Now employed as Assistant Professor and Co-Director, Biochemical Genetics Laboratory, Baylor College of Medicine, Houston, TX
Selection of division sites and coordination of cytokinesis with other cell cycle events are critical for every organism to proliferate. In E.coli, the nucleoid is proposed to exclude division from the site of the chromosome (nucleoid occlusion model). To study the effect of the nucleoid on timing and placement of cell division, we use localization of an early division protein, FtsZ, as a marker for formation of the division septum. The dynamics and quick assembly of FtsZ at potential division sites were observed in live cells in real time with a green fluorescent protein (GFP) fusion of FtsZ. FtsZ polymers or aggregates, probably representing the FtsZ depolymerization and immature FtsZ nucleation process, were also observed. We next examined whether the nucleoid inhibits division. We employed mutants in which chromosomes were trapped at the midcell division site. In a chromosomal partition mutant, multiple replicating chromosomes cannot segregate. FtsZ was excluded from midcell to the edge of the nucleoid. Interestingly, in anucleate cells produced from off-centered division, the FtsZ ring was localized in the central part of the cell, which indicates that the nucleoid is not required for either the division site maturation or FtsZ assembly. The negative effect of nucleoids was further confirmed in replication deficient dnaA mutants, in which only a single chromosome is present in the cell center. Our results suggest that the nucleoid, replicating or not, inhibits division in the area where the chromosome occupies. Increasing the level of FtsZ does not overcome the inhibition. Relaxation of chromosomes by reducing the gyrase activity or disruption of protein translation/translocation did not abort the division inhibition capacity of the nucleoid. However, preventing transcription did compromise the nucleoid occlusion effect, leading to formation of multiple FtsZ rings above the nucleoid. In summary, we demonstrate that FtsZ can assemble in the absence of the nucleoid. Nucleoids regulate the timing and position of division by inhibiting FtsZ assembly at unselected sites. Relief of this inhibition at midcell is coincident with the completion of DNA replication.
Temporal and Spatial Regulation of Cell Division by the Nucleoid in Escherichia coli