
Full text loading...
Category: Bacterial Pathogenesis; Microbial Genetics and Molecular Biology
Cell Division, Page 1 of 2
< Previous page | Next page > /docserver/preview/fulltext/10.1128/9781555815806/9781555813987_Chap10-1.gif /docserver/preview/fulltext/10.1128/9781555815806/9781555813987_Chap10-2.gifAbstract:
In Escherichia coli, septum assembly occurs at the midcell and involves coordinated inward growth of all three layers of the cell envelope—the cytoplasmic membrane, the peptidoglycan wall, and the outer membrane. How the division septum is assembled and how its assembly is coordinated with other events in the cell cycle, such as chromosome segregation, are the topics of this chapter. One situation is that regulation ensures that the Z ring assembles in the right place and at the right time in normally cycling cells—this sort of regulation is widely conserved among different bacterial species. Similarly, however, one could argue that FtsA too might function indirectly in recruitment as FtsA is not essential in B. subtilis and the normal requirement for FtsA in septal ring assembly in E. coli can be largely bypassed by artificially tethering FtsQ to the septal ring. Penicillin-binding protein 3 (PBP3) localizes to the septal ring, where it cross-links septal peptidoglycan and recruits FtsN. MgtA, in contrast, is a monofunctional transglycosylase. Curiously, overproduction of FtsN also rescues cell division in ftsK(Ts), ftsQ(Ts), ftsI(Ts), and ftsEX depletion strains too. The outer membrane is considered to invaginate simultaneously with the rest of the cell envelope in E. coli.
Full text loading...
Regulation of Z-ring assembly. The Min system and nucleoid occlusion (NO) work together to ensure that the only permissive site for Z-ring assembly is the gap that opens up at the midcell as nucleoids segregate late in the cell cycle. (Top) In a newborn cell, nucleoid occlusion prevents the Z ring from assembling at the midcell, while the Min system prevents Z rings from forming in the DNA-free region near either pole. Nucleoid occlusion in E. coli is mediated by a DNA-binding protein named SlmA. The MinCD inhibitor complex oscillates from pole to pole under control of MinE, which is concentrated near the midcell. See text for details. (Bottom) Chromosome segregation relieves the midcell of nucleoid occlusion. MinCD continues to oscillate, but its inhibitory effects do not reach as far as the midcell because MinE limits growth of MinD polymers.
Models for assembly of the septal ring. (Top) The order of assembly of proteins into the septal ring as determined by localization dependency. First, FtsZ forms the Z ring. FtsA and ZipA join next, independently of one another. Once both FtsA and ZipA have localized, the remaining proteins join the ring in the order indicated such that localization of each depends upon localization of the proteins to the left of it. ZapA and EnvC are omitted because their dependency relationships have not been established, but since neither is essential, none of the proteins shown are likely to depend on ZapA or EnvC for recruitment. Dependence of FtsK and other late proteins on FtsEX is leaky. (Bottom) Bacterial two-hybrid assays have identified a network of interactions among the division proteins. Lines connect proteins reported to interact in at least one assay, while circular arrows indicate self-interaction (e.g., dimerization). Adapted from Current Biology 15:R514—R526 ( Goehring and Beckwith, 2005 ), copyright 2005, with permission from Elsevier with inclusion of additional data (Arends and Weiss, unpublished).
Septal targeting domains in membrane proteins involved in cell division in E. coli. For clarity, proteins are not drawn to scale. Regions sufficient for septal localization are shown in black, while those known to be dispensable are white. Grey indicates a protein has not been studied in this regard. In many cases only a limited number of constructs have been characterized, so the targeting domains might be smaller than indicated. Note that targeting information can reside in any domain (cytoplasmic, transmembrane, or periplasmic) and there is no evidence that a targeting motif is shared among different proteins. These findings are consistent with the notion that division proteins localize by a cascade of protein-protein interactions rather than by binding to a common target. Because FtsL must associate with FtsB to localize to the septal ring, the targeting regions identified in FtsL probably mediate complex formation with FtsB rather than septal localization per se. Adapted with permission from the Journal of Bacteriology (Wissel et al. [2004]) with inclusion of new data from our lab (Arends and Weiss, unpublished).
Proteins found in the septal ring of E. coli
Proteins that regulate cell division in E. coli a