A GENE CODING FOR A PUTATIVE SIGMA-54 ACTIVATOR IS DEVELOPMENTALLY-REGULATED IN CAULOBACTER-CRESCENTUS

In Caulobacter crescentus, the alternative sigma factor sigma(54) play s an important role in the expression of late flagellar genes. sigma(5 4)-dependent genes are temporally and spatially controlled, being expr essed only in the swarmer pole of the predivisional cell. The only sig ma(54) activator described so far is the FlbD protein, which is involv ed in activation of the class III and IV flagellar genes and repressio n of the fliF promoter. To identify new roles for sigma(54) in the met abolism and differentiation of C. crescentus, we cloned and characteri zed a gene encoding a putative sigma(54) activator, named tacA. The de duced amino acid sequence from tacA has high similarity to the protein s from the NtrC family of transcriptional activators, including the as partate residues that are phosphorylated by histidine kinases in other activators. The promoter region of the tacA gene contains a conserved sequence element present in the promoters of class II flagellar genes , and tacA shows a temporal pattern of expression similar to the patte rns of these genes. We constructed an insertional mutant that is disru pted in tacA (strain SP2016), and an analysis of this strain showed th at it has all polar structures, such as pill, stalk, and flagellum, an d displays a motile phenotype, indicating that tacA is not involved in the flagellar biogenesis pathway. However, this strain has a high per centage of filamentous cells and shows a clear-plaque phenotype when i nfected with phage phi Cb5. These results suggest that the TacA protei n could mediate the effect of sigma(54) on a different pathway in C. c rescentus.