Regulation of periodic protein synthesis in the cell cycle: Control of initiation and termination of flagellar gene expression

Abstract

Conditional mutations and drugs that specifically block DNA synthesis and cell division in Caulobacter crescentus have been used to analyze the role of the cell cycle in differentiation. The results suggest that the DNA synthetic pathway, and not an intact cell division pathway, acts to time the periodic synthesis of flagellin A, flagellin B and the hook protein. The expression of these flagellar genes is apparently controlled at the level of transcription, with initiation of the synthetic periods dependent on a specific stage of chromosome replication. The required stage of replication, or DNA execution point, occurs near mid-S phase at approximately 0.65 of the swarmer cell cycle. Examination of a C. crescentus polyhook mutant PCM103 indicates, however, that the DNA synthetic pathway is only one level of regulation over flagellum formation. In the mutant, hook protein synthesis is coupled to DNA synthesis and is initiated at the expected time in the cell cycle, but hook protein synthesis is not repressed at the normal time, hook assembly fails to terminate and only a low level of flagellin is detected. Thus the defect in this strain, which grows and divides normally, appears to affect a sequence of developmental events at a point after the induction of the hook protein. These findings are consistent with a hierarchy of controls that regulates the periodic synthesis of flagellar proteins and their assembly in the flagellum. Initiation of hook protein and flagellin synthesis requires a specific stage of chromosome replication, while later stages of flagellum formation, including the termination of hook protein synthesis, also require a second level of regulation that is defective in strain PCM 103.