Switches and Signal Transduction Networks in theCaulobacter crescentusCell Cycle

Abstract

Cell differentiation in the gram-negative bacterium Caulobacter crescentus results from asymmetric cell division that produces two morphologically distinct progeny, a nonmotile stalked cell and a motile swarmer cell. Two examples are considered in this chapter. In the first, evidence is discussed that developmental events are coupled to the cell division cycle by a complex signal transduction pathway mediated by sensor histidine kinases and effector proteins. In the second, the role of the response regulator FlbD is examined in flagellum biosynthesis, where it functions as both a transcription activator and repressor to regulate the timing of flagellar (fla) gene expression in the cell cycle. As discussed in the second part of this chapter, there is also experimental evidence that DNA synthesis is required for initiation of the fla gene transcription cascade. Pseudoreversion analysis of a temperature sensitive pleC mutation identified cold-sensitive suppressors that map to three new cell division genes, divJ, divK, and divL. DNA sequence analysis of divJ and pleC show that both genes encode proteins with carboxyterminal domains homologous to the histidine kinases of the bacterial sensor proteins. It is a known fact that that flagellum biosynthesis, activation of motility, and pili formation require the completion of successive cell division cycle checkpoints. The nature of the regulated target genes in flagellum biosynthesis is much better understood, but nothing is known of the class I genes that respond to the cell cycle signal and initiate the fla gene cascade.