Analysis of the pleiotropic regulation of flagellar and chemotaxis gene expression in Caulobacter crescentus by using plasmid complementation.

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Abstract
The biosynthesis of the single polar flagellum and the proteins that comprise the chemotaxis methylation machinary are temporally and spacially regulated during the C. crescentus cell-division cycle. The genes involved in these processes are widely separated on the chromosome. The region of the chromosome defined flaE mutations contains at least 1 flagellin structural gene and appears to regulate flagellin synthesis and flagellar assembly. The protein product of the adjacent flaY gene was required to regulate the expression of several flagellin proteins and the assembly of a functional flagellum. Each of these genes is also required for the expression of chemotaxis methylation genes known to map elsewhere on the chromosome. In order to study the regulation of these genes, plasmids were constructed that contain an intact flaYe region or deletions in the region of flaY. These plasmids were mated into a wild-type strain and into strains containing various [transposon] Tn5 insertion and deletion mutations and a temperature-sensitive mutation in the flaYE region. The presence of a plasmid containing the flaYE region allowed the mutant strains to swim and to exhibit chemotaxis, to synthesize increased amounts of the flagellins, to methylate their methyl-accepting chemotaxis proteins (MCP) and to regain wild-type levels of methyltranferase activity. Chromosomal deletions that extend beyond the cloned region were not complemented by this plasmid. Plasmids containing small deletions in the flaY region failed to restore to any flaY or flaE mutants the ability to swim or to assemble a flagellar filament. When mated into a wild-type strain, plasmids bearing deletions in the flaY region were recessive. The pleiotropic regulation of flagellin synthesis, assembly and chemotaxis methylation functions exhibited by the flaY and flaE genes suggest that their gene products function in a regulatory hierarchy that controls flagellar and chemotaxis gene expression.

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