FtsZ rings in mukB mutants with or without the Min system

The site of cell division in Escherichia coli is defined by formation of th e Z ring between the two segregated daughter nucleoids. Positioning of the Z ring, composed of the highly conserved and tubulin-like FtsZ protein. app ears to be negatively regulated by both the nucleoid and the oscillating Mi nCD inhibitor proteins. MukB protein is probably involved in nucleoid conde nsation, and in the absence of MukB, the negative effect of the nucleoid on Z rings appears to be partially suppressed. In this study, we examined the localization of Z rings in cells lacking both the Min system and MukB. In the Delta min Delta mukB double null mutant, essentially all nucleoid-free zones, either at the cell poles or at non-polar sites between nucleoids. co ntained Z rings. However, a significant proportion of Z rings also formed o n top of nucleoids. Interestingly, Z ring clusters often formed at gaps bet ween nucleoids, and some of the rings within the clusters were clearly posi tioned on top of nucleoids. These results provide further evidence that the negative topological effect of nucleoids in cells lacking MukB is partiall y bur not totally suppressed, and that the absence of the Min system allows more promiscuous Z ring formation. (C) 2001 Societe francaise de biochimie et biologie moleculaire / Editions scientifiques et medicales Elsevier SAS .