Timed action of the gene products required for septum formation in the cell cycle of Bacillus subtilis.

Four isogenic strains of temperature-sensitive septationless mutants, whose mutations are located on different genes, were used to study the periods of action of the gene products required for the initiation of septum formation during the cell cycle of Bacillus subtilis. The shift-up experiments, in which portions of a synchronous culture of each mutant were transferred to the nonpermissive temperature, showed that the transition point, at which cells attained the ability to divide at the nonpermissive temperature in the cell cycle, was strain specific. Furthermore, the heat shock experiments, in which portions of a synchronous culture were subjected to the nonpermissive temperature before the transition point for a fixed period and shifted back to the permissive temperature, showed that the time interval between the shift-back and the subsequent cell division was specific to each strain but was independent of the age of heat shock. These results led us to the idea that the initiation of septum formation in B. subtilis requires the timed action of the four gene products, each of which functions at a specific stage in the cell cycle. In addition, the result with DNA elongation mutant MK-526, which is also septation defective, supported our previous findings that the initiation of septum formation requires the termination of DNA replication in the previous cell cycle.