CHARACTERIZATION OF CELL-CYCLE EVENTS DURING THE ONSET OF SPORULATIONIN BACILLUS-SUBTILIS

To elucidate the process of asymmetric division during sporulation of Bacillus subtilis, we have measured changes in cell cycle parameters d uring the transition from vegetative growth to sporulation. Because th e propensity of B. subtilis to grow in chains of cells precludes the u se of automated cell-scanning devices, we have developed a fluorescenc e microscopic method for analyzing cell cycle parameters in individual cells. From the results obtained, and measurements of DNA replication fork elongation rates and the escape time of sporulation from the inh ibition of DNA replication, we have derived a detailed time scale for the early morphological events of sporulation which is mainly consiste nt,vith the cell cycle changes expected following nutritional downshif t. The previously postulated sensitive stage in the DNA replication cy cle, beyond which the cell is unable to sporulate without a new cell c ycle, could represent a point in the division cycle at which the starv ed cell cannot avoid attaining the initiation mass for DNA replication and thus embarking on another round of the cell cycle. The final cell cycle event, formation of the asymmetric spore septum, occurs at abou t the time in the cell cycle at which the uninduced cell would have di vided centrally, in keeping with the view that spore septation is a mo dified version of vegetative division.