Copyright C) 1992, American Society for Microbiology Early Caulobacter crescentus Genes fliL and fliM Are Required for

The biogenesis of the Caulobacter crescentus polar flagellum requires the expression of more than 48 genes, which are organized in a regulatory hierarchy. TheflbO locus is near the top of the hierarchy, and consequently strains with mutations in this locus are nonmotile and lack the flagellar basal body complex. In addition to the motility phenotype, mutations in this locus also cause abnormal cell division. Complementing clones restore both motility and normal cell division. Sequence analysis of a complementing subclone revealed that this locus encodes at least two proteins that are homologs of the Salmonella typhimurium and Escherichia coli flagellar proteins FliL and FliM. FliM is thought to be a switch protein and to interface with the flagellum motor. The C. crescentusfliL andfliM genes form an operon that is expressed early in the cell cycle. TnS insertions in the fliM gene prevent the transcription of class II and class III flagellar genes, which are lower in the regulatory hierarchy. The start site of thefliLM operon lies 166 bp from the divergently transcribedflaCBD operon that encodes several basal body genes. Sequence comparison of thefliL transcription start site with those of other class I genes, flaS and flaO, revealed a highly conserved 29-bp sequence in a potential promoter region that differs from &o70 OS4, &V, and cr28 promoter sequences, suggesting that at least three class I genes share a unique 5 ' regulatory region. The developmental program of Caulobacter crescentu