Journal of Bacteriology, March 2000, p. 1714-1721, Vol. 182, No. 6
0021-9193/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.
Department of Microbiology and Immunology, School of Medicine, University of North Dakota, Grand Forks, North Dakota 58202-9037
Received 4 October 1999/Accepted 16 December 1999
Although general physiological functions have been ascribed to the
high-molecular-weight penicillin binding proteins (PBPs) of
Escherichia coli, the low-molecular-weight PBPs have no
well-defined biological roles. When we examined the morphology of a set
of E. coli mutants lacking multiple PBPs, we observed that
strains expressing active PBP 5 produced cells of normal shape,
while mutants lacking PBP 5 produced cells with altered diameters,
contours, and topological features. These morphological effects were
visible in untreated cells, but the defects were exacerbated in cells forced to filament by inactivation of PBP 3 or FtsZ. After
filamentation, cellular diameter varied erratically along the length of
individual filaments and many filaments exhibited extensive branching.
Also, in general, the mean diameter of cells lacking PBP 5 was
significantly increased compared to that of cells from isogenic strains
expressing active PBP 5. Expression of cloned PBP 5 reversed the
effects observed in
dacA mutants. Although deletion of
PBP 5 was required for these phenotypes, the absence of additional PBPs
magnified the effects. The greatest morphological alterations required
that at least 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 but 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.
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