Abstract
Bacteria that divide by binary fission form FtsZ rings at the geometric midpoint of the cell between the bulk of the replicated nucleoids. In Bacillus subtilis, the DNA- and membrane-binding Noc protein is thought to participate in nucleoid occlusion by preventing FtsZ rings from forming over the chromosome. To explore the role of Noc, we used time-lapse fluorescence microscopy to monitor FtsZ and the nucleoid of cells growing in microfluidic channels. Our data show that Noc does not prevent de novo FtsZ ring formation over the chromosome nor does Noc control cell division site selection. Instead, Noc corrals FtsZ at the cytokinetic ring and reduces migration of protofilaments over the chromosome to the future site of cell division. Moreover, we show that FtsZ protofilaments travel due to a local reduction in ZapA association, and the diffuse FtsZ rings observed in the Noc mutant can be suppressed by ZapA overexpression. Thus, Noc sterically hinders FtsZ migration away from the Z-ring during cytokinesis and retains FtsZ at the postdivisional polar site for full disassembly by the Min system.IMPORTANCE In bacteria, a condensed structure of FtsZ (Z-ring) recruits cell division machinery at the midcell, and Z-ring formation is discouraged over the chromosome by a poorly understood phenomenon called nucleoid occlusion. In B. subtilis, nucleoid occlusion has been reported to be mediated, at least in part, by the DNA-membrane bridging protein, Noc. Using time-lapse fluorescence microscopy of cells growing in microchannels, we show that Noc neither protects the chromosome from proximal Z-ring formation nor determines the future site of cell division. Rather, Noc plays a corralling role by preventing protofilaments from leaving a Z-ring undergoing cytokinesis and traveling over the nucleoid.
Highlights
Bacteria that divide by binary fission form FtsZ rings at the geometric midpoint of the cell between the bulk of the replicated nucleoids
One of the primary topological determinants of bacterial cell division is the phenomenon of nucleoid occlusion, in which FtsZ rings are prevented from being superimposed with the bulk of the chromosome [9, 10, 13]
In B. subtilis, nucleoid occlusion is thought to be mediated in part by the Noc protein, which, in turn, may directly or indirectly inhibit FtsZ dynamics
Summary
Bacteria that divide by binary fission form FtsZ rings at the geometric midpoint of the cell between the bulk of the replicated nucleoids. In Bacillus subtilis, the DNA- and membrane-binding Noc protein is thought to participate in nucleoid occlusion by preventing FtsZ rings from forming over the chromosome. IMPORTANCE In bacteria, a condensed structure of FtsZ (Z-ring) recruits cell division machinery at the midcell, and Z-ring formation is discouraged over the chromosome by a poorly understood phenomenon called nucleoid occlusion. In B. subtilis, nucleoid occlusion has been reported to be mediated, at least in part, by the DNA-membrane bridging protein, Noc. Using time-lapse fluorescence microscopy of cells growing in microchannels, we show that Noc neither protects the chromosome from proximal Z-ring formation nor determines the future site of cell division. The early cell division scaffolding protein FtsZ forms an intense ring-like structure in a space between the replicated nucleoids and recruits the machinery for cell envelope biosynthesis [5,6,7,8]. Nucleoid occlusion in E. coli seems to be an active mechanism, whereas nucleoid occlusion in B. subtilis is thought to be passive
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