Extracellular proteases modify cell wall turnover in Bacillus subtilis

Autoinduction AND Gate Inhibits Cell Lysis to Enhance Protein Production in Bacillus subtilis Controlled by Population Density and Cell Physiological State

The extracellular protease-deficient strain Bacillus subtilis WB600 is commonly used as a chassis cell for the production of industrial proteins. However, B. subtilis WB600 exhibits an increased susceptibility to cell lysis and a reduction in biomass. Inhibition of cell lysis by knocking out lytic genes will impair physiological function. Here, we dynamically inhibited cell lysis in B. subtilis WB600 to balance the impairment of physiological function with the accumulation of biomass. First, the inducible protein degradation systems (IPDSs) were constructed and used to investigate the effects of inhibiting cell lysis on biomass, cell morphology, and protein production at different times (using pullulanase as a test). The highest pullulanase activity was obtained at 20 h of inhibiting cell lysis, 184.8 U/mL, which was 44% higher than the activity of B. subtilis WB600. Then, to avoid addition of inducers, we introduced orthogonal quorum sensing and constructed autoinduction protein degradation systems (AIPDSs). The optimized AIPDS showed similar pullulanase activity to the optimal IPDS (20 h), 181.3 U/mL. Next, we constructed dual-signal input autoinduction protein degradation systems (DSI-AIPDSs) via AND gate to further address two deficiencies of AIPDS, one-time activation and damage to new cells. These DSI-AIPDSs were controlled by quorum sensing and stationary phase promoters that respond to population density and single-cell physiological state, respectively. Finally, the OD₆₀₀ and pullulanase activity of the strain with optimal DSI-AIPDS were 51% and 115% higher than those of B. subtilis WB600 in pullulanase production, respectively. We provided a B. subtilis chassis strain with considerable potential for biomass accumulation and enhanced protein production.

Research Progress on the Effect of Autolysis to Bacillus subtilis Fermentation Bioprocess

Bacillus subtilis is a gram-positive bacterium, a promising microorganism due to its strong extracellular protein secretion ability, non-toxic, and relatively mature industrial fermentation technology. However, cell autolysis during fermentation restricts the industrial application of B. subtilis. With the fast advancement of molecular biology and genetic engineering technology, various advanced procedures and gene editing tools have been used to successfully construct autolysis-resistant B. subtilis chassis cells to manufacture various biological products. This paper first analyses the causes of autolysis in B. subtilis from a mechanistic perspective and outlines various strategies to address autolysis in B. subtilis. Finally, potential strategies for solving the autolysis problem of B. subtilis are foreseen.

Pseudomonas savastanoi Two-Component System RhpRS Switches between Virulence and Metabolism by Tuning Phosphorylation State and Sensing Nutritional Conditions

Pseudomonas savastanoi uses a type III secretion system (T3SS) to invade host plants. Our previous studies have demonstrated that a two-component system (TCS), RhpRS, enables P. savastanoi to coordinate the T3SS gene expression, which depends on the phosphorylation state of RhpR under different environmental conditions. Orthologues of RhpRS are distributed in a wide range of bacterial species, indicating a general regulatory mechanism. How RhpRS uses external signals and the phosphorylation state to exercise its regulatory functions remains unknown. We performed chromatin immunoprecipitation sequencing (ChIP-seq) assays to identify the specific binding sites of RhpR and RhpR^D70A in either King's B medium (KB [a T3SS-inhibiting medium]) or minimal medium (MM [a T3SS-inducing medium]). We identified 125 KB-dependent binding sites and 188 phosphorylation-dependent binding sites of RhpR. In KB, RhpR directly and positively regulated cytochrome c ₅₅₀ production (via ccmA) and alcohol dehydrogenase activity (via adhB) but negatively regulated anthranilate synthase activity (via trpG) and protease activity (via hemB). In addition, phosphorylated RhpR (RhpR-P) directly and negatively regulated the T3SS (via hrpR and hopR1), swimming motility (via flhA), c-di-GMP levels (via PSPPH_2590), and biofilm formation (via algD). It positively regulated twitching motility (via fimA) and lipopolysaccharide production (via PSPPH_2653). Our transcriptome sequencing (RNA-seq) analyses identified 474 and 840 new genes that were regulated by RhpR in KB and MM, respectively. We showed nutrient-rich conditions allowed RhpR to directly regulate multiple metabolic pathways of P. savastanoi and phosphorylation enabled RhpR to specifically control virulence and the cell envelope. The action of RhpRS switched between virulence and regulation of multiple metabolic pathways by tuning its phosphorylation and sensing environmental signals in KB, respectively.IMPORTANCE The plant pathogen Pseudomonas savastanoi invades host plants through a type III secretion system, which is strictly regulated by a two-component system called RhpRS. The orthologues of RhpRS are widely distributed in the bacterial kingdom. The master regulator RhpR specifically depends on the phosphorylation state to regulate the majority of the virulence-related genes. Under nutrient-rich conditions, it modulates many important metabolic pathways, which consist of one-fifth of the genome. We propose that RhpRS uses phosphorylation- and nutrition-dependent mechanisms to switch between regulating virulence and metabolism, and this functionality is widely conserved among bacterial species.

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.

Characterization of the Highly Autolytic Lactococcus lactis subsp. cremoris Strains CO and 2250

Two highly autolytic Lactococcus lactis subsp. cremoris strains (CO and 2250) were selected and analyzed for their autolytic properties. Both strains showed maximum lysis when grown in M17 broth containing a limiting concentration of glucose (0.4 to 0.5%) as the carbohydrate source. Lysis did not vary greatly with pH or temperature but was reduced when strains were grown on lactose or galactose. Growth in M17 containing excess glucose (1%) prevented autolysis, although rapid lysis of L. lactis subsp. cremoris CO did occur in the presence of 1% glucose if sodium fluoride (an inhibitor of glycolysis) was added to the medium. Maximum cell lysis in a buffer system was observed early in the stationary phase, and for CO, two pH optima were observed for log-phase and stationary-phase cells (6.5 and 8.5, respectively). Autolysins were extracted from the cell wall fraction of each strain by using either 4% sodium dodecyl sulfate (SDS), 6 M guanidine hydrochloride, or 4 M lithium chloride, and their activities were analyzed by renaturing SDS-polyacrylamide gel electrophoresis on gels containing Micrococcus luteus or L. lactis subsp. cremoris CO cells as the substrate. More than one lytic band was observed on each substrate, with the major band having an apparent molecular mass of 48 kDa for CO. Each lytic band was present throughout growth and lysis. These results suggest that at least two different autolytic enzymes are present in the autolytic L. lactis subsp. cremoris strains. The presence of the lactococcal cell wall hydrolase gene, acmA (G. Buist, J. Kok, K. J. Leenhouts, M. Dabrowska, G. Venema, and A. J. Haandrikman, J. Bacteriol. 177:1554-1563, 1995), in strains 2250 and CO was confirmed by Southern hybridization. Analysis of an acmA deletion mutant of 2250 confirmed that the gene was involved in cell separation and had a role in cell lysis.

Global gene expression in Staphylococcus aureus following exposure to alcohol

It was recently shown that, as in yeast, alcohols selectively increase the hemolytic properties of certain staphylococci strains. This phenomenon has been called 'microbial alcohol-conferred hemolysis'(MACH). Here we present the changes in gene expression by Staphylococcus aureus 8325-4, in response to ethanol. Ethanol upregulated the expression of multiple toxins and increase the pathogen potential of S. aureus strain 8325-4. Ethanol also increased the level of genes considered necessary for production and viability of biofilm, such as: icaAD, sdrDE, pyr, and ure. Increased urease activity appeared to be an important factor in the ethanol response along with macromolecule repair mechanisms. Oxidative-stress responses, such as increased expression of sodA1, sodA2 and upregulation of zinc-containing alcohol dehydrogenase, alcohol-acetaldehyde dehydrogenase (adhE) and two aldehyde dehydrogenases (aldA1, aldA2), which can generate more reducing power, were also induced. Upregulation of fatty acid metabolism appears to be important in enabling the bacteria to handle excess amounts of ethanol which ultimately may lead to synthesis of lytic lypids. The patterns of regulation were confirmed by quantitive reverse transcriptase PCR (QRT-PCR). These results, taken together, suggest that exposure to ethanol increases pathogenic traits and induce oxidative-stress responses.

Interconnections between Sigma B, agr, and proteolytic activity in Staphylococcus aureus biofilm maturation

Staphylococcus aureus is a proficient biofilm former on host tissues and medical implants. We mutagenized S. aureus strain SH1000 to identify loci essential for ica-independent mechanisms of biofilm maturation and identified multiple insertions in the rsbUVW-sigB operon. Following construction and characterization of a sigB deletion, we determined that the biofilm phenotype was due to a lack of sigma factor B (SigB) activity. The phenotype was conserved in a sigB mutant of USA300 strain LAC, a well-studied community-associated methicillin-resistant S. aureus isolate. We determined that agr RNAIII levels were elevated in the sigB mutants, and high levels of RNAIII expression are known to have antibiofilm effects. By introducing an agr mutation into the SH1000 or LAC sigB deletion strain, S. aureus regained biofilm capacity, indicating that the biofilm phenotype was agr dependent. Protease activity is linked to agr activity and ica-independent biofilm formation, and we observed that the protease inhibitors phenylmethylsulfonyl fluoride and alpha-macroglobulin could reverse the sigB biofilm defect. Similarly, inactivating genes encoding both the aureolysin and Spl extracellular proteases in the sigB mutant restored biofilm capacity. Due to the growing link between murein hydrolase activity and biofilm maturation, autolysin zymography was performed, which revealed an altered profile in the sigB mutant; again, the phenotype could be repaired through protease inactivation. These findings indicate that the lack of SigB activity results in increased RNAIII expression, thus elevating extracellular protease levels and altering the murein hydrolase activity profile. Altogether, our observations demonstrate that SigB is an essential regulator of S. aureus biofilm maturation.

Living with an imperfect cell wall: compensation of femAB inactivation in Staphylococcus aureus

Background

Synthesis of the Staphylococcus aureus peptidoglycan pentaglycine interpeptide bridge is catalyzed by the nonribosomal peptidyl transferases FemX, FemA and FemB. Inactivation of the femAB operon reduces the interpeptide to a monoglycine, leading to a poorly crosslinked peptidoglycan. femAB mutants show a reduced growth rate and are hypersusceptible to virtually all antibiotics, including methicillin, making FemAB a potential target to restore β-lactam susceptibility in methicillin-resistant S. aureus (MRSA). Cis-complementation with wild type femAB only restores synthesis of the pentaglycine interpeptide and methicillin resistance, but the growth rate remains low. This study characterizes the adaptations that ensured survival of the cells after femAB inactivation.

Results

In addition to slow growth, the cis-complemented femAB mutant showed temperature sensitivity and a higher methicillin resistance than the wild type. Transcriptional profiling paired with reporter metabolite analysis revealed multiple changes in the global transcriptome. A number of transporters for sugars, glycerol, and glycine betaine, some of which could serve as osmoprotectants, were upregulated. Striking differences were found in the transcription of several genes involved in nitrogen metabolism and the arginine-deiminase pathway, an alternative for ATP production. In addition, microarray data indicated enhanced expression of virulence factors that correlated with premature expression of the global regulators sae, sarA, and agr.

Conclusion

Survival under conditions preventing normal cell wall formation triggered complex adaptations that incurred a fitness cost, showing the remarkable flexibility of S. aureus to circumvent cell wall damage. Potential FemAB inhibitors would have to be used in combination with other antibiotics to prevent selection of resistant survivors.

Biomass recycling from a riboflavin cultivation with B. subtilis: lysis, extract production and testing as substrate in riboflavin cultivation

Autolysis of riboflavin-producing B. subtilis can be induced by pH, lack of carbon source, and the buffer system. Stress factors like temperature shift or oxygen dearth enhance the autolysis process. After cultivation of a riboflavin-producing strain, the pH of the whole culture broth was adjusted to 6.5-7.5. At a temperature of 40 degrees C, autolysis started after 1 h. Adding a defined amount of commercially available endo- and exo-proteases enhanced both auto- and proteo-lysis. Optimization of endo- and exo-protease concentrations and of the time increased the degree of proteolysis. Additionally, the amount of DNA and Protein trapped in the riboflavin crystals could be significantly reduced by autolysis. After autolysis, the cultivation broth was centrifuged and the supernatant was cross-flow filtrated with a cut off of 10 kDa. Using this autolysate instead of yeast extract as a medium component for riboflavin production with B. subtilis, a riboflavin yield of 77% was obtained in comparison with the standard cultivation on yeast extract.

Effect of Bacillus subtilis spo0A mutation on cell wall lytic enzymes and extracellular proteases, and prevention of cell lysis

The Bacillus subtilis spo0A mutant is an adequate host for extracellular protein production (e.g., alpha-amylase). However the mutant was prone to cell lysis. SDS-PAGE and zymography of cell wall lytic proteins indicated that the spo0A mutant contained high amounts of two major autolysins (LytC [CwlB] and LytD [CwlG]) and two minor cell wall lytic enzymes (LytE [CwlF] and LytF [CwlE]). On the other hand, the expression of eight extracellular protease genes was very poor or absent in the spo0A mutant. An eight-extracellular-protease-deficient mutant (Dpr8 strain) was constructed and the strain also exhibited cell lysis. The autolysins from the spo0A mutant were degraded by the supernatant of the wild type but not degraded by that of the Dpr8 mutant. These results suggest that the extensive cell lysis of the spo0A mutant was partially caused by the stability of autolysins via the decrease of the extracellular proteases. The introduction of a major autolysin and/or SigD mutations into the spo0A mutant was effective for preventing cell lysis.

Characterization of novel Mycobacterium tuberculosis and Mycobacterium smegmatis mutants hypersusceptible to beta-lactam antibiotics

Our laboratory previously constructed mutants of Mycobacterium tuberculosis and Mycobacterium smegmatis with deletions in the genes for their major beta-lactamases, BlaC and BlaS, respectively, and showed that the mutants have increased susceptibilities to most beta-lactam antibiotics, particularly the penicillins. However, there is still a basal level of resistance in the mutants to certain penicillins, and the susceptibilities of the mutants to some cephalosporin-based beta-lactams are essentially the same as those of the wild types. We hypothesized that characterizing additional mutants (derived from beta-lactamase deletion mutants) that are hypersusceptible to beta-lactam antibiotics might reveal novel genes involved with other mechanisms of beta-lactam resistance, peptidoglycan assembly, and cell envelope physiology. We report here the isolation and characterization of nine beta-lactam antibiotic-hypersusceptible transposon mutants, two of which have insertions in genes known to be involved with peptidoglycan biosynthesis (ponA2 and dapB); the other seven mutants have insertions which affect novel genes. These genes can be classified into three groups: those involved with peptidoglycan biosynthesis, cell division, and other cell envelope processes. Two of the peptidoglycan-biosynthetic genes (ponA2 and pbpX) may encode beta-lactam antibiotic-resistant enzymes proposed to be involved with the synthesis of the unusual diaminopimelyl linkages within the mycobacterial peptidoglycan.

Strategies for efficient production of heterologous proteins in Escherichia coli

In recent years, the number of recombinant proteins used for therapeutic applications has increased dramatically. Production of these proteins has a remarkable demand in the market. Escherichia coli offers a means for the rapid and economical production of recombinant proteins. These advantages, coupled with a wealth of biochemical and genetic knowledge, have enabled the production of such economically therapeutic proteins such as insulin and bovine growth hormone. These demands have driven the development of a variety of strategies for achieving high-level expression of protein, particularly involving several aspects such as expression vectors design, gene dosage, promoter strength (transcriptional regulation), mRNA stability, translation initiation and termination (translational regulation), host design considerations, codon usage, and fermentation factors available for manipulating the expression conditions, which are the major challenges is obtaining the high yield of protein at low cost.

Restricted translocation across the cell wall regulates secretion of the broad-range phospholipase C of Listeria monocytogenes

The virulence of Listeria monocytogenes is directly related to its ability to spread from cell to cell without leaving the intracellular milieu. During cell-to-cell spread, bacteria become temporarily confined to secondary vacuoles. Among the bacterial factors involved in escape from these vacuoles is a secreted broad-range phospholipase C (PC-PLC), the activation of which requires processing of an N-terminal prodomain. Mpl, a secreted metalloprotease of Listeria, is involved in the proteolytic activation of PC-PLC. We previously showed that, during intracellular growth, bacteria maintain a pool of PC-PLC that is not accessible to antibodies and that is rapidly released in its active form in response to a decrease in pH. pH-regulated release of active PC-PLC is Mpl dependent. To further characterize the mechanism regulating secretion of PC-PLC, the bacterial localization of PC-PLC and Mpl was investigated. Both proteins were detected in the bacterial supernatant and lysate with no apparent changes in molecular weight. Extraction of bacteria-associated PC-PLC and Mpl required cell wall hydrolysis, but there was no indication that either protein was covalently bound to the bacterial cell wall. Results from pulse-chase experiments performed with infected macrophages indicated that the rate of synthesis of PC-PLC exceeded the rate of translocation across the bacterial cell wall and confirmed that the pool of PC-PLC associated with bacteria was efficiently activated and secreted upon acidification of the host cell cytosol. These data suggest that bacterially associated PC-PLC and Mpl localize at the cell wall-membrane interface and that translocation of PC-PLC across the bacterial cell wall is rate limiting, resulting in the formation of a bacterially associated pool of PC-PLC that would readily be accessible for activation and release into nascent secondary vacuoles.

Virulence of a hemB mutant displaying the phenotype of a Staphylococcus aureus small colony variant in a murine model of septic arthritis

Persistence of Staphylococcus aureus during invasive infections has been associated with a small-colony variant (SCV) phenotype. SCVs are frequently auxotrophic for menadione or hemin, two compounds involved in the biosynthesis of the electron transport chain. SCVs have been shown to be more resistant to antibiotics such as aminoglycosides, grow slowly and persist intracellularly. The aim of this study was to assess the virulence of an hemB mutant, which has been shown to display the typical characteristics of clinical SCVs, in a murine model of septic arthritis. NMRI mice were inoculated intravenously with either the wild type strain Newman or with its mutant displaying the SCV phenotype. The clinical, bacteriological, and histopathological progression of the disease was studied. Mice inoculated with the hemB mutant displayed a higher frequency and a significantly higher severity of arthritis than mice inoculated with the wild type Newman strain. Despite that, the mutant inoculated mice displayed significantly lower bacterial burden in their kidneys and joints compared with mice exposed to the wild parental strain. Notably, the hemB mutant produced almost 20 times more protease in vitro than the parental strain. We conclude that the small colony variants of S. aureus are more virulent on a per organism basis than its isogenic parental strain in the model of septic arthritis. This can at least in part be explained by the ability of SCV to produce high amounts of destructive proteases.

Regulation of autolysins in teichuronic acid-containing Bacillus subtilis cells

Bacillus subtilis cells grown under phosphate starvation induce teichuronic acid (TUA) synthesis while simultaneously repressing teichoic acid synthesis (TA). The turnover rates of TA-containing and TUA-containing walls are similar, indicating that autolysin function is similar and suggesting that modulation of autolytic function may be similar. In this study, it is demonstrated, utilizing fluorescein isothiocyanate (FITC)-dextran to probe the wall pH, that a low pH exists in the wall matrix. A second probe, cationized ferritin (CF), was used to observe cell surface protonation. Suspensions of B. subtilis cells containing either TA or TUA were aggregated with CF only after the addition of a proton-motive-force-dissipating agent. Respiring B. subtilis TUA-containing cells labelled with FITC-dextran exhibited little fluorescence. Conversely, fluorescence intensities exhibited by cells de-energized with nitrogen gas were significantly greater. The effects of protonmotive force on autolytic activity were studied by adding cell wall protein extract containing concentrated autolysin to exponentially growing TA-containing and TUA-containing B. subtilis cells. Both TUA-containing and TA-containing cells were lysed only after the addition of sodium azide. These data suggest that during normal growth the wall of TUA-containing B. subtilis cells is protonated, and proton-motive force influences autolytic regulation in both TUA-containing and TA-containing B. subtilis cells.

Identification of Rgg-regulated exoproteins of Streptococcus pyogenes

Streptococcus pyogenes secretes many proteins that influence host-pathogen interactions. Despite their importance, relatively little is known about the regulation of these proteins. The rgg gene (also known as ropB) is required for the expression of streptococcal erythrogenic toxin B (SPE B), an extracellular cysteine protease that contributes to virulence. Proteomics was used to determine if rgg regulates the expression of additional exoproteins. Exponential- and stationary-phase culture supernatant proteins made by S. pyogenes NZ131 rgg and NZ131 speB were separated by two-dimensional electrophoresis. Differences were identified in supernatant proteins from both exponential- and stationary-phase cultures, although considerably more differences were detected among stationary-phase supernatant proteins. Forty-two proteins were identified by peptide fingerprinting with matrix-assisted laser desorption mass spectrometry. Mitogenic factor, DNA entry nuclease (open reading frame [ORF 226]), and ORF 953, which has no known function, were more abundant in the culture supernatants of the rgg mutant compared to the speB mutant. ClpB, lysozyme, and autolysin were detected in the culture supernatant of the speB mutant but not the rgg mutant. To determine if Rgg affected protein expression at the transcriptional level, real-time (TaqMan) reverse transcription (RT)-PCR was used to quantitate Rgg-regulated transcripts from NZ131 wild-type and speB and rgg mutant strains. The results obtained with RT-PCR correlated with the proteomic data. We conclude that Rgg regulates the transcription of several genes expressed primarily during the stationary phase of growth.

Description of staphylococcus serine protease (ssp) operon in Staphylococcus aureus and nonpolar inactivation of sspA-encoded serine protease

Signature tagged mutagenesis has recently revealed that the Ssp serine protease (V8 protease) contributes to in vivo growth and survival of Staphylococcus aureus in different infection models, and our previous work indicated that Ssp could play a role in controlling microbial adhesion. In this study, we describe an operon structure within the ssp locus of S. aureus RN6390. The ssp gene encoding V8 protease is designated as sspA, and is followed by sspB, which encodes a 40.6-kDa cysteine protease, and sspC, which encodes a 12.9-kDa protein of unknown function. S. aureus SP6391 is an isogenic derivative of RN6390, in which specific loss of SspA function was achieved through a nonpolar allelic replacement mutation. In addition to losing SspA, the culture supernatant of SP6391 showed a loss of 22- to 23-kDa proteins and the appearance of a 40-kDa protein corresponding to SspB. Although the 40-kDa SspB protein could degrade denatured collagen, our data establish that this is a precursor form which is normally processed by SspA to form a mature cysteine protease. Culture supernatant of SP6391 also showed a new 42-kDa glucosaminidase and enhanced glucosaminidase activity in the 29 to 32 kDa range. Although nonpolar inactivation of sspA exerted a pleiotropic effect, S. aureus SP6391 exhibited enhanced virulence in a tissue abscess infection model relative to RN6390. Therefore, we conclude that SspA is required for maturation of SspB and plays a role in controlling autolytic activity but does not by itself exert a significant contribution to the development of tissue abscess infections.

A new two-component regulatory system involved in adhesion, autolysis, and extracellular proteolytic activity of Staphylococcus aureus

A transposition mutant of Staphylococcus aureus was selected from the parent strain MT23142, a derivative of strain 8325. The site of transposition was near the 5' terminus of the gene arlS. ArlS exhibits strong similarities with histidine protein kinases. Sequence analysis suggested that arlS forms an operon with upstream gene arlR. The predicted product of arlR is a member of the OmpR-PhoB family of response regulators. The arlS mutant formed a biofilm on a polystyrene surface unlike the parent strain and the complemented mutant. Biofilm formation was associated with increased primary adherence to polystyrene, whereas cellular adhesion was only slightly decreased. In addition, the arlS mutant exhibited increased autolysis and altered peptidoglycan hydrolase activity compared to the parental strain and to the complemented mutant. As it has been shown for coagulase-negative staphylococci that some autolysins are able to bind polymer surfaces, these data suggest that the two-component regulatory system ArlS-ArlR may control attachment to polymer surfaces by affecting secreted peptidoglycan hydrolase activity. Finally, the arlS mutant showed a dramatic decrease of extracellular proteolytic activity, including serine protease activity, in comparison to the wild-type strain and the complemented mutant, and cells grown in the presence of phenylmethylsulfonyl fluoride (a serine protease inhibitor) showed an increased autolysin activity. Since the locus arlR-arlS strikingly modifies extracellular proteolytic activity, this locus might also be involved in the virulence of S. aureus.

Spectroscopic properties of a novel neutral proteinase from Saccharomonospora canescens

A neutral proteinase (NPS) was purified from the culture broth of Saccharomonospora canescens sp. novus, strain 5, using DEAE cellulose and a POROS HQ/M 4.6 x 100 mm column. The stability towards thermal and chemical (guanidine hydrochloride, Gdn.HCl) denaturation of NPS was investigated by kinetic and equilibrium studies. The unfolding processes were monitored by circular dichroism and fluorescence spectroscopy. The free energy of stabilization in water was calculated to be 2.1 kcal mol-1. The thermostability was determined by the critical temperature Tc from fluorescence measurements (69 degrees C) and the melting temperature Tm (70 degrees C) from (1) measurements. Quenching with acrylamide, iodide and cesium gives information about the microenvironment of intrinsic protein fluorophores. The Ksv constant for NPS is 4.6 and classifies the emitting tryptophans as 'buried' in the hydrophobic interior of the investigated protein.

Physiological characterization of Streptococcus bovis mutants that can resist 2-deoxyglucose-induced lysis

Streptococcus bovis JB1 does not normally lyse, but stationary phase lysis can be induced by including 2-deoxyglucose (2DG) in the growth medium. Isolates deficient in glucose/2DG phosphotransferase activity (PTS-) also lysed when 2DG was present (Lys+) and this result indicated that 2DG phosphorylation via the PTS was not an obligate requirement for 2DG-induced lysis. Cells and cell walls from 2DG-grown cultures lysed faster when proteinase K was added, but glucose-grown cultures and cell walls were not affected. A lipoteichoic acid (LTA) extract (aqueous phase from hot phenol treatment) from glucose-grown cells inhibited the lysis of 2DG-grown cultures, but a similar extract prepared from 2DG-grown cells was without effect. Thin-layer chromatography and differential staining indicated that wild-type and Lys+ PTS- cells incorporated 2DG into LTA, but lysis-resistant cultures (Lys- PTS+ and Lys- PTS-) did not. LTA from lysis-resistant (Lys- PTS+ and Lys- PTS-) cells grown with glucose and 2DG also prevented 2DG-dependent lysis of the wild-type. LTA could not inhibit degradation of cell walls isolated from 2DG-grown cultures, but LTA inhibited the lysis of Micrococcus lysodeikticus (Micrococcus luteus) cells that were exposed to supernatants from 2DG-grown S. bovis cultures. Group D streptococci (including S. bovis) normally have an alpha-1,2 linked glucose disaccharide (kojibiose) in their LTA, but kojibiose cannot be synthesized from 2DG. This observation suggested that the kojibiose moiety of LTA was involved in autolysin inactivation. Wild-type S. bovis had ATP- as well as PEP-dependent mechanisms of 2DG phosphorylation and one lysis-resistant phenotype (Lys- PTS-) had reduced levels of both activities. However, the Lys- PTS+ phenotype was still able to phosphorylate 2DG via ATP and PEP and this result indicated that some other step of 2DG incorporation into LTA was being inhibited. Based on these results, growth in the presence of 2DG appears to prevent synthesis of normal LTA, which is involved in the regulation of autolytic enzymes.

Autolysis of Lactococcus lactis is influenced by proteolysis

The autolysin AcmA of Lactococcus lactis was shown to be degraded by the extracellular lactococcal proteinase PrtP. Autolysis, as evidenced by reduction in optical density of a stationary-phase culture and concomitant release of intracellular proteins, was greatly reduced when L. lactis MG1363 cells expressed the cell wall-anchored lactococcal proteinase PrtP of the PI-type caseinolytic specificity (PI). On the other hand, lactococcal strains that did not produce the proteinase showed a high level of autolysis, which was also observed when the cells produced the secreted form of PI or a cell wall-anchored proteinase with PIII-type specificity. Autolysis was also increased when MG1363 expressed the cell wall-anchored hybrid PI/PIII-type proteinase PIac. Zymographic analysis of AcmA activity during stationary phase showed that AcmA was quickly degraded by PI and much more slowly by PrtP proteinases with PIII-type and intermediate specificities. Autolysis of L. lactis by AcmA was influenced by the specificity, amount, and location of the lactococcal proteinase. No autolysis was observed when the various proteinases were expressed in an L. lactis acmA deletion mutant, indicating that PrtP itself did not cause lysis of cells. The chain length of a strain was significantly shortened when the strain expressed a cell wall-anchored active proteinase.

A novel thermostable neutral proteinase from Saccharomonospora canescens

A novel thermostable neutral proteinase, called NPS, was purified to electrophoretic homogeneity from the culture broth of Saccharomonospora canescens sp. novus, strain 5. The molecular mass was determined by SDS-polyacrylamide gel electrophoresis to be 35,000 Da. The enzyme exhibits a sharp pH optimum of proteolytic activity at pH 6.7. NPS was completely inactivated with inhibitors, typical for metalloendopeptidases, EDTA and 1,10-phenantroline, whereas the serine proteinase inhibitor PMSF had no effect. Atomic absorption measurements showed that the proteinase binds a single zinc and four calcium ions. The enzyme thermostability was characterized in the absence and presence of added calcium. Melting temperature, Tm = 77 degrees C and an activation energy, Ea, for the thermal deactivation of the excited protein fluorophores of 72.13 kJ mol-1 were calculated in the presence of 100 mM CaCl2. The Ea-value is considerably higher than those obtained for a number of proteinases from microorganisms and was explained by the thermostable structure of the enzyme. Effective radiationless energy transfer from phenol groups to indole rings was observed. 68% of the light absorbed by tyrosyl residues is transferred to tryptophyl side chains. No homology was found after comparison of the NPS N-terminal sequence, including the first 26 residues, with those of other neutral proteinases from microorganisms. In contrast to the well-known bacterial neutral proteinase thermolysin and related enzymes from microorganisms, NPS possesses arylamidase and esterase activities. Further crystallographic studies will reveal the structural reasons for this specificity. Epoxy and epithio pyranosides are inhibitors of the proteinase arylamidase activity.

Stabilization of the glucan-binding lectin of Streptococcus sobrinus by specific ligand

Cell suspensions of Streptococcus sobrinus can be aggregated by high molecular-weight alpha-1,6 glucans. The aggregation depends on the fidelity of a cell wall-bound, glucan-binding lectin (GBL). It is thought that the lectin may play a part in the sucrose-dependent accretion of streptococci in dental plaques. Results showed that the anionic detergent, sodium dodecyl sulphate (SDS) was a potent inhibitor of the lectin. When cells were incubated in SDS and washed to remove the detergent, lectin activity was diminished. Following incubation of the cells with SDS in the presence of glucan T-10, a low molecular-weight alpha-1,6 glucan, the loss of activity was less pronounced, suggesting that the glucan afforded partial protection against denaturation. Urea and guanidine hydrochloride were good inhibitors of the lectin, but, unlike SDS, were not able to inhibit it irreversibly, except at very high concentrations. Cationic detergents, such as cetylpyridinium bromide (and chloride), also irreversibly denatured the streptococcal lectin, but were not as effective as SDS in abolishing its activity. The results suggest that alpha-1,6 glucan stabilizes the GBL of S. sobrinus, rendering it more resistant to the effect of chaotropes. This may be one reason why dental plaques tend to resist detergents in dentrifices.

The role of autolysins during vegetative growth of Bacillus subtilis 168

A set of isogenic mutants of Bacillus subtilis 168, insertionally inactivated in the genes encoding a number of lytic enzymes and a sigma factor (sigma D, which controls the expression of a number of autolysins) was constructed. Phenotypic analysis of the mutants determined the individual and combined roles of the autolysins in vegetative growth. The major vegetative autolysins of B. subtilis, LytC (50 kDa amidase) and LytD (90 kDa glucosaminidase), were shown to have roles in cell separation, cell wall turnover, antibiotic-induced lysis and motility. LytC was also shown to have a role in general cell lysis induced by sodium azide. Renaturing SDS-PAGE of cell-wall-binding protein extracts of the mutant strains revealed the presence of a novel autolysin that was previously masked by LytC. This 49 kDa enzyme was shown to be sigma D-controlled and was identified as a candidate cell separation and cell wall turnover enzyme. A multiple mutant strain, lacking LytC, LytD and the 49 kDa enzyme, retained at least ten bands of autolytic activity. These may correspond to individual or proteolytically processed novel autolysins, the functions of which are unknown. The multiple mutant strains facilitate the study of these, and other lytic enzymes, to determine their cellular functions.

Autolysis of Lactococcus lactis ssp. lactis and Lactobacillus casei ssp. casei. Cell lysis induced by a crude bacteriocin

Autolytic properties of Lactococcus lactis subsp. lactis IFPL359, its Lac Prt derivative Lc. lactis Tl and Lactobacillus casei subsp. casei IFPL731, used as starter and adjunct starter in goat's milk cheese making, have been studied. The lytic effect of a bacteriocin produced by a lactic acid bacterium isolated from raw goat's milk has also been analyzed. Lactococcal cells resuspended in phosphate buffer showed a peak of autolysis when they were harvested in the early growth phase. A more stable autolytic pattern through the exponential growth was obtained for Lb. casei IFPL731. Optimal autolysis was found in 0.1 M sodium phosphate buffer during incubation at 40 degrees C for Lb. casei IFPL731 and at 35 degrees C for the lactococci. Thermoinduction of cell lysis was not obtained in any of the cases under the conditions studied. Lytic effect of the crude bacteriocin assayed was strongest against Lc. lactis Tl. Lysis response to the bacteriocin seemed to be strain-dependent and related to growth conditions.

Modification of the Staphylococcus aureus fibronectin binding phenotype by V8 protease

The amount of cell surface fibronectin (Fn)-binding protein (FnBP) adhesin expressed by Staphylococcus aureus is maximal during exponential growth but disappears rapidly as the culture progresses into stationary phase. To identify factors responsible for the loss of cell surface FnBP, a culture of S. aureus L170, which shows high levels of Fn binding, was supplemented at the time of inoculation with concentrated stationary-phase supernatant from S. aureus L530, a strain which binds Fn poorly. The resulting exponential-phase cells were devoid of FnBP. The factor responsible for this activity was purified from the culture supernatant and identified as V8 protease. When cultured with 375 ng of exogenous V8 protease ml(-1), exponential-phase cells of S. aureus L170 were devoid of cell surface FnBP, and concentrations as low as 23 ng x ml(-1) resulted in reduced amounts of FnBP. Addition of the protease inhibitor alpha2-macroglobulin to the culture medium prevented the growth-phase-dependent loss of cell surface FnBP, whereas growth with exogenous V8 protease resulted in reduced adherence to the solid-phase N-terminal fragment of Fn and to the extracellular matrix synthesized by fetal rabbit lung fibroblasts. Although FnBP was extremely sensitive to V8 protease, exogenous protease did not exert a significant influence on the amount of cell surface protein A. However, a limited number of other high-molecular-weight cell surface proteins were also sensitive to V8 protease. Therefore, both the adhesive phenotype and cell surface protein profile of S. aureus can be modified by V8 protease activity.

Expression in bacteria other than Escherichia coli

Although many bacteria are used for the expression of foreign genes, there is still a need to develop better expression systems. Advances have been made in the stabilization of gene maintenance and in the control of expression, therefore increasing the potential usefulness of some of these bacteria.

The effect of growth and starvation on the lysis of the ruminal cellulolytic bacterium Fibrobacter succinogenes

Growing cultures of Fibrobacter succinogenes assimilated more ammonia than could be accounted for by cellular protein, RNA, or DNA and released large amounts of nonammonia nitrogen. The difference between net and true growth was most dramatic at low dilution rates, but mathematical derivations indicated that the lysis rate was a growth rate-independent function. The lysis rate was sevenfold greater than the true maintenance rate (0.07 h-1 versus 0.01 h-1). Because slowly growing cells had as much proton motive force and ATP as fast-growing cells, lysis was not a starvation response per se. Stationary-phase cells had a lysis rate that was 10-fold less than that of growing cells. Rapidly growing cells were not susceptible to phenylmethylsulfonyl fluoride, but phenylmethylsulfonyl fluoride increased the lysis rate of the cultures when they reached the stationary phase. This latter result indicated that autolysins of stationary-phase cells were being inactivated by a serine proteinase. When growing cells were treated with the glycolytic inhibitor iodoacetate, the proteinase-dependent transition to the stationary phase was circumvented, and the rate of lysis could be increased by as much as 50-fold.

Molecular cloning and nucleotide sequence of the gene encoding the major peptidoglycan hydrolase of Lactococcus lactis, a muramidase needed for cell separation

A gene of Lactococcus lactis subsp. cremoris MG1363 encoding a peptidoglycan hydrolase was identified in a genomic library of the strain in pUC19 by screening Escherichia coli transformants for cell wall lysis activity on a medium containing autoclaved, lyophilized Micrococcus lysodeikticus cells. In cell extracts of L. lactis MG1363 and several halo-producing E. coli transformants, lytic bands of similar sizes were identified by denaturing sodium dodecyl sulfate (SDS)-polyacrylamide gels containing L. lactis or M. lysodeikticus cell walls. Of these clearing bands, corresponding to the presence of lytic enzymes with sizes of 46 and 41 kDa, the 41-kDa band was also present in the supernatant of an L. lactis culture. Deletion analysis of one of the recombinant plasmids showed that the information specifying lytic activity was contained within a 2,428-bp EcoRV-Sau3A fragment. Sequencing of part of this fragment revealed a gene (acmA) that could encode a polypeptide of 437 amino acid residues. The calculated molecular mass of AcmA (46,564 Da) corresponded to that of one of the lytic activities detected. Presumably, the enzyme is synthesized as a precursor protein which is processed by cleavage after the Ala at position 57, thus producing a mature protein with a size of 40,264 Da, which would correspond to the size of the enzyme whose lytic activity was present in culture supernatants of L. lactis. The N-terminal region of the mature protein showed 60% identity with the N-terminal region of the mature muramidase-2 of Enterococcus hirae and the autolysin of Streptococcus faecalis. Like the latter two enzymes, AcmA contains C-terminal repeated regions. In AcmA, these three repeats are separated by nonhomologous intervening sequences highly enriched in serine, threonine, and asparagine. Genes specifying identical activities were detected in various strains of L. lactis subsp. lactis and L. lactis subsp. cremoris by the SDS-polyacrylamide gel electrophoresis detection assay and PCR experiments. By replacement recombination, an acmA deletion mutant which grew as long chains was constructed, indicating that AcmA is required for cell separation.

Anomalies in cell wall turnover associated with the growth temperature of Bacillus subtilis

Cell wall turnover appeared to be anomalously fast in Bacillus subtilis when the cells were grown at temperatures below 29 degrees C. Turnover rates k(generation-1), of exponential cultures at 25 degrees were approximately double those of cells grown at 37 degrees C. When autolysin levels were assayed in cell walls, it was found that the enzyme activities were constant between 25 degrees C and 40 degrees C, suggesting that there was no greater synthesis of autolysin at the lower temperature. Analyses of walls for individual components, extent of aminosugar substitution and extent of crosslinking, did not reveal significant differences between samples obtained from 25 degrees C or 37 degrees C cultures. The N-acetylmuramoyl-L-alanine amidase was stable over the temperature range studied. Lysis of cells, induced by carbonylcyanide-m-chlorophenylhydrazone, occurred at a faster rate for cells obtained at 25 degrees C than for cells obtained at 37 degrees C. In addition, the lysis of cells by hen egg white lysozyme was slightly faster when the cells were obtained from 25 degrees C cultures than from 37 degrees C cultures. It is possible the autolysin(s) responsible for cell wall turnover are cold-activated.

Mapping and characterization of multiple chromosomal factors involved in methicillin resistance in Staphylococcus aureus

Chromosomal factors, termed fem or aux factors, are needed for the expression of methicillin resistance in methicillin-resistant (Mcr) Staphylococcus aureus; also needed is the mec-encoded low-affinity penicillin-binding protein PBP 2'. These factors make up part of the normal set of genes present in susceptible and resistant strains of S. aureus and can be identified by Tn551-mediated insertional inactivation of the methicillin resistance. In this study, we characterized different Tn551 inserts and mapped them into four distinct loci on the SmaI chromosomal map of S. aureus NCTC 8325, thereby identifying two new loci which code for fem factors. The largest fragment, SmaI-A, carries three loci, two coding for both closely linked factors femA and femB and a novel third locus (femC) that is not linked to the other two. An additional, fourth, locus, femD, was identified in fragment SmaI-I. femA and femB inactivation reduced overall methicillin resistance, whereby femB had less of an influence on the resistance level. femC and femD inactivation reduced mainly the basal resistance level in heterogeneously Mcr strains and had less of an impact on the subpopulation with high-level resistance. Inactivation of either of these factors was shown to have no influence on the production of PBP 2', the main factor mediating methicillin resistance. In addition, no changes were observed in the banding patterns of the major autolysins in whole-cell extracts of the fem mutants, suggesting that the reduced cell wall turnover and autolysis observed in some of the insertionally inactivated strains were due to changes either of the substrate or in the autolysin control.

Zonal turnover of cell poles of Bacillus subtilis

Turnover of cell walls of Bacillus subtilis occurs in three distinct phases: a lag phase, a relatively rapid phase persisting for 2-3 generations and a much slower phase continuing for several additional generations. A lectin probe revealed that cell pole material was lost during the slow phase of turnover and that the loss of wall occurred in zones, beginning at the cylinder-pole junction and continuing to the cell tip. This is in contrast to cell wall turnover in cylinders where turnover occurs randomly at many surface sites.

Secretion and processing of staphylococcal nuclease by Bacillus subtilis

We have studied the secretion and processing of Staphylococcus aureus nuclease in Bacillus subtilis. We show that the initial species of nuclease found in the cell supernatants during short-term radioactive labeling (pulse-chase) had a molecular weight of approximately 18,800 and comigrated in a sodium dodecyl sulfate-polyacrylamide gel with staphylococcal nuclease B. This nuclease B form was processed to the mature nuclease A extracellularly by a phenylmethylsulfonyl fluoride-sensitive protease. The nuclease B-processing site is a consensus signal peptidase site, and the processing of nuclease B was coupled to secretion as judged by pulse-chase experiments. The nuclease A was shown by microsequencing of the N terminus to be 2 amino acid residues shorter than the nuclease A described for S. aureus Foggi. The nuclease B form was still the first species found in the culture supernatant after removal of the N-terminal 26 amino acids of the native 60-amino-acid signal peptide. However, removal of the N-terminal 72 amino acids abolishes secretion of any nuclease form and leads to the intracellular accumulation of nuclease.

The functions of autolysins in the growth and division of Bacillus subtilis

Some bacteria, such as streptococci, exhibit growth from discrete and well-defined zones. In Streptococcus faecalis, growth zones can be observed in the electron microscope, and the position of the zone can be used as a marker for cell cycle events. Growth of the cell surface of Bacillus subtilis appears to be by a much different mechanism from that of streptococci. Cell elongation takes place by the insertion at many sites in the cell cylinder of peptidoglycan components. The insertion occurs on the inner face of the wall, and upon cross linking, the new wall material becomes stress bearing and older wall is pushed to the surface. When old wall reaches the surface, it becomes susceptible to excision by autolysins, resulting in wall turnover; cell elongation, due to the stretching of the cross-linked peptidoglycan, therefore, accompanies turnover and does not require a specialized growth zone.

Characterization of a cloned Bacillus subtilis gene that inhibits sporulation in multiple copies

We have isolated a 1.0-kilobase fragment of the Bacillus subtilis chromosome which, when present in high-copy-number plasmids, caused a sporulation-proficient strain to become phenotypically sporulation deficient. This is referred to as the sporulation inhibition (Sin) phenotype. This DNA fragment, in multicopy, also inhibited the production of extracellular protease activity, which normally appears at the beginning of stationary growth. The origin of the fragment was mapped between the dnaE and spo0A genes on the B. subtilis chromosome, and its complete DNA sequence has been determined. By analysis of various deletions and a spontaneous mutant the Sin function was localized to an open reading frame (ORF) predicted from the DNA sequence. Inactivation of this ORF in the chromosome did not affect the ability of cells to sporulate. However, the late-growth-associated production of proteases and alpha-amylase was elevated in these cells. The predicted amino acid sequence of the protein encoded by this ORF had a DNA-binding domain, typically present in several regulatory proteins. We propose that the sin ORF encodes a regulatory protein that is involved in the transition from vegetative growth to sporulation.

Isolation and initial characterization of a Bacillus subtilis mutant with novel protease secretion capability

A bank of pTV32 (Tn 917 lacZ) - generated Bacillus subtilis mutants were examined on milk agar for the ability to produce proteases at 48 degrees C. A single mutant, BUL786, was isolated, which could hydrolyze casein after overnight incubation at 48 degrees C. This mutant secreted protease 10 fold more at 48 degrees C when compared to 37 degrees C, and part of the activity appears to be 48 degrees C-specific. At high temperatures, other strains of B. subtilis, including hyperprotease secretors, were unable to secrete protease to any significant degree. The BUL786 strain is missing the 97K major heat shock protein. Since a number of other proteins also appear to be secreted at 48 degrees C, this mutant may be a hypersecretor of exported proteins at temperatures greater than 45 degrees C.

Molecular cloning and nucleotide sequence of a DNA fragment from Bacillus natto that enhances production of extracellular proteases and levansucrase in Bacillus subtilis

A DNA fragment from Bacillus natto IFO3936 has been cloned which enhances the production of both extracellular alkaline and neutral proteases in Bacillus subtilis. The DNA sequence analysis around the gene responsible for the hyperproduction, prtR, revealed one open reading frame (comprising 60 amino acid residues) which was bounded by potential transcriptional and translational regulatory signals in its preceding and following regions. This open reading frame was not homologous to the published sequences of the structural genes of the two proteases. The calculated molecular weight (7,109) of the polypeptide predicted from the DNA sequence is much smaller than those of the two proteases, indicating that the gene product is distinct from those enzymes. In-frame fusion between the N-terminal region of the coding sequence and the lacZ gene of Escherichia coli demonstrated that the coding region was indeed translated in vivo. By deletion analysis it was suggested that prtR was the structural gene for the 60-amino-acid polypeptide. Cells carrying a prtR plasmid secreted both proteases 40 to 400 times more than the cells carrying the vector alone. Furthermore, it was found that prtR also enhanced the production of levansucrase by 1 or 2 orders of magnitude. There was no difference, however, in the amount of the other extracellular enzymes such as alpha-amylase, RNase, and alkaline phosphatase. These results indicate that prtR is specific for the hyperproduction of the proteases and levansucrase.

Inside-to-outside growth and turnover of the wall of gram-positive rods

Gram-positive, rod-shaped bacteria, given a pulse of peptidoglycan precursors, first exhibit a lag before the second or turnover phase of peptidoglycan commences. This is because new material is inserted on the inner face of the wall and gradually displaced through the wall. Based on this experimental observation, a mathematical model was constructed and compared with experimental data obtained in several laboratories for the first and second phases of wall turnover of Bacillus subtilis. The model allows the parameters of the process to be estimated for experiments with any labeling time. According to the surface stress theory the wall which is layed down immediately outside the cytoplasmic layer is in an unextended conformation. As subsequent additions of murein occur, the wall moves outward, becomes stretched, and bears the stress due to hydrostatic pressure. Ultimately, peptide and glycosyl bonds become cleaved. At the end of the lag phase the cleavage becomes so extensive that wall fragments are liberated into the medium. This strategy permits rod-shaped growth. In some experimental situations the half-life of wall radioactivity in this second phase roughly equals the doubling time; consequently, the exponential release probably does not represent random turnover but instead is the result of expansion of the underlying wall that continues to create strain which favors autolysis action. The slower turnover of the third phase, where there is a much slower loss, is also included in the analysis.

Secretion of staphylococcal nuclease by Bacillus subtilis

The staphylococcal nuclease (nuc) gene from Staphylococcus aureus has been cloned and expressed in Bacillus subtilis. The nuclease protein was expressed either from its own promoter and translation start signals, or from a combination of a B. subtilis promoter, ribosome binding site, and a signal peptide sequence. Greater than 80% of the active gene product was secreted into the medium, whereas, when a signal peptide sequence was absent, as little as 4% of the nuclease activity was found in the culture medium. Intracellular (or cell-bound) nuclease, as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and Western blotting, was shown to have the molecular weight of the predicted precursor protein with the signal peptide. Levels of nuclease reached 50 mg per liter in the culture medium, depending on the growth medium and the strain used. These findings indicate the prospective use of nuclease as a model system for studying secretion of heterologous proteins in B. subtilis.

Cloning of the neutral protease gene of Bacillus subtilis and the use of the cloned gene to create an in vitro-derived deletion mutation

The neutral protease gene of Bacillus subtilis has been cloned, and its nucleotide sequence has been determined. The cloned gene was used to create an in vitro-derived deletion mutation, which was used to replace the wild-type copy of the gene. This deletion, in combination with a deletion of the alkaline protease gene, completely abolished protease production. The loss of the proteases had no detectable effect on growth, morphology, or sporulation.

Insertion and fate of the cell wall in Bacillus subtilis

Cell wall assembly was studied in autolysin-deficient and -sufficient strains of Bacillus subtilis. Two independent probes, one for peptidoglycan and the other for surface-accessible teichoic acid, were employed to monitor cell surface changes during growth. Cell walls were specifically labeled with N-acetyl-D-[3H]glucosamine, and after growth, autoradiographs were prepared for both cell types. The locations of silver grains revealed that label was progressively lost from numerous sites on the cell cylinders, whereas label was retained on the cell poles, even after several generations. In the autolysin-deficient and chain-forming strain, it was found that the distance between densely labeled poles approximately doubled after each generation of growth. In the autolysin-sufficient strain, it was found that the numbers of labeled cell poles remained nearly constant for several generations, supporting the premise that completed septa and poles are largely conserved during growth. Fluorescein-conjugated concanavalin A was also used to determine the distribution of alpha-D-glucosylated teichoic acid on the surfaces of growing cells. Strains with temperature-sensitive phosphoglucomutase were used because in these mutants, glycosylation of cell wall teichoic acids can be controlled by temperature shifts. When the bacteria were grown at 45 degrees C, which stops the glucosylation of teichoic acid, the cells gradually lost their ability to bind concanavalin A on their cylindrical surfaces, but they retained concanavalin A-reactive sites on their poles. Discrete areas on the cylinder, defined by the binding of fluorescent concanavalin A, were absent when the synthesis of glucosylated teichoic acid was inhibited during growth for several generations at the nonpermissive temperature. When the mutant was shifted from a nonpermissive to a permissive temperature, all areas of the cylinder became able to bind the labeled concanavalin A after about one-half generation. Old cell poles were able to bind the lectin after nearly one generation at the permissive temperature, showing that new wall synthesis does occur in the cell poles, although it occurs slowly. These data, based on both qualitative and quantitative experiments, support a model for cell wall assembly in B. subtilis, in which cylinders elongate by inside-to-outside growth, with degradation of the stress-bearing old wall in wild-type organisms. Loss of wall material, by turnover, from many sites on the cylinder may be necessary for intercalation of new wall and normal length extension. Poles tend to retain their wall components during division and are turned over much more slowly.

Absence of correlation between rates of cell wall turnover and autolysis shown by Bacillus subtilis mutants

Bacillus subtilis mutants with reduced rates of cell wall autolysis reached a constant rate of wall turnover after a longer lag than the standard strain but eventually showed the same turnover rate. In reverse, a turnover-deficient mutant autolysed at a slightly higher rate than the standard strain. Consequently, there is no correlation between the rates of cell wall turnover and autolysis.