Transcriptional control of flagellar genes in Escherichia coli K-12.

Autoregulation of the expression of flagellar genes was investigated by the technique of operon fusion. The results suggested that the flaU gene is a repressor and the flaD gene is an activator of transcription of the hag, flaS, and Mocha operons. The action of the putative flaU repressor appears to be masked by its interaction with other flagellar proteins during assembly; thus, repression is apparent only when the interacting proteins are absent. This hypothesis is supported by the phenotype of an unusual flaU mutant, which represses even though it is unable to promote flagellar assembly. Presumably, the mutant synthesizes a repressor whose activity is no longer masked by interaction with other flagellar proteins.