Deletion of the min Operon Results in Increased Thermosensitivity of an ftsZ84 Mutant and Abnormal FtsZ Ring Assembly, Placement, and Disassembly

To investigate the interaction between FtsZ and the Min system during cell division ofEscherichia coli , we examined the effects of combining a well-known thermosensitive mutation offtsZ ,ftsZ84 , with ΔminCDE , a deletion of the entiremin locus. Because the Min system is thought to down-regulate Z-ring assembly, the prediction was that removingminCDE might at least partially suppress the thermosensitivity offtsZ84 , which can form colonies below 42°C but not at or above 42°C. Contrary to expectations, the double mutant was significantly more thermosensitive than theftsZ84 single mutant. When shifted to the new lower nonpermissive temperature, the double mutant formed long filaments mostly devoid of Z rings, suggesting a likely cause of the increased thermosensitivity. Interestingly, even at 22°C, many Z rings were missing in the double mutant, and the rings that were present were predominantly at the cell poles. Of these, a large number were present only at one pole. These cells exhibited a higher than expected incidence of polar divisions, with a bias toward the newest pole. Moreover, some cells exhibited dramatically elongated septa that stained for FtsZ, suggesting that the double mutant is defective in Z-ring disassembly, and providing a possible mechanism for the polar bias. Thermoresistant suppressors of the double mutant arose that had modestly increased levels of FtsZ84. These cells also exhibited elongated septa and, in addition, produced a high frequency of branched cells. A thermoresistant suppressor of theftsZ84 single mutant also synthesized more FtsZ84 and produced branched cells. The evidence from this study indicates that removing the Min system exposes and exacerbates the inherent defects of the FtsZ84 protein, resulting in clear septation phenotypes even at low growth temperatures. Increasing levels of FtsZ84 can suppress some, but not all, of these phenotypes.