Functional Studies of Penicillin-binding Protein 1 in Bacillus subtilis
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The penicillin-binding proteins (PBPs) synthesize and modify peptidoglycan (PG), the main structural element of the bacterial cell wall. PBPs and PG synthesis are highly conserved in all bacteria and both have been important targets for antibiotic and antibacterial development. In the Gram positive bacterium Bacillus subtilis, PBP1 is composed of the four domains S, N, P, and C in order from the N- to C-terminus. It plays important roles in vegetative PG synthesis. Compared to the wild type B. subtilis, the PBP1 null mutant has decreased growth rate, cell diameter, and PG crosslinking; the cell population has more long cells; and the colonies have raised and smooth edges. In this work, we constructed six mutant forms of PBP1 that were tagged with a C-terminal FLAG epitope, to complement the wild type gene. We examined the colony and cell morphologies, and PBP1 localization in the mutant strains. The removal of the cytoplasmic region of the PBP1 S domain and the replacement of PBP1 S domain by PBP4 S domain did not change the colony morphologies, and each of these two mutations had minor effects on growth rate, cell diameter, PG crosslinking and generation of long cells in the cell population. The single point mutation in the active site of the N or P domain presumably removed the enzymatic activity, and each mutation caused slower growth rate, decreased cell diameter and PG crosslinking. The point mutation in the P domain had a minor effect on the colony morphology and formation of long cells; while the mutation in the N domain altered the colony morphology, and resulted in high percentage of long cells that is comparable to the PBP1 null mutant. The C domain of PBP1 has no apparent enzymatic activity, but the loss of it altered the colony morphology, and caused slower growth rate, decreased cell diameter, and PG crosslinking. In the wild type B. subtilis, PBP1 localizes to the septum. This septum localization specificity was lost in strains expressing PBP1 without the C domain, with PBP4 S domain, or with a point mutation in the active site of the N domain. PBP1 with a point mutation in the active site of the P domain, or without the cytoplasmic region of the S domain, had decreased septum localization specificity. These findings were used to develop a model of how PBP1 domain functioning in B. subtilis.
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