FtsZ rings in mukB mutants with or without the Min system

The site of cell division in Escherichia coli is defined by formation of the Z ring between the two segregated daughter nucleoids. Positioning of the Z ring, composed of the highly conserved and tubulin-like FtsZ protein, appears to be negatively regulated by both the nucleoid and the oscillating MinCD inhibitor proteins. MukB protein is probably involved in nucleoid condensation, 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 Deltamin DeltamukB double null mutant, essentially all nucleoid-free zones, either at the cell poles or at non-polar sites between nucleoids, contained Z rings. However, a significant proportion of Z rings also formed on top of nucleoids. Interestingly, Z ring clusters often formed at gaps between nucleoids, and some of the rings within the clusters were clearly positioned on top of nucleoids. These results provide further evidence that the negative topological effect of nucleoids in cells lacking MukB is partially but not totally suppressed, and that the absence of the Min system allows more promiscuous Z ring formation.