Journal of Bacteriology, October 2001, p. 6046-6053, Vol. 183, No. 20
0021-9193/01/$04.00+0 DOI: 10.1128/JB.183.20.6046-6053.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.
Department of Biology, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061,1 and Department of Microbiology and Immunology, Loyola University Medical Center, Maywood, Illinois 601532
Received 14 June 2001/Accepted 19 July 2001
The four class A penicillin-binding proteins (PBPs) of
Bacillus subtilis appear to play functionally redundant
roles in polymerizing the peptidoglycan (PG) strands of the
vegetative-cell and spore walls. The ywhE product was
shown to bind penicillin, so the gene and gene product were renamed
pbpG and PBP2d, respectively. Construction of mutant
strains lacking multiple class A PBPs revealed that, while PBP2d plays
no obvious role in vegetative-wall synthesis, it does play a role in
spore PG synthesis. A pbpG null mutant produced spore PG
structurally similar to that of the wild type; however, electron
microscopy revealed that in a significant number of these spores the PG
did not completely surround the spore core. In a pbpF
pbpG double mutant this spore PG defect was apparent in every
spore produced, indicating that these two gene products play partially
redundant roles. A normal amount of spore PG was produced in the double
mutant, but it was frequently produced in large masses on either side
of the forespore. The double-mutant spore PG had structural alterations
indicative of improper cortex PG synthesis, including twofold decreases
in production of muramic
-lactam and L-alanine side
chains and a slight increase in cross-linking. Sporulation gene
expression in the pbpF pbpG double mutant was normal,
but the double-mutant spores failed to reach dormancy and subsequently
degraded their spore PG. We suggest that these two
forespore-synthesized PBPs are required for synthesis of the spore germ
cell wall, the first layer of spore PG synthesized on the surface of
the inner forespore membrane, and that in the absence of the germ cell
wall the cells lack a template needed for proper synthesis of the spore
cortex, the outer layers of spore PG, by proteins on the outer
forespore membrane.
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