The Gram-negative cell envelope is composed of outer and inner membranes surrounding a semirigid peptidoglycan (PG) wall. The wall protects cells from osmotic lysis and maintains cell shape, but it must be modified for cells to grow and divide. Thus, the PG is synthesized and modified by penicillin binding proteins (PBPs) and hydrolyzed by amidases. Here, we investigate how PG modifications induce the Rcs and Cpx stress responses in Escherichia coli and how the Tol-Pal complex monitors PG integrity during division.
We first describe an E. coli mutant lacking PBPs 4, 5, 7, and AmpH (strain CS448) that was nonmotile. A wild-type copy of any one of these PBPs restored motility. Surprisingly, we found that motility was inhibited because the Rcs and Cpx stress responses were induced in the absence of these four PBPs. These results imply that a specific cell wall fragment may be the stress signal that induces these pathways.
To further characterize these Rcs- or Cpx-inducing signals, we determined the muropeptide composition of CS448. We found a significant decrease in muropeptides modified by Lpp. Cells lacking Lpp induced the Rcs phosphorelay, and this was reversed by overproducing the outer membrane lipoprotein Pal. While Rcs induction depended partially on the absence of bound-form Lpp, our results suggest that free-form Lpp is also required to maintain envelope integrity and may be part of the stress signal.
The synthesis and degradation of septal PG during division is regulated carefully in Gram-negative bacteria, and perturbing this process can cause lysis. One regulator, the Tol-Pal complex, helps coordinate PG and outer membrane invagination during division. We report that an E. coli mutant lacking components of the Tol-Pal system lysed rapidly when exposed to the PBP3-specific β-lactam antibiotic, aztreonam. Lysis depended on divisome assembly and required the presence of inactive PBP3. The lethal event was mediated by the PG amidase AmiA or by AmiB and AmiC. Aztreonam also triggered lysis in ybgC, cpoB, or lpp mutants, even though these proteins are not considered part of the Tol-Pal complex. The results suggest that all of these elements influence PG degradation during cell division.
|Commitee:||Forrest, Craig, Kelly, Thomas, Morrison, Richard, Voth, Daniel|
|School:||University of Arkansas for Medical Sciences|
|Department:||Microbiology and Immunology|
|School Location:||United States -- Arkansas|
|Source:||DAI-B 77/11(E), Dissertation Abstracts International|
|Keywords:||Cell division, Cpx pathway, Escherichia coli, Penicillin binding protein, Rcs phosphorelay, Tol-pal complex|
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