Escherichia coli fliAZY operon.

We have cloned the Escherichia coli fliAZY operon, which contains the fliA gene (the alternative sigma factor sigma F) and two novel genes, fliZ and fliY. Transcriptional mapping of this operon shows two start sites, one of which is preceded by a canonical E sigma F-dependent consensus and is dependent on sigma F for expression in vivo and in vitro. We have overexpressed and purified sigma F and demonstrated that it can direct core polymerase to E sigma F-dependent promoters. FliZ and FliY are not required for motility but may regulate sigma F activity, perhaps in response to a putative cell density signal that may be detected by FliY, a member of the bacterial extracellular solute-binding protein family 3.