Characterization of the rate-limiting step of the secretion of Bacillus subtilis alpha-amylase overproduced during the exponential phase of growth

Protein secretion zones during overexpression of amylase within the Gram-positive cell wall

BACKGROUND

Whereas the translocation of proteins across the cell membrane has been thoroughly investigated, it is still unclear how proteins cross the cell wall in Gram-positive bacteria, which are widely used for industrial applications. We have studied the secretion of α-amylase AmyE within two different Bacillus strains, B. subtilis and B. licheniformis.

RESULTS

We show that a C-terminal fusion of AmyE with the fluorescent reporter mCherry is secreted via discrete patches showing very low dynamics. These are visible at many places within the cell wall for many minutes. Expression from a high copy number plasmid was required to be able to see these structures we term "secretion zones". Zones corresponded to visualized AmyE activity on the surface of cells, showing that they release active enzymes. They overlapped with SecA signals but did not frequently co-localize with the secretion ATPase. Single particle tracking showed higher dynamics of SecA and of SecDF, involved in AmyE secretion, at the cell membrane than AmyE. These experiments suggest that SecA initially translocates AmyE molecules through the cell membrane, and then diffuses to a different translocon. Single molecule tracking of SecA suggests the existence of three distinct diffusive states of SecA, which change during AmyE overexpression, but increased AmyE secretion does not appear to overwhelm the system.

CONCLUSIONS

Because secretion zones were only found during the transition to and within the stationary phase, diffusion rather than passive transport based on cell wall growth from inside to outside may release AmyE and, thus, probably secreted proteins in general. Our findings suggest active transport through the cell membrane and slow, passive transition through the cell wall, at least for overexpressed proteins, in bacteria of the genus Bacillus.

Enhancing Extracellular Pullulanase Production in Bacillus subtilis Through dltB Disruption and Signal Peptide Optimization

Bacillus subtilis has many attributes that make it a popular host for recombinant protein production. Although its protein production ability has been enhanced through protease gene disruption, residual proteases like quality control HtrA and HtrB can limit protein yield by degrading inadequately folded proteins present during overexpression. In this study, two strategies were employed to increase production of industrial enzyme pullulanase: enhancing extracellular pullulanase folding and optimizing its signal peptide. The hypothesis was that disruption of dltB gene of expression host B. subtilis WS9 would enhance recombinant extracellular folding by increasing cation binding to the cell's outer envelope. Consistent with this hypothesis, disrupting dltB enhanced pullulanase production by 49% in shake-flask cultures. The disruption also enhanced extracellular α-CGTase and β-CGTase production by 25% and 35%, respectively. Then, more effective signal peptide for pullulanase production was identified through high-throughput screening of 173 unique B. subtilis signal peptides. Replacing the native signal peptide of pullulanase with that encoded by ywtF increased extracellular pullulanase activity by an additional 12%. Three-liter fermenter scale-up production yielded the highest extracellular pullulanase activity reported to date: 8037.91 U·mL-1. This study highlights the usefulness of dltB deletion and signal peptide optimization in enhancing extracellular protein production.

Statistical Optimization of the Production of NaCl-Tolerant Proteases by a Moderate Halophile, Virgibacillus sp. SK37

The objectives of this study are to optimize the conditions for providing high yield of NaCl-tolerant extracellular protease from Virgibacillus sp. SK37 based on a fish-based medium and to investigate the effects of the key factors (mass per volume ratios of dried anchovy, yeast extract and NaCl, and initial pH of the medium) on the secretion pattern of proteases. Based on the predicted response model, the optimized medium contained 1.81% of dried anchovy, 0.33% of yeast extract and 1.25% of NaCl at pH=7.8. Under these conditions, a 5.3-fold increase in protease production was achieved, compared with the broth containing only 1.2% of dried anchovy (5% of NaCl at pH=7). The cubic regression adequately described the protease production. Protease activity was determined using sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) on the synthetic substrate (Suc-Ala-Ala-Pro-Phe-AMC). Proteases of molecular masses of 19, 34, 35 and 44 kDa were secreted in the presence of NaCl, whereas those of 22 and 42 kDa were the main proteases detected in the absence of NaCl. In addition, no secreted proteases were detected when initial pH of the medium was pH=6. The peptide mass fingerprint of the medium cultured with 10% NaCl showed a higher abundance of peptides with lower mass of 500-1000 m/z compared with the medium containing 0% NaCl, indicating the higher proteolytic activity of the high-salt medium. The Virgibacillus sp. SK37 proteases showed a marked preference towards Lys, Arg and Tyr in the presence of NaCl and towards Lys and Arg in the absence of NaCl.

Evidence of a bacterial receptor for lysozyme: binding of lysozyme to the anti-σ factor RsiV controls activation of the ecf σ factor σV

σ factors endow RNA polymerase with promoter specificity in bacteria. Extra-Cytoplasmic Function (ECF) σ factors represent the largest and most diverse family of σ factors. Most ECF σ factors must be activated in response to an external signal. One mechanism of activation is the stepwise proteolytic destruction of an anti-σ factor via Regulated Intramembrane Proteolysis (RIP). In most cases, the site-1 protease required to initiate the RIP process directly senses the signal. Here we report a new mechanism in which the anti-σ factor rather than the site-1 protease is the sensor. We provide evidence suggesting that the anti-σ factor RsiV is the bacterial receptor for the innate immune defense enzyme, lysozyme. The site-1 cleavage site is similar to the recognition site of signal peptidase and cleavage at this site is required for σ^V activation in Bacillus subtilis. We reconstitute site-1 cleavage in vitro and demonstrate that it requires both signal peptidase and lysozyme. We demonstrate that the anti-σ factor RsiV directly binds to lysozyme and muramidase activity is not required for σ^V activation. We propose a model in which the binding of lysozyme to RsiV activates RsiV for signal peptidase cleavage at site-1, initiating proteolytic destruction of RsiV and activation of σ^V. This suggests a novel mechanism in which conformational change in a substrate controls the cleavage susceptibility for signal peptidase. Thus, unlike other ECF σ factors which require regulated intramembrane proteolysis for activation, the sensor for σ^V activation is not the site-1 protease but the anti-σ factor.

All cells sense and respond to changes in their environments by transmitting information across the membrane. In bacteria, σ factors provide promoter specificity to RNA polymerase. Bacteria encode Extra-Cytoplasmic Function (ECF) σ factors, which often respond to extracellular signals. Activation of some ECF σ factors is controlled by stepwise proteolytic destruction of an anti-σ factor which is initiated by a site-1 protease. In most cases, the site-1 protease required to initiate the RIP process is thought to be the signal sensor. Here we report that the anti-σ factor RsiV, and not the site-1 protease, is the sensor for σ^V activation. Activation of the ECF σ factor σ^V is induced by lysozyme, an innate immune defense enzyme. We identify the site-1 protease as signal peptidase, which is required for general protein secretion. The anti-σ factor RsiV directly binds lysozyme. Binding of lysozyme to RsiV allows signal peptidase to cleave RsiV at site-1 and this leads to activation of σ^V. Thus, the anti-σ factor functions as a bacterial receptor for lysozyme. RsiV homologs from C. difficile and E. faecalis also bind lysozyme, suggesting they may utilize this receptor-ligand mechanism to control activation of σ^V to induce lysozyme resistance.

Protein transport across the cell wall of monoderm Gram-positive bacteria

In monoderm (single-membrane) Gram-positive bacteria, the majority of secreted proteins are first translocated across the cytoplasmic membrane into the inner wall zone. For a subset of these proteins, final destination is within the cell envelope as either membrane-anchored or cell wall-anchored proteins, whereas another subset of proteins is destined to be transported across the cell wall into the extracellular milieu. Although the cell wall is a porous structure, there is evidence that, for some proteins, transport is a regulated process. This review aims at describing what is known about the mechanisms that regulate the transport of proteins across the cell wall of monoderm Gram-positive bacteria.

Antibody production in Bacillus megaterium: strategies and physiological implications of scaling from microtiter plates to industrial bioreactors

Bacillus megaterium was used as an alternative high potential microbial production system for the production of antibody fragment D1.3 scFv. The aim of the study was to follow a holistic optimization approach from medium screening in small scale microtiter platforms, gaining deeper process understanding in the bioreactor scale and implementing advanced process strategies at larger scales (5-100 L). Screening and optimization procedures were supported by statistical design of experiments and a genetic algorithm approach. The process control relied on a soft-sensor for biomass estimation to establish a μ-oscillating time-dependent fed-batch strategy. Several cycles of growth phases and production phases, equal to starving phases, were performed in one production. Flow cytometry was used to monitor and characterize the dynamics of secretion and cell viability. Besides the biosynthesis of the product, secretion was optimized by an appropriate medium design considering different carbon sources, metal ions, (NH(4))(2)SO(4), and inductor concentrations. For bioprocess design, an adapted oscillating fed-batch strategy was conceived and successfully implemented at an industrially relevant scale of 100 L. In comparison to common methods for controlling fed-batch profiles, the developed process delivered increased overall productivities. Thereby measured process parameters such as growth stagnation or productivity fluctuations were directly linked to single cell or population behavior leading to a more detailed process understanding. Above all, the importance of single cell analysis as key scale-free tool to characterize and optimize recombinant protein production is highlighted, since this can be applied to all development stages independently of the cultivation platform.

Differentiation of propeptide residues regulating the compartmentalization, maturation and activity of the broad-range phospholipase C of Listeria monocytogenes

The intracellular bacterial pathogen Listeria monocytogenes secretes a broad-range phospholipase C enzyme called PC-PLC (phosphatidylcholine phospholipase C) whose compartmentalization and enzymatic activity is regulated by a 24-amino-acid propeptide (Cys28-Ser51). During intracytosolic multiplication, bacteria accumulate the proform of PC-PLC at their membrane-cell-wall interface, whereas during cell-to-cell spread vacuolar acidification leads to maturation and rapid translocation of PC-PLC across the cell wall in a manner that is dependent on Mpl, the metalloprotease of Listeria. In the present study, we generated a series of propeptide mutants to determine the minimal requirement to prevent PC-PLC enzymatic activity and to identify residues regulating compartmentalization and maturation. We found that a single residue at position P1 (Ser51) of the cleavage site is sufficient to prevent enzymatic activity, which is consistent with P1' (Trp52) being located within the active-site pocket. We observed that mutants with deletions at the N-terminus, but not the C-terminus, of the propeptide are translocated across the cell wall more effectively than wild-type PC-PLC at a physiological pH, and that individual amino acid residues within the N-terminus influence Mpl-mediated maturation of PC-PLC at acidic pH. However, deletion of more than 75% of the propeptide was required to completely prevent Mpl-mediated maturation of PC-PLC. These results indicate that the N-terminus of the propeptide regulates PC-PLC compartmentalization and that specific residues within the N-terminus influence the ability of Mpl to mediate PC-PLC maturation, although a six-residue propeptide is sufficient for Mpl to mediate PC-PLC maturation.

Changing a single amino acid in Clostridium perfringens beta-toxin affects the efficiency of heterologous secretion by Bacillus subtilis

Achieving efficient heterologous protein production and secretion by Bacillus subtilis is an attractive prospect, although often disappointingly low yields are reached. The expression of detoxified Clostridium perfringens beta-toxin (beta-toxoid) is exemplary for this. Although beta-toxin can be efficiently expressed and secreted by Bacillus subtilis, the genetically detoxified, and industrially interesting, beta-toxoid variant is difficult to obtain in high amounts. To optimize the expression of this putative vaccine component, we studied the differences in the global gene regulation responses of B. subtilis to overproduction of either beta-toxin or beta-toxoid by transcriptomics. A clear difference was the upregulation of the CssRS regulon, known to be induced upon secretion stress, when beta-toxoid is produced. YkoJ, a protein of unknown function, was also upregulated, and we show that its expression is dependent on cssS. We then focused on the heterologous protein itself and found that the major secretion bottleneck can be traced back to a single amino acid substitution between the beta-toxin and the beta-toxoid, which results in the rapid degradation of beta-toxoid following secretion across the cytoplasmic membrane. In contrast to beta-toxin, beta-toxoid protein is more prone to degradation directly after secretion, most likely due to poor folding characteristics introduced with point mutations. Our results show that although the host can be adapted in many ways, the intrinsic properties of a heterologous protein can play a decisive role when optimizing heterologous protein production.

Heterologous production and secretion of Clostridium perfringens β-toxoid in closely related Gram-positive hosts

The spore forming bacterium Clostridium perfringens is a widely occurring pathogen. Vaccines against C. perfringens type B and C are currently manufactured using beta-toxin secreted by virulent C. perfringens strains. Large-scale production of vaccines from virulent strains requires stringent safety conditions and costly detoxification and control steps. Therefore, it would be beneficial to produce this toxin in a safe production host and in an immunogenic, but non-toxic form (toxoid). For high-level expression of beta-toxoid, we cloned the highly active ribosomal rpsF promoter of Bacillus subtilis in a broad host range multicopy plasmid. In B. subtilis, we obtained high intracellular production, up to 200 microg ml(-1) culture. However, the beta-toxoid was poorly secreted. The employed rpsF expression system allowed using the same expression plasmids in other heterologous hosts such as Lactococcus lactis and Streptococcus pneumoniae. In these organisms secretion of beta-toxoid was ten times higher compared to the best producing B. subtilis strain. These results show the usefulness of the rpsF based broad host range expression system.

Increasing the stability of sacB transcript improves levansucrase production in Bacillus subtilis

AIMS

To develop a strategy to increase the stability of transcripts of structural genes expressed under the control of sacR, the leader region of Bacillus subtilis levansucrase gene.

METHODS AND RESULTS

Insertion of Shine Dalgarno like sequences in the 5'-untranslated sacR region controlling the expression of sacB. Depending on the number of stabilizing sequences inserted and the position of these sequences with respect to the translation start codon, it was observed that the mRNA stability and the final protein production could be increased or decreased.

CONCLUSIONS

This mRNA stabilization can be used to increase exocellular protein production in the degU32 (Hy) mutant.

SIGNIFICANCE AND IMPACT OF THE STUDY

This approach can be applied to the expression of heterologous genes of biotechnological interest.

Autogenous modulation of the Bacillus subtilis sacB–levB–yveA levansucrase operon by the levB transcript

Silencing oflevB, the second structural gene of the tricistronic levansucrase operon encoding the endolevanase LevB, decreases the level of levansucrase expression inBacillus subtilis. Conversely, independent expression oflevBgreatly stimulates operon expression. This autogenous effect is mediated by thelevBtranscript, which carries an internal sequence (5′-AAAGCAGGCAA-3′) involved in the enhancing effect.In vitro, thelevBtranscript displays an affinity for the N-terminal fragment of SacY (KD0·2 μM), the regulatory protein that prevents transcription termination of the levansucrase operon. This positive-feedback loop leads to an increase in the operon expression whenB. subtilisis growing in the presence of high sucrose concentrations. Under these conditions, extracellular levan synthesized by the fructosyl polymerase activity of levansucrase can be degraded mainly into levanbiose by the action of LevB. Levanbiose is neither taken up nor metabolized by the bacteria. This work modifies the present view of the status of levansucrase inB. subtilisphysiology.

Influence of lipoteichoic acid D-alanylation on protein secretion in Lactococcus lactis as revealed by random mutagenesis

Lactococcus lactis, a food-grade nonpathogenic lactic acid bacterium, is a good candidate for the production of heterologous proteins of therapeutic interest. We examined host factors that affect secretion of heterologous proteins in L. lactis. Random insertional mutagenesis was performed with L. lactis strain MG1363 carrying a staphylococcal nuclease (Nuc) reporter cassette in its chromosome. This cassette encodes a fusion protein between the signal peptide of the Usp45 lactococcal protein and the mature moiety of a truncated form of Nuc (NucT). The Nuc secretion efficiency (secreted NucT versus total NucT) from this construct is low in L. lactis (approximately 40%). Twenty mutants affected in NucT production and/or in secretion capacity were selected and identified. In these mutants, several independent insertions mapped in the dltA gene (involved in D-alanine transfer in lipoteichoic acids) and resulted in a NucT secretion defect. Characterization of the dltA mutant phenotype with respect to NucT secretion revealed that it is involved in a late secretion stage by causing mature NucT entrapment at the cell surface.

Bacillus subtilis α-amylase: interactions of a partially folded conformer with small unilamellar vesicles

We studied the interactions between conformers of exocellular alpha-amylase and small unilamellar vesicles (SUV) composed of the major membrane lipids of Bacillus subtilis under physiological conditions of pH, temperature and ionic strength. Using fluorescence spectroscopy, surface plasmon resonance (SPR) and phase separation, we show that the native alpha-amylase has no affinity for the SUV, whereas a partially folded form, displaying structural properties in common with the competent state for secretion, binds to the vesicles (KA approximately 10(5) M(-1)). This association prevented its subsequent folding. The complex was destabilized in the presence of PrsA, a major peripheric lipoprotein of B. subtilis which displays a strong affinity for SUV (KA approximately 1.5x10(8) M(-1)). Vesicles coated with PrsA lost their ability to bind the partially folded conformer. The approach in vitro, in which our aim was to mimic the last stage of alpha-amylase translocation, indicates that PrsA possibly helps, in vivo, the secreted protein to acquire its native conformation by modulating the interaction between the latter and the lipid polar heads on the trans side of the cytoplasmic membrane.

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.

The extracytoplasmic folding factor PrsA is required for protein secretion only in the presence of the cell wall in Bacillus subtilis

Pulse-chase labelling was used to study the role of the cell wall microenvironment in the functioning of Bacillus subtilis PrsA, an extracellular lipoprotein and member of the parvulin family of peptidylprolyl cis/trans-isomerases. It was found that in protoplasts, and thus in the absence of a cell wall matrix, the post-translocational folding, stability and secretion of the AmyQ alpha-amylase were independent of PrsA, in contrast to the strict dependency found in rods. The results indicate that PrsA is dedicated to assisting the folding and stability of exported proteins in the particular microenvironment of the cytoplasmic membrane-cell wall interface, possibly as a chaperone preventing unproductive interactions with the wall. The data also provide evidence for a crucial role of the wall in protein secretion. The presence of the wall directly or indirectly facilitates the release of AmyQ from the cell membrane and affects the rate of the signal peptide processing.

Use of a promoter trap system in Bacillus anthracis and Bacillus subtilis for the development of recombinant protective antigen-based vaccines

We have recently reported Bacillus anthracis attenuated live vaccine strains efficiently expressing recombinant protective antigen (rPA) and have shown a direct correlation between the level of rPA secreted by these cells and efficacy (S. Cohen, I. Mendelson, Z. Altboum, D. Kobiler, E. Elhanany, T. Bino, M. Leitner, I. Inbar, H. Rosenberg, Y. Gozes, R. Barak, M. Fisher, C. Kronman, B. Velan, and A. Shafferman, Infect. Immun. 68:4549-4558, 2000). To isolate more potent Bacillus promoters for a further increase in the production of rPA, we developed a promoter trap system based on various gfp reporter genes adapted for use in both Bacillus subtilis and B. anthracis backgrounds. Accordingly, a B. anthracis library of 6,000 clones harboring plasmids with chromosomal B. anthracis DNA fragments inserted upstream from gfpuv was constructed. Based on fluorescence intensity, 57 clones carrying potentially strong promoters were identified, some of which were DNA sequenced. The most potent B. anthracis promoter identified (Pntr; 271 bp) was 500 times more potent than the native pagA promoter and 70 times more potent than the alpha-amylase promoter (Pamy). This very potent promoter was tested along with the other promoters (which are three, six, and eight times more potent than Pamy) for the ability to drive expression of rPA in either B. subtilis or B. anthracis. The number of cell-associated pre-PA molecules in B. anthracis was found to correlate well with the strength of the promoter. However, there appeared to be an upper limit to the amount of mature PA secreted into the medium, which did not exceed that driven by Pamy. Furthermore, the rPA constructs fused to the very potent promoters proved to be deleterious to the bacterial hosts and consequently led to genetic instability of the PA expression plasmid. Immunization with attenuated B. anthracis expressing rPA under the control of promoters more potent than Pamy was less efficient in eliciting anti-PA antibodies than that attained with Pamy. The results are consistent with the notion that overexpression of PA leads to severe secretion stress and have practical implications for the design of second-generation rPA-based vaccines.

S-layer protein transport across the cell wall of Corynebacterium glutamicum: in vivo kinetics and energy requirements

Corynebacteria are Gram-positive bacteria with a very peculiar cell envelope structure as it is constituted of an inner membrane and an outer membrane-like structure. Protein secretion in Corynebacterium glutamicum was studied in vivo, using the S-layer protein PS2 as a model. We show that different variants of PS2 protein are exported through the whole cell envelope with a half-life ranging between 2 and 4 min, by a two-step mechanism. The first step, which is over after about 1.5 min, is ATP- and proton motive force-dependent and may correspond to translocation across the inner membrane via the 'Sec' machinery. The second step, across the cell wall and the outer mycolate layer, is rapid but independent of energy sources. This very efficient secretion process across the mycolate layer raises the question of the existence in this layer of a specific machinery.

Simultaneous inactivation of the wprA and dltB genes of Bacillus subtilis reduces the yield of alpha-amylase

AIMS

In Gram-positive bacteria, signal peptide-bearing secretory proteins are translocated through the cytoplasmic membrane and fold into their native conformation on the outside of the cell. The products of the Bacillus subtilis wprA and dltB genes separately influence post-translocational stages of the secretion process by mediating proteolytic degradation and folding of secretory proteins. Inactivation of either wprA or dltB in B. subtilis increases the yield of secretory proteins released into the culture medium in an intact and biologically active conformation. The aim of this work was to study the combined influence of these genes.

METHODS AND RESULTS

A wprA/dltB double mutant was constructed, but did not have an additive effect on secretion and caused a significant reduction in the yield of alpha-amylase.

CONCLUSIONS AND SIGNIFICANCE

The activities of the wprA gene and the dlt operon interact in a negative way to influence the growth cycle and protein secretion. The mechanism by which this may occur, and its potential significance for the secretion of native and non-native proteins from B. subtilis and related bacteria, is discussed.

Engineering of a Bacillus subtilis strain with adjustable levels of intracellular biotin for secretory production of functional streptavidin

Streptavidin is a biotin-binding protein which has been widely used in many in vitro and in vivo applications. Because of the ease of protein recovery and availability of protease-deficient strains, the Bacillus subtilis expression-secretion system is an attractive system for streptavidin production. However, attempts to produce streptavidin using B. subtilis face the problem that cells overproducing large amounts of streptavidin suffer poor growth, presumably because of biotin deficiency. This problem cannot be solved by supplementing biotin to the culture medium, as this will saturate the biotin binding sites in streptavidin. We addressed this dilemma by engineering a B. subtilis strain (WB800BIO) which overproduces intracellular biotin. The strategy involves replacing the natural regulatory region of the B. subtilis chromosomal biotin biosynthetic operon (bioWAFDBIorf2) with an engineered one consisting of the B. subtilis groE promoter and gluconate operator. Biotin production in WB800BIO is induced by gluconate, and the level of biotin produced can be adjusted by varying the gluconate dosage. A level of gluconate was selected to allow enhanced intracellular production of biotin without getting it released into the culture medium. WB800BIO, when used as a host for streptavidin production, grows healthily in a biotin-limited medium and produces large amounts (35 to 50 mg/liter) of streptavidin, with over 80% of its biotin binding sites available for future applications.

Optimization of the cell wall microenvironment allows increased production of recombinant Bacillus anthracis protective antigen from B. subtilis

The stability of heterologous proteins secreted by gram-positive bacteria is greatly influenced by the microenvironment on the trans side of the cytoplasmic membrane, and secreted heterologous proteins are susceptible to rapid degradation by host cell proteases. In Bacillus subtilis, degradation occurs either as the proteins emerge from the presecretory translocase and prior to folding into their native conformation or after the native conformation has been reached. The former process generally involves membrane- and/or cell wall-bound proteases, while the latter involves proteases that are released into the culture medium. The identification and manipulation of factors that influence the folding of heterologous proteins has the potential to improve the yield of secreted heterologous proteins. Recombinant anthrax protective antigen (rPA) has been used as a model secreted heterologous protein because it is sensitive to proteolytic degradation both before and after folding into its native conformation. This paper describes the influence of the microenvironment on the trans side of the cytoplasmic membrane on the stability of rPA. Specifically, we have determined the influence of net cell wall charge and its modulation by the extent to which the anionic polymer teichoic acid is D-alanylated on the secretion and stability of rPA. The potential role of the dlt operon, responsible for D-alanylation, was investigated using a Bacillus subtilis strain encoding an inducible dlt operon. We show that, in the absence of D-alanylation, the yield of secreted rPA is increased 2.5-fold. The function of D-alanylation and the use of rPA as a model protein are evaluated with respect to the optimization of B. subtilis for the secretion of heterologous proteins.

yveB, Encoding endolevanase LevB, is part of the sacB-yveB-yveA levansucrase tricistronic operon in Bacillus subtilis

Transcription of sacB, yveB and yveA, three clustered genes on the Bacillus subtilis chromosome, is simultaneously induced by sucrose. Northern blotting analyses with specific probes showed three distinct mRNAs: a monocistronic 1.7 kb sacB mRNA, a bicistronic 3.3 kb sacB-yveB mRNA and a tricistronic 4.9 kb sacB-yveB-yveA mRNA. These results indicate that sacB, encoding levansucrase, is the proximal gene of a sucrose-inducible operon that includes the two other genes. The yield of the full-length transcript is lower than that of the bicistronic transcript, whose yield is itself lower than that of the monocistronic transcript. This suggested that the 3' terminal parts of sacB and yveB genes worked as internal terminator structures. The protein encoded by yveB, which remains anchored to the membrane, displays an endolevanase activity, which, coupled with exolevanase activity of SacB, leads to a complete degradation of levan, a branched fructosyl polymer. It is proposed to rename yveB as levB.

Study of mycoloyl transferase transport across the cell envelope of Corynebacterium glutamicum

PS1 is a major exported protein of Corynebacterium glutamicum homologous to mycobacterial antigen 85. It is largely associated with the mycolic acid-containing cell wall and acts as a mycoloyl transferase. The transport of PS1 to the cell wall is slow and occurs through two energetically distinct steps: the first one, which includes processing by signal peptidase, is rapid and inhibited by sodium azide or carbonyl cyanide m-chlorophenylhydrazone. This step is probably associated with translocation across the cytoplasmic membrane. The kinetics of the second step depend on the size of the polypeptide chain to be transported but neither ATP nor proton motive force is required. This step may correspond to the diffusion of PS1 across the cell wall to its final location.

Transcripts of the genes sacB, amyE, sacC and csn expressed in Bacillus subtilis under the control of the 5' untranslated sacR region display different stabilities that can be modulated

When Bacillus subtilis levanase (SacC), alpha-amylase (AmyE) and chitosanase (Csn) structural genes were expressed under the regulated control of sacR, the inducible levansucrase (SacB) leader region in a degU32(Hy) mutant, it was observed that the production yields of the various extracellular proteins were quite different. This is mainly due to differences in the stabilities of their corresponding mRNAs which lead to discrepancies between the steady-state level of mRNA of sacB and csn on the one hand and amyE and sacC on the other. In contrast to levansucrase mRNA, the decay curves of alpha-amylase and levanase mRNAs obtained by Northern blotting analysis did not match the decay curves of their functional mRNA. This suggested that only a part of the population of the amyE and sacC transcripts was fully translated, while the others were possibly poorly bound to ribosomes and thus were only partially translated or not at all and consequently submitted to rapid endonuclease degradation. This hypothesis was substantiated by the finding that the introduction of a Shine-Dalgarno sequence upstream from the ribosome-binding site in the sacC transcript resulted in a fourfold increase in both the half-life of this transcript and the production of levanase. An additional cause of low-level levanase production is the premature release of mRNA by the polymerase. It was attempted to correlate this event with internal secondary structures of sacC mRNA.

Quantitation of the capacity of the secretion apparatus and requirement for PrsA in growth and secretion of alpha-amylase in Bacillus subtilis

Regulated expression of AmyQ alpha-amylase of Bacillus amyloliquefaciens was used to examine the capacity of the protein secretion apparatus of B. subtilis. One B. subtilis cell was found to secrete maximally 10 fg of AmyQ per h. The signal peptidase SipT limits the rate of processing of the signal peptide. Another limit is set by PrsA lipoprotein. The wild-type level of PrsA was found to be 2 x 10(4) molecules per cell. Decreasing the cellular level of PrsA did not decrease the capacity of the protein translocation or signal peptide processing steps but dramatically affected secretion in a posttranslocational step. There was a linear correlation between the number of cellular PrsA molecules and the number of secreted AmyQ molecules over a wide range of prsA and amyQ expression levels. Significantly, even when amyQ was expressed at low levels, overproduction of PrsA enhanced its secretion. The finding is consistent with a reversible interaction between PrsA and AmyQ. The high cellular level of PrsA suggests a chaperone-like function. PrsA was also found to be essential for the viability of B. subtilis. Drastic depletion of PrsA resulted in altered cellular morphology and ultimately in cell death.

The Bacillus subtilis 168 csn gene encodes a chitosanase with similar properties to a streptomyces enzyme

The Bacillus subtilis 168 csn gene encodes a chitosanase. It was found that transcription of the csn gene was temporally regulated and was not subject to metabolic repression. Chitosanase synthesis was abolished in a csn mutant strain. Csn was overproduced in B. subtilis, partially purified and characterized. The deduced amino acid sequence, K(m), and optimal pH and temperature of the B. subtilis enzyme were closer to those of a chitosanase from Streptomyces sp. N174 than to those of chitosanases from other Bacillus strains.

The carboxyl terminus of the Bacillus subtilis SecA is dispensable for protein secretion and viability

The Escherichia coli secretion-dedicated chaperone SecB targets a subset of proteins to the translocase by interacting with the carboxyl (C-) terminus of SecA. This region of SecA is highly conserved in Eubacteria, but despite its presence in the Bacillus subtilis SecA, the B. subtilis genome does not appear to contain a gene for a clear homologue of SecB. Deletion of the C-terminus of the B. subtilis SecA yields cells that have normal viability, but that exhibit a response reminiscent of oxidative stress and the loss of a number of secretory proteins from the culture supernatant. Semi-quantitative RT-PCR demonstrates that these proteins are expressed at lower levels. The C-terminus of SecA fused to glutathione S:-transferase (GST) specifically binds a cytosolic protein, termed MrgA. This protein has been reported to function in relation to oxidative stress, but deletion of the mrgA gene does not result in a secretion defect nor does it cause an oxidative stress response. It is concluded that the C-terminus of the B. subtilis SecA is not essential for secretion and viability.

Cell-associated degradation affects the yield of secreted engineered and heterologous proteins in the Bacillus subtilis expression system

A series of chimeric alpha-amylase genes derived from amyL, which encodes the liquefying alpha-amylase from Bacillus licheniformis, were constructed in vitro using gene splicing techniques. The gene constructs were cloned in Bacillus subtilis, where their ability to direct the synthesis and secretion of active alpha-amylase was determined. Detectable alpha-amylase activity was observed for some, but not all, of the chimeric proteins. Studies on the secretion of wild-type AmyL and its chimeric derivatives revealed that, whilst these proteins were stable in the extracellular milieu, all were subject to some degree of degradation during secretion. The chimeric enzymes were degraded to a greater extent than the native enzyme. These findings suggest that cell-associated proteolysis is a significant problem affecting the use of B. subtilis as host bacterium for the production of heterologous proteins.

Kinetics of alpha-amylase secretion in Aspergillus oryzae

Pulse and pulse-chase experiments have been performed to study L-[(35)S] methionine incorporation and protein secretion kinetics in Aspergillus oryzae. Pulse experiments confirmed the mechanism of methionine uptake reported previously for Penicillium chrysogenum (Benko et al., 1967). Pulse-chase experiments were carried out to investigate the alpha-amylase secretion kinetics in A. oryzae. No unglycosylated alpha-amylase was detected neither intracellularly nor extracellularly demonstrating that glycosylation was not the rate controlling step in the secretory pathway. The pulse chase experiments indicated that there are two pools of intracellular alpha-amylase: a fast secreted and a slow secreted. The secretion of those two pools were described with a kinetic model, which was fitted to the pulse chase experiments.

Anionic polymers of Bacillus subtilis cell wall modulate the folding rate of secreted proteins

In order to characterize the dynamics of the interaction between the emergent membrane translocated exoprotein and the components of Bacillus subtilis cell wall, we examined the kinetics of the in vitro refolding of levansucrase and alpha-amylase, at pH 7 and 37 degrees C, in the presence of polyphosphates (polyP) of various chain lengths (2</=n</=65). These soluble anionic polymers are considered here to mimic the role of teichoic acids. Even in the absence of calcium, levansucrase rapidly refolded in the presence of polyP of n>/=16. In contrast, polyP modulate indirectly the rate of alpha-amylase refolding via their affinity for calcium. These differential effects might explain that the rate of the cell wall translocation of alpha-amylase secretion was found to be half that of levansucrase.

Evaluation of bottlenecks in the late stages of protein secretion in Bacillus subtilis

Despite a high capacity for secretion of homologous proteins, the secretion of heterologous proteins by Bacillus subtilis is frequently inefficient. In the present studies, we have investigated and compared bottlenecks in the secretion of four heterologous proteins: Bacillus lichenifomis alpha-amylase (AmyL), Escherichia coli TEM beta-lactamase (Bla), human pancreatic alpha-amylase (HPA), and a lysozyme-specific single-chain antibody. The same expression and secretion signals were used for all four of these proteins. Notably, all identified bottlenecks relate to late stages in secretion, following translocation of the preproteins across the cytoplasmic membrane. These bottlenecks include processing by signal peptidase, passage through the cell wall, and degradation in the wall and growth medium. Strikingly, all translocated HPA was misfolded, its stability depending on the formation of disulfide bonds. This suggests that the disulfide bond oxidoreductases of B. subtilis cannot form the disulfide bonds in HPA correctly. As the secretion bottlenecks differed for each heterologous protein tested, it is anticipated that the efficient secretion of particular groups of heterologous proteins with the same secretion bottlenecks will require the engineering of specifically optimized host strains.

Bacillus subtilis alpha-amylase: the rate limiting step of secretion is growth phase-independent

When Bacillus subtilis alpha-amylase was expressed under the control of sacR in a degU32(Hy) strain, the production of exoenzyme occurred during both the exponential and stationary phases of growth. In each phase, pulse-chase experiments showed that the rate-limiting step of the secretion process was the release of the processed form of the protein in each physiological context. The rate of this event was slightly slower (t(1/2) = 3.2 min) during the stationary phase than during the exponential phase (t(1/2) = 2 min). The effectors which possibly control the efficiency of the release stage, the level of PrsA or the calcium binding properties of the cell wall, remained unchanged throughout growth phases.

Differential dependence of levansucrase and alpha-amylase secretion on SecA (Div) during the exponential phase of growth of Bacillus subtilis

SecA, the translocation ATPase of the preprotein translocase, accounts for 0.25% of the total protein in a degU32(Hy) Bacillus subtilis strain in logarithmic phase. The SecA level remained constant irrespective of the demand for exoprotein production but dropped about 12-fold during the late stationary phase. Modulation of the level of functional SecA during the exponential phase of growth affected differently the secretion of levansucrase and alpha-amylase overexpressed under the control of the sacB leader region. The level of SecA was reduced in the presence of sodium azide and in the div341 thermosensitive mutant at nonpermissive temperatures. Overproduction of SecA was obtained with a multicopy plasmid bearing secA. The gradual decrease of the SecA level reduced the yield of secreted levansucrase with a concomitant accumulation of unprocessed precursor in the cells, while an increase in the SecA level resulted in an elevation of the production of exocellular levansucrase. In contrast, alpha-amylase secretion was almost unaffected by high concentrations of sodium azide or by very low levels of SecA. Secretion defects were apparent only under conditions of strong SecA deprivation of the cell. These data demonstrate that the alpha-amylase and levansucrase precursors markedly differ in their dependency on SecA for secretion. It is suggested that these precursors differ in their binding affinities for SecA.

The influence of protein folding on late stages of the secretion of alpha-amylases from Bacillus subtilis

A derivative of the alpha-amylase from Bacillus licheniformis (AmyL) engineered to give an active enzyme with increased net positive charge is secreted by Bacillus subtilis with a yield that is significantly lower than that of the native enzyme. This reduction in yield is the result of increased proteolysis during or shortly after translocation through the cytoplasmic membrane. When we compared the overall rate of folding of the engineered derivative (AmyLQS50.5) with that of AmyL it exhibited a greater dependency on Ca2+ ions for in vitro folding. When the concentration of Ca2+ in the growth medium was increased, so too did the relative yield of AmyLQS50.5. We discuss the importance of secretory protein folding at the membrane/cell wall interface with respect to the yield of native and heterologous proteins from B. subtilis.

Characterization of a stable intermediate trapped during reversible refolding of Bacillus subtilis alpha-amylase

Bacillus subtilis exocellular alpha-amylase is reversibly refolded after denaturation by guanidine hydrochloride at pH 7 and 37 degrees C. The unfolding-folding transition monitored by intrinsic fluorescence changes and resistance to proteolysis was resolved into a two-state transition. The first step (t1/2 < 1 s) led from D, the totally unfolded state, to C, a stable partially structured state of the protein. This folding intermediate was devoid of any enzyme activity and partially resistant to protease degradation. Calcium was required for the transition from C to N, the native state. This metal did not remain associated with the native form and could be replaced by barium or strontium, but not by magnesium. We discuss the hypothesis that C, the folding intermediate whose further transformation is under kinetic control, is the competent state involved in the secretion process of alpha-amylase.