Stability and synthesis of the penicillin-binding proteins during sporulation

Spore Peptidoglycan

Bacterial endospores possess multiple integument layers, one of which is the cortex peptidoglycan wall. The cortex is essential for the maintenance of spore core dehydration and dormancy and contains structural modifications that differentiate it from vegetative cell peptidoglycan and determine its fate during spore germination. Following the engulfment stage of sporulation, the cortex is synthesized within the intermembrane space surrounding the forespore. Proteins responsible for cortex synthesis are produced in both the forespore and mother cell compartments. While some of these proteins also contribute to vegetative cell wall synthesis, others are sporulation specific. In order for the bacterial endospore to germinate and resume metabolism, the cortex peptidoglycan must first be degraded through the action of germination-specific lytic enzymes. These enzymes are present, yet inactive, in the dormant spore and recognize the muramic-δ-lactam modification present in the cortex. Germination-specific lytic enzymes across Bacillaceae and Clostridiaceae share this specificity determinant, which ensures that the spore cortex is hydrolyzed while the vegetative cell wall remains unharmed. Bacillus species tend to possess two redundant enzymes, SleB and CwlJ, capable of sufficient cortex degradation, while the clostridia have only one, SleC. Additional enzymes are often present that cannot initiate the cortex degradation process, but which can increase the rate of release of small fragments into the medium. Between the two families, the enzymes also differ in the enzymatic activities they possess and the mechanisms acting to restrict their activation until germination has been initiated.

Peptidoglycan transformations during Bacillus subtilis sporulation

While vegetative Bacillus subtilis cells and mature spores are both surrounded by a thick layer of peptidoglycan (PG, a polymer of glycan strands cross-linked by peptide bridges), it has remained unclear whether PG surrounds prespores during engulfment. To clarify this issue, we generated a slender ΔponA mutant that enabled high-resolution electron cryotomographic imaging. Three-dimensional reconstructions of whole cells in near-native states revealed a thin PG-like layer extending from the lateral cell wall around the prespore throughout engulfment. Cryotomography of purified sacculi and fluorescent labelling of PG in live cells confirmed that PG surrounds the prespore. The presence of PG throughout engulfment suggests new roles for PG in sporulation, including a new model for how PG synthesis might drive engulfment, and obviates the need to synthesize a PG layer de novo during cortex formation. In addition, it reveals that B. subtilis can synthesize thin, Gram-negative-like PG layers as well as its thick, archetypal Gram-positive cell wall. The continuous transformations from thick to thin and back to thick during sporulation suggest that both forms of PG have the same basic architecture (circumferential). Endopeptidase activity may be the main switch that governs whether a thin or a thick PG layer is assembled.

Penicillin-binding protein sensitive to cephalexin in sporulation of Bacillus cereus

Cephalexin, cefaclor, cefadroxil, and cefotaxime strongly inhibited sporulation of Bacillus cereus ts-4 at 1 microgram/ml. Cephalexin was most inhibitory on the sporulation of B. cereus when the antibiotic was added at 3 h after induction of sporulation by nutrient downshift technique. Examination of 4',6-diamidino-2-phenylindole-stained cells by fluorescence-phase contrast microscopy showed that cephalexin inhibited the formation of asymmetric septum. By using [3H]penicillin, eight penicillin-binding proteins (PBPs) were detected from the cells of B. cereus ts-4. Among them, four PBPs were also detected in sporulating cells. Affinity of cephalexin to PBPs were measured indirectly by competition for subsequent binding of radioactive penicillin G. Cephalexin strongly bound to PBP 4 with molecular weight of 72,000 in sporulating cells.

The Bacillus subtilis spoVD gene encodes a mother-cell-specific penicillin-binding protein required for spore morphogenesis

The Bacillus subtilis spoVD gene has been cloned and sequenced. It encodes a 71,262 Da protein with extensive sequence similarity to penicillin-binding proteins from various organisms. The context of this gene in the B. subtilis chromosome, immediately upstream of the mur operon, suggests that it is related to the pbpB gene of Escherichia coli, which is involved in the synthesis of septal peptidoglycan during cell division. Expression of spoVD in E. coli leads to the synthesis of a membrane-associated protein of the size expected for SpoVD, which can bind labelled penicillin. However, insertional disruption of the spoVD gene has no effect on vegetative growth or division: a second pbp-like gene immediately upstream of spoVD is probably the functional homologue of E. coli pbpB. spoVD seems instead to have a specialized role in the morphogenesis of the spore cortex, which is a modified form of peptidoglycan. spoVD transcription appears to occur from a promoter recognized by the sigma E form of RNA polymerase. Analysis of the expression of a spoVD'-lacZ reporter gene supports this notion and indicates that a second level of negative regulation is dependent on the SpoIIID protein. SpoVD synthesis probably occurs only in the mother cell since both sigma E and SpoIIID are thought to be specific to this cell type. Such localization of SpoVD synthesis was supported by the results of a genetic test showing that expression of spoVD only in the mother cell is sufficient for spore formation. The results support the proposition that spore cortex formation is determined primarily by the mother cell.

Mutagenesis and mapping of the gene for a sporulation-specific penicillin-binding protein in Bacillus subtilis

Penicillin-binding protein (PBP) 5* is produced by Bacillus subtilis only during sporulation and is believed to be required for synthesis of the peptidoglycan-like cortex layer of the spore. The structural gene (dacB) for PBP 5* was insertionally mutagenized by integration of a plasmid bearing an internal fragment of the gene, and the phenotype of the null mutant was characterized. The mutant had no apparent vegetative growth or germination defect, but it produced extremely heat-sensitive spores. This property is consistent with a defect in the amount or assembly of the cortex and supports the hypothesis that PBP 5* is required for synthesis of this structure. Analysis of the progeny after spontaneous excision of the integrated plasmid led to the conclusion that expression of the dacB gene was required only in the mother cell compartment during sporulation, which is also consistent with a role for PBP 5* in cortex synthesis and with its location in the outer forespore membrane. Genetic mapping located dacB midway between aroC (206 degrees) and lys (210 degrees) on the B. subtilis chromosome. This is a region where there are no other known spo, ger, or PBP genes. In related studies, we found that a null mutant of dacA, the structural gene for vegetative PBP 5, produced normal heat-resistant spores, which suggests that this PBP is not essential for cortex synthesis. In addition, a candidate for another sporulation-specific PBP was revealed on gels at approximately the same position as PBP 5*. The two PBPs could be distinguished by immunoassays.

Isolation and sequence analysis of dacB, which encodes a sporulation-specific penicillin-binding protein in Bacillus subtilis

A novel penicillin-binding protein (PBP 5*) with D,D-carboxypeptidase activity is synthesized by Bacillus subtilis, beginning at about stage III of sporulation. The complete gene (dacB) for this protein was cloned by immunoscreening of an expression vector library and then sequenced. The identity of dacB was verified not only by the size and cross-reactivity of its product but also by the presence of the nucleotide sequence that coded for the independently determined NH2 terminus of PBP 5*. Analysis of its complete amino acid sequence confirmed the hypothesis that this PBP is related to other active-site serine D,D-peptidases involved in bacterial cell wall metabolism. PBP 5* had the active-site domains common to all PBPs, as well as a cleavable amino-terminal signal peptide and a carboxy-terminal membrane anchor that are typical features of low-molecular-weight PBPs. Mature PBP 5* was 355 amino acids long, and its mass was calculated to be 40,057 daltons. What is unique about this PBP is that it is developmentally regulated. Analysis of the sequence provided support for the hypothesis that the sporulation specificity and mother cell-specific expression of dacB can be attributed to recognition of the gene by a sporulation-specific sigma factor. There was a good match of the putative promoter of dacB with the sequence recognized by sigma factor E (sigma E), the subunit of RNA polymerase that is responsible for early mother cell-specific gene expression during sporulation. Analysis of PBP 5* production by various spo mutants also suggested that dacB expression is on a sigma E-dependent pathway.

Suppression of defective-sporulation phenotypes by mutations in transcription factor genes of Bacillus subtilis

Mutations in the Bacillus subtilis major RNA polymerase sigma factor gene (rpoD/crsA47) and a sensory receiver gene (spoOA/rvtA11) are potent intergenic suppressors of several stage 0 sporulation mutations (spoOB, OE, OF & OK). We show here that these suppressors also rescue temperature-sensitive sporulation phenotypes (Spots) caused by mutations in RNA polymerase, ribosomal protein, and protein synthesis elongation factor EF-G genes. The effects of the crsA and rvtA suppressors on RNA polymerase and ribosomal protein spots mutations are similar to those previously described for mutations in another intergenic suppressor gene rev. We have examined the effects of rvtA and crsA mutations on the expression of sporulation-associated membrane proteins, including flagellin and penicillin binding protein 5* (PBP 5*). Both suppressors restored sporulation and synthesis of PBP 5* in several spoO mutants. However, only rvtA restored flagellin synthesis in spoO suppressed backgrounds. The membrane protein phenotypes resulting from the presence of crsA or rvtA suppressors in spoO strains suggests that these suppressors function via distinct molecular mechanisms. The rvtA and crsA mutations are also able to block the ability of ethanol to induce spoO phenocopies at concentrations of ethanol which prevent sporulation in wild type cells. The effects of ethanol on sporulation-associated membrane protein synthesis in wild type and suppressor containing strains have been examined.

The life-cycle proteins RodA of Escherichia coli and SpoVE of Bacillus subtilis have very similar primary structures

Comparison of the predicted amino acid sequence of the cell-cycle RodA protein with the National Research Foundation protein sequence database shows that the 370-amino-acid RodA, a protein that is essential for wall elongation in Escherichia coli and maintenance of the rod shape of the cell, is highly analogous, in terms of primary structure, with the Bacillus subtilis SpoVE protein involved in stage V of sporulation.

Absence of penicillin-binding protein 4 from an apparently normal strain of Bacillus subtilis

The phenotype of a Bacillus subtilis 168 strain with no detectable penicillin-binding protein 4 was examined. Despite the fact that penicillin-binding protein 4 is one of the most penicillin-sensitive proteins in the species, its apparent loss had no obvious effect on the organism or its susceptibility to various beta-lactam antibiotics.

Correlation of penicillin-binding protein composition with different functions of two membranes in Bacillus subtilis forespores

The distribution of penicillin-binding proteins (PBPs) within different membranes of sporulating cells of Bacillus subtilis was examined in an effort to correlate the location of individual PBPs with their proposed involvement in either cortical or vegetative peptidoglycan synthesis. The PBP composition of forespores was determined by two methods: examination of isolated forespore membranes and assay of the in vivo accessibility of the PBPs to penicillin. In both cases, it was apparent that PBP 5*, the major PBP synthesized during sporulation, was present primarily, but not exclusively, in the forespore. The membranes from mature dormant spores were prepared by either chemically stripping the integument layers of the spores, followed by lysozyme digestion, or lysozyme digestion alone of coat-defective gerE spores. PBP 5* was detected in membranes from unstripped spores but was never found in stripped ones, which suggests that the primary location of this PBP is the outer forespore membrane. This is consistent with a role for PBP 5* exclusively in cortex synthesis. In contrast, vegetative PBPs 1 and 2A were only observed in stripped spore preparations that were greatly enriched for the inner forespore membrane, which supports the proposed requirement for these PBPs early in germination. The apparent presence of PBP 3 in both membranes of the spore reinforces the suggestion that it catalyzes a step common to both cortical and vegetative peptidoglycan synthesis.

Induction of penicillin-binding proteins under catabolite-repressed conditions

Decoyinine, an inhibitor of GMP synthetase, was used to induce sporulation under catabolite-repressed conditions in Bacillus subtilis. Sporulation-specific penicillin-binding proteins 4* and 5* were produced in abundance, and there was an increase in vegetative penicillin-binding proteins 2B and 3. These results, which were completely blocked by addition of guanosine, suggest that synthesis of penicillin-binding proteins is neither catabolite repressed nor directly dependent on the stringent response.

Structural modifications in the peptidoglycan of Escherichia coli associated with changes in the state of growth of the culture

By determining the composition in muropeptides of the murein of a number of strains of Escherichia coli, purified from cells at various states of growth, the sacculus was found to be considerably modified when cells stop active growth. Murein from resting cells becomes hypercross-linked and richer in covalently bound lipoprotein, whereas the mean length of the glycan chains is considerably reduced. The alteration of the sacculus occurs progressively during the transitions from active growth to stationary phase and vice versa.

Restoration of vegetative penicillin-binding proteins during germination and outgrowth of Bacillus subtilis spores: relationship of individual proteins to specific cell cycle events

The order in which the vegetative penicillin-binding proteins (PBPs) are first synthesized and the rate of their return to normal levels during germination and outgrowth of Bacillus subtilis spores were determined. The rate of synthesis of most of the PBPs was much faster than that of the majority of other membrane proteins, which is consistent with the involvement of PBPs in biosynthesis of the rapidly expanding peptidoglycan. The pattern of PBP changes that occurred during the cell cycle, including sporulation, suggests a likely role for PBP 2A in cell elongation and a unique requirement for PBP 2B during both symmetric and asymmetric septum formation. PBP 3 is the only PBP that appears to be equally necessary for vegetative and cortical peptidoglycan synthesis.