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 of Escherichia coli, we examined the effects of combining a well-k nown thermosensitive mutation of ftsZ, ftsZ84, with Delta minCDE, a deletio n of the entire min locus. Because the Min system is thought to down-regula te Z-ring assembly, the prediction was that removing minCDE might at least partially suppress the thermosensitivity of ftsZ84, which can form colonies below 42 degrees C but not at or above 42 degrees C. Contrary to expectati ons, the double mutant was significantly more thermosensitive than the ftsZ 84 single mutant. When shifted to the new lower nonpermissive temperature, the double mutant formed long filaments mostly devoid of Z rings, suggestin g a likely cause of the increased thermosensitivity. Interestingly, even at 22 degrees C, many Z rings were missing in the double mutant, and the ring s 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 fo r FtsZ, suggesting that the double mutant is defective in Z-ring disassembl y, and providing a possible mechanism for the polar bias. Thermoresistant s uppressors of the double mutant arose that had modestly increased levels of FtsZ84. These cells also exhibited elongated septa and, in addition, produ ced a high frequency of branched cells. A thermoresistant suppressor of the ftsZ84 single mutant also synthesized more FtsZ84 and produced branched ce lls. The evidence from this study indicates that removing the Min system ex poses and exacerbates the inherent defects of the FtsZS4 protein, resulting in clear septation phenotypes even at low growth temperatures. Increasing levels of FtsZ84 can suppress some, but not all, of these phenotypes.