Penicillin binding protein 5 affects cell diameter, contour, and morphology of Escherichia coli

Although general physiological functions have been ascribed to the high-mol ecular-weight penicillin binding proteins (PBPs) of Escherichia coli, the l ow-molecular-weight PBPs have no well-defined biological roles, When we exa mined the morphology of a set of E. coli mutants lacking multiple PBPs, we observed that strains expressing active PBP 5 produced cells of normal shap e, while mutants lacking PBP 5 produced cells with altered diameters, conto urs, and topological features. These morphological effects were visible in untreated cells, but the defects were exacerbated in cells forced to filame nt by inactivation of PBP 3 or FtsZ. After filamentation, cellular diameter varied erratically along the length of individual filaments and many filam ents exhibited extensive branching. Also, in general, the mean diameter of cells lacking PBP 5 was significantly increased compared to that of cells f rom isogenic strains expressing active PBP 5, Expression of cloned PBP 5 re versed the effects observed in Delta dacA mutants, Although deletion of PBP 5 was required for these phenotypes, the absence of additional PBPs magnif ied the effects. The greatest morphological alterations required that at le ast three PBPs in addition to PBP 5 be deleted from a single strain. In the extreme cases in which six or seven PBPs were deleted from a single mutant , cells and cell filaments expressing PBP 5 retained a normal morphology bu t cells and filaments lacking PBP 5 were aberrant. In no case did mutation of another PBP produce the same drastic: morphological effects. We conclude that among the low-molecular-weight PBPs, PBP 5 plays a principle role in determining cell diameter, surface uniformity, and overall topology of the peptidoglycan sacculus.