TRANSCRIPTIONAL AND MUTATIONAL ANALYSES OF THE RPON OPERON IN CAULOBACTER-CRESCENTUS

The alternative sigma Factor sigma(54) is required for the biogenesis of both the flagellum and the stalk, in Caulobacter crescentus. The DN A sequence downstream of the sigma(54) gene (rpoN) has been determined , revealing three open reading frames (ORFs) encoding peptides of 203, 208, and 159 amino acids. ORF208 and ORF159 are homologous to ORFs fo und downstream of rpoN in other microorganisms. The organization of th is region in C crescentus Is similar to that in other bacteria, with t he exception of an additional ORF, ORF203, immediately downstream from rpoN. There is a single temporally regulated promoter that drives the expression of both rpoN and ORF203. Promoter probe analysis indicates the presence of another promoter downstream from ORF203 which exhibit s a temporal control that is different from that of the rpoN promoter. Mutational analysis was used to address the function of the proteins encoded by these three downstream ORFs. The mutations have no effect o n the transcription of previously known sigma(54)-dependent flagellar promoters except for a slight effect of an ORF159 mutation on transcri ption of fliK.