Temporal regulation and overlap organization of two Caulobacter flagellar genes

The biogenesis of the bacterial flagellum and chemotaxis apparatus in both Escherichia coli and Caulobacter crescentus requires the ordered expression of over 40 genes whose expression is controlled by a trans-acting regulatory hierarchy. In C. crescentus, additional control mechanisms ensure that the transcription of these genes is initiated at the correct time in the cell cycle. We demonstrate here that two flagellar genes, flaE and fla Y, whose products function in trans to modulate the level of transcription of other flagellar genes, are themselves temporally controlled. DNA sequence analysis of the 3413 base-pairs encompassing the flaE and fla Y coding sequences and the 5′ regulatory region showed that flaE encodes a protein of 16,000 M_r and fla Y a, protein of 17,000 M_r. Evidence that flaE and fla Y are transcribed as a polycistronic message includes (1) the polar effect of Tn5 insertions; (2) deletion analysis showing that the flaE promoter is essential for complementation of both flaE and fla Y alleles; and (3) nuclease S₁ assays showing protection of a transcript spanning both genes. The transcript start site in front of flaE was determined and the -10 region conforms to the E. coli sigma 28 promoter consensus sequence. Nuclease S₁ analysis also revealed a protected fragment whose size was consistent with a transcript initiating in vivo at a consensus "nif" promoter sequence in front of the fla Y gene. The entire promoter region and an upstream consensus sequence that might be a regulatory element for the fla Y gene lies within the carboxyl-terminal coding sequence of the flaE gene.