Characterization of two Bacillus subtilis penicillin-binding protein-coding genes, ykuA (pbpH) and yrrR (pbpI)

Penicillin-binding proteins (PBPs) are required in the synthesis of the cell wall of bacteria. In Bacillus subtilis, PBPs play important roles in the life cycle, including both vegetative growth and sporulation, and contribute to the formation of the different structures of vegetative cell wall and spore cortex. The B. subtilis genome sequencing project revealed there were two uncharacterized genes, ykuA and yrrR, with extensive sequence similarity to class B PBPs. These two genes are renamed and referred to henceforth as pbpH and pbpI, respectively.

A sequence alignment of the predicted product of pbpH against the microbial protein database demonstrated that the most similar protein in B. subtilis is PBP2A and in E. coli is PBP2. This suggested that PbpH belongs to a group of the genes required for maintaining the rod shape of the cell. Study of a pbpH-lacZ fusion showed that pbpH was expressed weakly during vegetative growth and the expression reached the highest level at the transition from exponential phase to stationary phase. The combination of a pbpA deletion and the pbpH deletion was lethal and double mutant strains lacking pbpH and pbpC or pbpI (also named yrrR) were viable. The viable mutants were indistinguishable from the wild-type except that the vegetative PG of the pbpC pbpH strain had a slightly slightly lower amount of disaccharide tetrapeptide with 1 amidation and higher amount of disaccharide tripeptide tetrapeptide with 2 amidations when compared to others strains. This suggests that PbpC (PBP3) is involved in vegetative PG synthesis but only affects the PG structure with a very low efficiency.

A pbpA pbpH double mutant containing a xylose-regulated pbpH gene inserted into the chromosome at the amyE locus was constructed. Depletion of PbpH resulted in an arrest in cell growth and a dramatic morphological change in both vegetative cells and outgrowing spores. Vegetative cells lacking pbpA and pbpH expression swelled and cell elongation was arrested, leading to the formation of pleiomorphic spherical cells and eventual lysis. In these cells, cell septations were randomly localized, cell walls and septa were thicker than those seen in wild type cells, and the average cell width and volume were larger than those of cells expressing pbpA or pbpH. The vegetative PG had an increased abundance of one unidentified muropeptide. Spores produced by the pbpA pbpH double mutant were able to initiate germination but the transition of the oval-shaped spores to rod-shape cells was blocked. The outgrowing cells were spherical, gradually enlarged, and eventually lysed. Outgrowth of these spores in the presence of xylose led to the formation of helical cells. Thus, PbpH is apparently required for maintenance of cell shape, specifically for cell elongation. PbpH and PBP2a play a redundant role homologous to that of PBP2 in E. coli.

A sequence alignment of the predicted product of pbpI against the microbial protein database demonstrated that the most similar protein in B. subtilis is SpoVD and in E. coli is PBP3. This suggested that PbpI belongs to the group of the genes required for synthesis of the spore or septum PG. PbpI was identified using radio-labeled penicillin and found to run underneath PBP4 on SDS-PAGE. PbpI is therefore renamed PBP4b. Study of a pbpI-lacZ fusion showed that pbpI was expressed predominantly during early sporulation. A putative sigma F recognition site is present in the region upstream of pbpI and studies using mutant strains lacking sporulation-specific sigma factors demonstrated that the expression of pbpI is mainly dependent on sigma factor F. A pbpI single mutant, a pbpI pbpG double mutant, and a pbpI pbpF double mutant were indistinguishable from the wild-type. The sporulation defect of a pbpI pbpF pbpG triple mutant was indistinguishable from that of a pbpF pbpG double mutant. Structure parameters of the forespore PG in a pbpI spoVD strain are similar to that of a spoVD strain. These results indicate that PBP4b plays a unknown redundant role.