Influence of the nucleoid on placement of FtsZ and MinE rings in Escherichia coli

We previously presented evidence that replicating but unsegregated nucleoid s, along with the Min system, act as topological inhibitors to restrict ass embly of the FtsZ ring (Z ring) to discrete sites in the cell. To test if n onreplicating nucleoids have similar exclusion effects, we examined Z rings in dnaA (temperature sensitive) mutants. Z rings were excluded from centra lly localized nucleoids and were often observed at nucleoid edges. Cells wi th nonreplicating nucleoids formed filaments, some of which contained large nucleoid-free areas in which Z rings were positioned at regular intervals. Because MinE may protect FtsZ from the action of the MinC inhibitor in the se nucleoid-free zones, we examined the localization of a MinE-green fluore scent protein (GFP) fusion with respect to Z rings and nucleoids. Like Z ri ngs, MinE-GFP appeared to localize independently of nucleoid position, form ing rings at regular intervals in nucleoid-free regions. Unlike FtsZ, howev er, MinE GFP often localized on top of nucleoids, replicating or not, sugge sting that MinE is relatively insensitive to the nucleoid inhibition effect . These data suggest that both replicating and nonreplicating nucleoids are capable of topologically excluding Z rings but not MinE.