Noc corrals migration of FtsZ protofilaments during cytokinesis inBacillus subtilis

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 B. subtilis, the DNA- and membrane-binding Noc protein is thought to mediate nucleoid occlusion to prevent 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 FtsZ formation over the chromosome or control cell division site selection. Instead, Noc inhibits migration of FtsZ protofilaments from one FtsZ structure to the next. Moreover, we show that FtsZ protofilaments travel due to a local reduction in ZapA association, and the Noc mutant phenotype can be suppressed by ZapA overexpression. Thus, Noc maintains a high local concentration of FtsZ to stabilize FtsZ rings during cytokinesis.IMPORTANCEIn bacteria, a condensed structure of FtsZ (Z-ring) recruits cell division machinery, and Z-ring formation is inhibited over the chromosome by a poorly understood phenomenon called nucleoid occlusion. In B. subtilis, nucleoid occlusion has been reported to be mediated 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.

The division defect of aBacillus subtilis minD nocdouble mutant can be suppressed by Spx-dependent and Spx-independent mechanisms

“…B. subtilis divides by plate formation (septation) with subsequent cell separation, and thus, cell division events were conservatively defined as a local 20% decrease in mRFPmars cell body intensity. 51,65 Prior to IPTG washout, cells of the minD noc double mutant induced for noc grew in a manner similar to that reported for a minD mutant alone such that cells were elongated, and frequent minicells were observed (Fig 2A). 51 Over the next 5-6 hours, the cells grew but gradually stopped dividing until the entire population became filamentous and ultimately lysed (Fig 2B ) (Movie S1).…”

“…Data from tracking the four events were then used to calculate various parameters of cell division. 18,51,65 Each of the cell division parameters in the minD yjbH mutant were similar to that reported for minD (Fig 6C…”

“…62 Recent microfluidic analysis indicated that the decondensed FtsZ filaments originated as a subpopulation of FtsZ that departed the cytokinetic ring and migrated along the membrane to the future site of cell division. 65 Thus, multiple points of Noc recruitment of DNA to the membrane is thought to act as a passive palisade that occludes FtsZ migration over the chromosome and thereby corrals FtsZ between nucleoids. Cells mutated for the Noc system do not suffer a growth rate defect but Noc is nonetheless important for restricting FtsZ migration and concentrating FtsZ to improve division efficiency.…”

CTP regulates membrane-binding activity of the nucleoid occlusion protein Noc

“…The nucleoid occlusion protein Noc increases cell division efficiency 22 by directing the division machinery towards mid-cell either by inhibiting FtsZ formation over the nucleoid 18,21 and/or by concentrating FtsZ in the vicinity of a pre-existing mid-cell Z-ring 23 . The extensive Noc-mediated DNA-membrane interaction is at the heart of both models for nucleoid occlusion 21,23 . In this study, we show that CTP regulates the nucleoid occlusion activity of Noc.…”

“…In Firmicutes, the nucleoid occlusion protein Noc is a paralog of ParB [15][16][17] ; however, the role of Noc is different from that of a canonical ParB [18][19][20] . Noc helps to direct the assembly of the cell division machinery towards the middle of a dividing cell where the concentration of chromosomal DNA (the nucleoid) is the least, thus ensuring a binary cell division 18,[21][22][23] . Noc does so by nucleating on 16-bp NBS (Nocbinding site) sites scattered around the chromosome before spreading to neighboring DNA to form large Noc-DNA nucleoprotein complexes 24 .…”