The division defect of a Bacillus subtilis minD noc double mutant can be suppressed by Spx-dependent and Spx-independent mechanisms

During growth, bacteria increase in size and divide. Division is initiated by the formation of the Z-ring, an intense ring-like cytoskeletal structure formed by treadmilling protofilaments of the tubulin homolog FtsZ. FtsZ localization is thought to be controlled by the Min and Noc systems, and here, we explore why cell division fails at high temperature when the Min and Noc systems are simultaneously mutated. Microfluidic analysis of a minD noc double mutant indicated that FtsZ formed proto-Z-rings at periodic inter-chromosome locations but that the rings failed to mature and become functional. Extragenic suppressor analysis indicated that a variety of mutations restored high temperature growth to the minD noc double mutant, and while many were likely pleiotropic, others implicated the proteolysis of the transcription factor Spx. Further analysis indicated that a Spx-dependent pathway activated the expression of ZapA, a protein that primarily compensates for the absence of Noc. Additionally, an Spx-independent pathway increased the activity of the divisome to reduce the length of the cytokinetic period. Finally, we provide evidence of an as-yet-unidentified protein that is activated by Spx and governs the frequency of polar division and minicell formation.IMPORTANCEBacteria must properly position the location of the cell division machinery in order to grow, divide, and ensure each daughter cell receives one copy of the chromosome. In B. subtilis, cell division site selection is thought to depend on two systems called Min and Noc, and while neither is individually essential, cells fail to grow at high temperature when both are mutated. Here, we show that cell division fails in the absence of Min and Noc, not due to a defect in FtsZ localization, but rather a failure in the maturation of the cell division machinery. To understand what happens when the division machinery fails to mature, suppressor mutations that bypass the need for Min, Noc, or both were selected. Some of the mutants activated the Spx stress response pathway while others appeared to directly enhance divisome activity.