The flagellar-specific transcription factor, σ28, is the Type III secretion chaperone for the flagellar-specific anti-σ28 factor FlgM

Abstract

The σ²⁸ protein is a member of the bacterial σ⁷⁰-family of transcription factors that directs RNA polymerase to flagellar late (class 3) promoters. The σ²⁸ protein is regulated in response to flagellar assembly by the anti-σ²⁸ factor FlgM. FlgM inhibits σ²⁸-dependent transcription of genes whose products are needed late in assembly until the flagellar basal motor structure, the hook-basal body (HBB), is constructed. A second function for the σ²⁸ transcription factor has been discovered: σ²⁸ facilitates the secretion of FlgM through the HBB, acting as the FlgM Type III secretion chaperone. Transcription-specific mutants in σ²⁸ were isolated that remained competent for FlgM-facilitated secretion separating the transcription and secretion-facilitation activities of σ ²⁸. Conversely, we also describe the isolation of mutants in σ²⁸ that are specific for FlgM-facilitated secretion. The data demonstrate that σ²⁸ is the Type III secretion chaperone for its own anti-sigma factor FlgM. Thus, a novel role for a σ⁷⁰-family transcription factor is described.