in stress responses.

The two strains exhibited considerable variations in expression levels of specific proteins during the different phases of the cultivation process, whereas induction of DsrS production had, in general, little effect. The largely differing behaviour of the two strains with regard to DsrS expression can be attributed, at least in part, to changes observed in the proteome which predominantly concern biosynthetic enzymes and proteins belonging to the membrane translocation system, which were strongly down-regulated at high cell densities in MS941 compared with WH320. At the same time a cell envelope-associated quality control protease and two peptidoglycan-binding proteins related to cell wall turnover were strongly expressed in MS941 but not found in WH320. However, to further explain the very different physiological responses of the two strains to the same cultivation conditions, it is necessary to identify the mutated genes in WH320 in addition to the known lacZ.

In view of the results of this proteomic study it seems that at high cell density conditions and hence low growth rates MS941, in contrast to WH320, does not maintain a vegetative growth which is essential for the expression of the foreign dsrS gene by using the xylA promoter. It is conceivable that applications of a promoter which is highly active under nutrient-limited cultivation conditions is necessary, at least for MS941, for the overexpression of recombinant genes in such B. megaterium fed-batch cultivation process. However to obtain a heterologous protein in secreted and properly folded form stills remains a big challenge.

Single, double and triple mutants of Salmonella enterica serovar Typhimurium degP (htrA), degQ (hhoA) and degS (hhoB) have diverse phenotypes on exposure to elevated temperature and their growth in vivo is attenuated to different extents

DegP (HtrA) is a well-studied protease involved in survival of bacteria under stress conditions in vitro and in vivo. There are two paralogues of DegP in the Salmonella enterica serovar Typhimurium genome, DegQ and DegS. In order to understand more about the biological significance of this gene family, a series of deg-deletion mutants was generated in S. Typhimurium strain SL3261 by allelic replacement. At elevated temperature in vitro, the viability of degP and degS mutants was reduced when compared with the parent strain whereas the viability of a degQ mutant was not significantly affected. The viability of a double degP-degS mutant at elevated temperature was severely decreased when compared with the respective single mutants or, interestingly, with a triple degP-degQ-degS mutant. All the deg deletions were transduced into the mouse-virulent strain SL1344 and the resultant mutants were injected intravenously into BALB/c mice to test virulence. degP and degS single mutants and all combinations of double and triple mutants were attenuated to different degrees, whereas the single degQ mutant was as virulent as the wild-type strain. Thus, within this gene family, degP and degS appear important for survival at elevated temperature and are necessary for full virulence, whereas a single degQ deletion appears to have no clear role in survival and growth at elevated temperature or in mice.

The CssRS two-component regulatory system controls a general secretion stress response in Bacillus subtilis

Bacillus species are valuable producers of industrial enzymes and biopharmaceuticals, because they can secrete large quantities of high-quality proteins directly into the growth medium. This requires the concerted action of quality control factors, such as folding catalysts and 'cleaning proteases'. The expression of two important cleaning proteases, HtrA and HtrB, of Bacillus subtilis is controlled by the CssRS two-component regulatory system. The induced CssRS-dependent expression of htrA and htrB has been defined as a protein secretion stress response, because it can be triggered by high-level production of secreted alpha-amylases. It was not known whether translocation of these alpha-amylases across the membrane is required to trigger a secretion stress response or whether other secretory proteins can also activate this response. These studies show for the first time that the CssRS-dependent response is a general secretion stress response which can be triggered by both homologous and heterologous secretory proteins. As demonstrated by high-level production of a nontranslocated variant of the alpha-amylase, AmyQ, membrane translocation of secretory proteins is required to elicit this general protein secretion stress response. Studies with two other secretory reporter proteins, lipase A of B. subtilis and human interleukin-3, show that the intensity of the protein secretion stress response only partly reflects the production levels of the respective proteins. Importantly, degradation of human interleukin-3 by extracellular proteases has a major impact on the production level, but only a minor effect on the intensity of the secretion stress response.

The deg proteases protect Synechocystis sp. PCC 6803 during heat and light stresses but are not essential for removal of damaged D1 protein during the photosystem two repair cycle

Members of the DegP/HtrA (or Deg) family of proteases are found widely in nature and play an important role in the proteolysis of misfolded and damaged proteins. As yet, their physiological role in oxygenic photosynthetic organisms is unclear, although it has been widely speculated that they participate in the degradation of the photodamaged D1 subunit in the photosystem two complex (PSII) repair cycle, which is needed to maintain PSII activity in both cyanobacteria and chloroplasts. We have examined the role of the three Deg proteases found in the cyanobacterium Synechocystis sp. PCC 6803 through analysis of double and triple insertion mutants. We have discovered that these proteases show overlap in function and are involved in a number of key physiological responses ranging from protection against light and heat stresses to phototaxis. In previous work, we concluded that the Deg proteases played either a direct or an indirect role in PSII repair in a glucose-tolerant version of Synechocystis 6803 (Silva, P., Choi, Y. J., Hassan, H. A., and Nixon, P. J. (2002) Philos. Trans. R. Soc. Lond. B Biol. Sci. 357, 1461-1467). In this work, we have now been able to demonstrate unambiguously, using a triple deg mutant created in the wild type strain of Synechocystis 6803, that the Deg proteases are not obligatory for PSII repair and D1 degradation. We therefore conclude that although the Deg proteases are needed for photoprotection of Synechocystis sp. PCC 6803, they do not play an essential role in D1 turnover and PSII repair in vivo.

Secretion of heterologous proteins in Bacillus subtilis can be improved by engineering cell components affecting post-translocational protein folding and degradation

AIMS

To explore the potential to enhance secretion of heterologous proteins in Bacillus subtilis by engineering cell factors affecting extracytoplasmic protein folding and degradation.

METHODS AND RESULTS

Bottleneck components affecting the extracytoplasmic phase of protein secretion were genetically engineered and their effects on the secretion of 11 industrially interesting heterologous proteins were studied by Western blotting and enzymatic assays. Overproduction of PrsA lipoprotein enhanced the secretion of alpha-amylase of Bacillus stearothermophilus (fourfold) and pneumolysin (1.5-fold). Increasing the net negative charge of the cell wall because of lack of the d-alanine substitution of anionic cell wall polymers enhanced the secretion of pneumolysin c. 1.5-fold. Decreasing the level of HtrA-type quality control proteases caused harmful effects on growth and did not enhance secretion. Pertussis toxin subunit, S1 was found to be a substrate for HtrA-type proteases and its secretion was dependent on these proteases.

CONCLUSIONS

Secretion of heterologous proteins can be enhanced by engineering components involved in late stages of secretion in a protein-dependent manner.

SIGNIFICANCE AND IMPACT OF THE STUDY

The study revealed both possibilities and limitations of modulating the post-translocational phase of secretion as a means to improve the yield of heterologous proteins.

Role for HtrA in stress induction and virulence potential in Listeria monocytogenes

In silico analysis of the Listeria monocytogenes genome revealed lmo0292, a gene predicted to encode a HtrA-like serine protease. A stable insertion mutant was constructed, revealing a requirement for htrA in the listerial response to heat, acid, and penicillin stress. Transcriptional analysis revealed that htrA is not induced in response to heat shock but is induced in response to low pH and penicillin G stress. Furthermore, htrA expression was shown to be dependent upon the LisRK two-component sensor-kinase, a system known to respond to changes in integrity of the cell envelope. In addition, we demonstrated that a second in-frame start codon, upstream of that previously annotated for L. monocytogenes htrA, incorporating a putative signal sequence appears to influence virulence potential. Finally, a significant virulence defect was observed for the htrA mutant, indicating that this gene is required for full virulence in mice. Our findings suggest that L. monocytogenes lmo0292 encodes an HtrA-like serine protease that is not part of the classical heat shock response but is involved in stress responses and virulence.

Role of HtrA in surface protein expression and biofilm formation by Streptococcus mutans

The HtrA surface protease in gram-positive bacteria is involved in the processing and maturation of extracellular proteins and degradation of abnormal or misfolded proteins. Inactivation of htrA has been shown to affect the tolerance to thermal and environmental stress and to reduce virulence. We found that inactivation of Streptococcus mutans htrA by gene-replacement also resulted in a reduced ability to withstand exposure to low and high temperatures, low pH, and oxidative and DNA damaging agents. The htrA mutation affected surface expression of several extracellular proteins including glucan-binding protein B (GbpB), glucosyltransferases, and fructosyltransferase. In addition, htrA mutation also altered the surface expression of enolase and glyceraldehyde-3-phosphate dehydrogenease, two glycolytic enzymes that are known to be present on the streptococcal cell surface. As expected, microscopic analysis of in vitro grown biofilm structure revealed that the htrA deficient biofilms adopted a much more granular patchy appearance, rather than the relatively smooth confluent layer normally seen in the wild type. These results suggest that HtrA plays an important role in the biogenesis of extracellular proteins including surface associated glycolytic enzymes and in biofilm formation of S. mutans.

Transcriptome analysis of the secretion stress response of Bacillus subtilis

Transcription profiling of all protein-encoding genes of Bacillus subtilis was carried out under several secretion stress conditions in the exponential growth phase. Cells that secreted AmyQ alpha-amylase at a high level were stressed only moderately: seven genes were induced, most significantly htrA and htrB, encoding quality control proteases, and yqxL, encoding a putative CorA-type Mg(2+) transporter. These three genes were induced more strongly by severe secretion stress (prsA3 mutant secreting AmyQ), suggesting that their expression responds to protein misfolding. In addition, 17 other genes were induced, including the liaIHGFSR (yvqIHGFEC) operon, csaA and ffh, encoding chaperones involved in the pretranslocational phase of secretion, and genes involved in cell wall synthesis/modification. Severe secretion stress caused downregulation of 23 genes, including the prsA paralogue yacD. Analysis of a cssS knockout mutant indicated that the absence of the CssRS two-component system, and consequently the absence of the HtrA and HtrB proteases, caused secretion stress. The results also suggest that the htrA and htrB genes comprise the CssRS regulon. B. subtilis cells respond to secretion/folding stress by various changes in gene expression, which can be seen as an attempt to combat the stress condition.

Autolysis of Lactococcus lactis is increased upon D-alanine depletion of peptidoglycan and lipoteichoic acids

Mutations in the genes encoding enzymes responsible for the incorporation of D-Ala into the cell wall of Lactococcus lactis affect autolysis. An L. lactis alanine racemase (alr) mutant is strictly dependent on an external supply of D-Ala to be able to synthesize peptidoglycan and to incorporate D-Ala in the lipoteichoic acids (LTA). The mutant lyses rapidly when D-Ala is removed at mid-exponential growth. AcmA, the major lactococcal autolysin, is partially involved in the increased lysis since an alr acmA double mutant still lyses, albeit to a lesser extent. To investigate the role of D-Ala on LTA in the increased cell lysis, a dltD mutant of L. lactis was investigated, since this mutant is only affected in the D-alanylation of LTA and not the synthesis of peptidoglycan. Mutation of dltD results in increased lysis, showing that D-alanylation of LTA also influences autolysis. Since a dltD acmA double mutant does not lyse, the lysis of the dltD mutant is totally AcmA dependent. Zymographic analysis shows that no degradation of AcmA takes place in the dltD mutant, whereas AcmA is degraded by the extracellular protease HtrA in the wild-type strain. In L. lactis, LTA has been shown to be involved in controlled (directed) binding of AcmA. LTA lacking D-Ala has been reported in other bacterial species to have an improved capacity for autolysin binding. Mutation of dltD in L. lactis, however, does not affect peptidoglycan binding of AcmA; neither the amount of AcmA binding to the cells nor the binding to specific loci is altered. In conclusion, D-Ala depletion of the cell wall causes lysis by two distinct mechanisms. First, it results in an altered peptidoglycan that is more susceptible to lysis by AcmA and also by other factors, e.g., one or more of the other (putative) cell wall hydrolases expressed by L. lactis. Second, reduced amounts of D-Ala on LTA result in decreased degradation of AcmA by HtrA, which results in increased lytic activity.

Comparative analysis of the roles of HtrA-like surface proteases in two virulent Staphylococcus aureus strains

The HtrA surface protease is involved in the virulence of many pathogens, mainly by its role in stress resistance and bacterial survival. Staphylococcus aureus encodes two putative HtrA-like proteases, referred to as HtrA(1) and HtrA(2). To investigate the roles of HtrA proteins in S. aureus, we constructed htrA(1), htrA(2), and htrA(1) htrA(2) insertion mutants in two genetically different virulent strains, RN6390 and COL. In the RN6390 context, htrA(1) inactivation resulted in sensitivity to puromycin-induced stress. The RN6390 htrA(1) htrA(2) mutant was affected in the expression of several secreted virulence factors comprising the agr regulon. This observation was correlated with the disappearance of the agr RNA III transcript in the RN6390 htrA(1) htrA(2) mutant. The virulence of this mutant was diminished in a rat model of endocarditis. In the COL context, both HtrA(1) and HtrA(2) were essential for thermal stress survival. However, only HtrA(1) had a slight effect on exoprotein expression. The htrA mutations did not diminish the virulence of the COL strain in the rat model of endocarditis. Our results indicate that HtrA proteins have different roles in S. aureus according to the strain, probably depending on specific differences in the regulation of virulence factor and stress protein expression. We propose that HtrA(1) and HtrA(2) contribute to pathogenicity by controlling the production of certain extracellular factors that are crucial for bacterial dissemination, as revealed in the RN6390 background. We speculate that HtrA proteins act in the agr-dependent regulation pathway by assuring folding and/or maturation of some surface components of the agr system.

The Bacillus secretion stress response is an indicator for alpha-amylase production levels

AIMS

Overproduced alpha-amylases in Bacillus subtilis provoke a specific stress response involving the CssRS two-component system, which controls expression of the HtrA and HtrB proteases. Previously, the B. subtilis TepA protein was implicated in high-level alpha-amylase secretion. Our present studies were aimed at investigating a possible role of TepA in secretion stress management, and characterizing the intensity of the secretion stress response in relation to alpha-amylase production.

METHODS AND RESULTS

The expression of a transcriptional htrB-lacZ gene fusion, and the levels of alpha-amylase production were monitored simultaneously using tepA mutant B. subtilis strains. TepA was shown to be dispensable for secretion stress management. Importantly, however, the levels of htrB-lacZ expression can be correlated with the levels of alpha-amylase production.

CONCLUSION

Our observations show that the secretion stress response can serve as an indicator for alpha-amylase production levels.

SIGNIFICANCE AND IMPACT OF STUDY

Conceivably, this stress response can be employed to monitor the biotechnological production of various secretory proteins by the Bacillus cell factory.

Proteomics of protein secretion by Bacillus subtilis: separating the "secrets" of the secretome

Secretory proteins perform a variety of important "remote-control" functions for bacterial survival in the environment. The availability of complete genome sequences has allowed us to make predictions about the composition of bacterial machinery for protein secretion as well as the extracellular complement of bacterial proteomes. Recently, the power of proteomics was successfully employed to evaluate genome-based models of these so-called secretomes. Progress in this field is well illustrated by the proteomic analysis of protein secretion by the gram-positive bacterium Bacillus subtilis, for which approximately 90 extracellular proteins were identified. Analysis of these proteins disclosed various "secrets of the secretome," such as the residence of cytoplasmic and predicted cell envelope proteins in the extracellular proteome. This showed that genome-based predictions reflect only approximately 50% of the actual composition of the extracellular proteome of B. subtilis. Importantly, proteomics allowed the first verification of the impact of individual secretion machinery components on the total flow of proteins from the cytoplasm to the extracellular environment. In conclusion, proteomics has yielded a variety of novel leads for the analysis of protein traffic in B. subtilis and other gram-positive bacteria. Ultimately, such leads will serve to increase our understanding of virulence factor biogenesis in gram-positive pathogens, which is likely to be of high medical relevance.

Role of HtrA in the virulence and competence of Streptococcus pneumoniae

HtrA is a major virulence factor of Streptococcus pneumoniae (the pneumococcus). Deletion of the gene for HtrA from strain D39 of the pneumococcus completely abolished its virulence in mouse models of pneumonia and bacteremia, while the virulence of a second strain (TIGR4) was dramatically reduced. HtrA-negative mutants induced much less inflammation in the lungs during pneumonia than the wild type. HtrA is involved in the ability of the pneumococcus to grow at high temperatures, to resist oxidative stress, and to undergo genetic transformation. The expression and cellular location of several known virulence factors of the pneumococcus were not affected by the lack of HtrA.

Role for serine protease HtrA (DegP) of Streptococcus pyogenes in the biogenesis of virulence factors SpeB and the hemolysin streptolysin S

The serine protease HtrA is involved in the folding and maturation of secreted proteins, as well as in the degradation of proteins that misfold during secretion. Depletion of HtrA has been shown to affect the sensitivity of many organisms to thermal and environmental stresses, as well as being essential for virulence in many pathogens. In the present study, we compared the behaviors of several different HtrA mutants of the gram-positive pathogen Streptococcus pyogenes (group A streptococcus). Consistent with prior reports, insertional inactivation of htrA, the gene that encodes HtrA, resulted in a mutant that grew poorly at 37 degrees C. However, an identical phenotype was observed when a similar polar insertion was placed immediately downstream of htrA in the streptococcal chromosome, suggesting that the growth defect of the insertion mutant was not a direct result of insertional inactivation of htrA. This conclusion was supported by the observation that a nonpolar deletion mutation of htrA did not produce the growth defect. However, this mutation did affect the production of several secreted virulence factors whose biogenesis requires extensive processing. For the SpeB cysteine protease, the loss of HtrA was associated with a failure to proteolytically process the zymogen to an active protease. For the streptolysin S hemolysin, a dramatic increase in hemolytic activity resulted from the depletion of HtrA. Interestingly, HtrA-deficient mutants were not attenuated in a murine model of subcutaneous infection. These data add to the growing body of information that implies an important role for HtrA in the biogenesis of secreted proteins in gram-positive bacteria.