Abstract
Caulobacter crescentus contains a 25- and a 27-kDa flagellin, which are assembled into the flagellar filament, and a 29-kDa flagellin, which is related in sequence but is of unknown function. We have used DNA sequence analysis and nuclease S1 assays to map the in vivo transcription start sites of the three flagellin genes and to study their regulation. These experiments lead to several conclusions. First, copies of the 29-, 25-, and 27-kDa flagellin genes are organized in a tandem array in the flaEY gene cluster of C. crescentus. Second, flagellin genes are under transcriptional control and each gene is expressed with a characteristic periodicity in the cell cycle. Third, flagellin gene promoters contain conserved nucleotide sequence elements at -13, -24, and -100 that are homologous to the fla genes in the hook gene cluster. The -13 and -24 sequences conform to a fla gene promoter consensus sequence (C/TTGGCC/GC-N5-TTGC) that is similar in sequence to the -12, -24 consensus sequence of the Klebsiella pneumonia nif gene promoters. Fourth, the sequence element at approximately -100 in the 25- and the 27-kDa flagellin genes is homologous to a 19-base-pair sequence [designated previously as II-1; see Chen, L.-S., Mullin, D. M. & Newton, A. (1986) Proc. Natl. Acad. Sci. USA 83, 2860-2864]at -101 in the promoter of transcription unit II of the hook gene cluster; the two flagellin genes, like the fla genes examined in the hook gene cluster that contain the -100 element, are under positive control by transcription unit III of the hook gene cluster. This result supports a model in which the timing of fla gene transcription in the C. crescentus cell cycle is determined in part by a cascade of trans-acting regulatory gene products.