CELL-CYCLE ARREST OF A CAULOBACTER-CRESCENTUS SECA MUTANT

Cell differentiation is an inherent component of the Caulobacter cresc entus cell cycle. The transition of a swarmer cell, with a single pola r flagellum, into a sessile stalked cell includes several morphogeneti c events. These include the release of the flagellum and pill, the pro teolysis of chemotaxis proteins, the biogenesis of the polar stalk, an d the initiation of DNA replication. We have isolated a group of tempe rature-sensitive mutants that are unable to complete this process at t he restrictive temperature. We show here that one of these strains has a mutation in a homolog of the Escherichia coli secA gene, whose prod uct is involved in protein translocation at the cell membrane. This C. crescentus secA mutant has allowed the identification of morphogeneti c events in the swarmer-to-stalked cell transition that require SecA-d ependent protein translocation. Upon shift to the nonpermissive temper ature, the mutant secA swarmer cell is able to release the polar flage llum, degrade chemoreceptors, and initiate DNA replication, but it is unable to form a stalk, complete DNA replication, or carry out cell di vision. At the nonpermissive temperature, the cell cycle blocks prior to the de novo synthesis of flagella and chemotaxis proteins that norm ally occurs in the predivisional cell. Although interactions between t he chromosome and the cytoplasmic membrane are believed to be a functi onal component of the temporal regulation of DNA replication, the abil ity of this secA mutant to initiate replication at the nonpermissive t emperature suggests that SecA dependent events are not involved in thi s process. However, both cell division and stalk formation, which is a nalogous to a polar division event, require SecA function.