Alternative sigma factors and the regulation of flagellar gene expression

Synthesis of bacterial flagella and the accompanying array of chemotaxis receptors and transducers represents a major commitment of energy and resources for a growing bacterial cell and is subject to numerous levels of regulation. Genes for flagellar and chemotaxis proteins are expressed in a complex transcriptional cascade. This regulatory hierarchy acts to ensure that the highly expressed filament structural protein, flagellin, is synthesized only after a prerequisite set of structural proteins has been expressed and properly assembled. Recent evidence suggests that many bacteria utilize an alternative sigma (sigma) subunit, similar in specificity to the Bacillus subtilis sigma 28 protein, to direct transcription of flagellin, chemotaxis and motility genes. In Caulobacter crescentus and Campylobacter spp., both a sigma 54-like factor and a sigma 28-like factor participate in the transcription of flagellar and chemotaxis genes. Conversely, a sigma 28-like factor controls non-motility functions in at least one non-flagellated organism.