Expression of an early gene in the flagellar regulatory hierarchy is sensitive to an interruption in DNA replication

Abstract

Genes involved in the biogenesis of the flagellum in Caulobacter crescentus are expressed in a temporal order and are controUed by a trans-acting regulatory hierarchy. Strains with mutations in one of these genes, flaS, cannot transcribe flageUlar structural genes and divide abnormally. This gene was cloned, and it was found that its transcription is initiated early in the cell cycle. Subclones that restored motility toflaS mutants also restored normal cell division. Although transcription offlaS was not dependent on any other known gene in the flagellar hierarchy, it was autoregulated and subject to mild negative control by other genes at the same level of the hierarchy. An additional level of control was revealed when it was found that an interruption of DNA replication caused the inhibition offlaS transcription. TheflaS transcript initiation site was identified, and an apparently unique promoter sequence was found to be highly conserved among the genes at the same level of the hierarchy. The flagellar genes with this conserved 5' region all initiate transcription eariy in the cell cycle and are all sensitive to a disruption in DNA replication. Mutations in these genes also cause an aberrant cell division phenotype. Therefore, flagellar genes at or near the top of the hierarchy may be controlled, in part, by a unique transcription factor and may be responsive to the same DNA replication cues that mediate other cell cycle events, such as cell division.Each Caulobacter crescentus cell division produces morphologically distinct progeny that exhibit different programs of gene expression ( Fig. 1) (14, 23, 40). The progeny stalked cell immediately initiates DNA replication, whereas chromosome replication is delayed in the progeny swarmer cell until its transition to a stalked cell later in the cell cycle (12,34). This transition is marked by the loss of the polar flagellum and initiation of stalk formation at the site previously occupied by the flagellum. Shortly after DNA replication begins, the stalked cell embarks upon the ordered transcription of a group of flagellar genes that culminates in the biogenesis of a new flagellum at the cell pole opposite the stalk.The biogenesis and function of the C. crescentus flagellum require the coordinated expression of at least 48 genes (17). These genes are organized in a trans-acting regulatory hierarchy in which the temporal order of transcription of the structural genes reflects the order of flagellar component assembly (14,23,40). Differential activation of or'4 promoters contributes to the temporal sequence of class II and class III flagellar gene transcription (13,22,24,(37)(38)(39)43) and the localized positioning of flagellar gene products (21,33). An additional level of control was revealed when Newton and coworkers showed that the synthesis of the hook protein and the flagellins is dependent on DNA replication (45)(46)(47)(48)(49)52).

Coupling between gene expression and DNA synthesis early during development in Bacillus subtilis.

“…Conceivably, any of the steps in the phosphotransfer pathway, as well as the stability of one of the phosphorylated proteins could be regulated. It has been proposed that the phospho-relay serves to integrate several different developmental signals that are required for initiation of sporulation (12,13 whether this coupling involves recA and the SOS response or the regulated activity of a transcription factor (69). Late gene expression in bacteriophage T4 requires DNA replication, and the replication fork seems to function directly as a mobile enhancer of transcription initiation (70,71).…”

New members of the ctrA regulon: the major chemotaxis operon in Caulobacter is CtrA dependent

“…Additionally class II flagellar gene promoters fliQp, fliFp and fliLp are sensitive to an interruption in DNA replication (Dingwall et al, 1992b). Like the class II flagellar genes, mcpA and cagA are expressed differentially during the cell cycle ; therefore it is important to ascertain whether replication of the chromosome is a prerequisite for transcription.…”

Caulobacter FliQ and FliR membrane proteins, required for flagellar biogenesis and cell division, belong to a family of virulence factor export proteins

“…Other class II genes encode proteins required for flagellar assembly and function (24, 45a, 65). The order of flagellar gene expression approximates the order of assembly of the gene products into the nascent structure (11,13,17,43,62). In C. crescentus, Salmonella typhimurium, and Escherichia coli, if flagellar assembly is aborted, transcription of the remaining flagellar genes is blocked, suggesting that the two processes are coupled (19,23,24,31).…”

“…We reported previously that a nonmotile mutant with a deletion in the flaS locus not only diminished or abolished the transcription of class III and class IV genes but also exhibited a cell division defect (13). To understand the role of this class II flaS locus in the assembly of the flagellum and in cell divi-FIG.…”

“…The fliQR operon is activated early in the cell cycle (13), and studies were initiated to define the cis-acting sites that might contribute to its temporal regulation. Mutational analysis has revealed that the cis-acting sites that mediate the temporal activation of fliQR transcription reside within 59 bp of the transcription start site.…”