The atypical two-subunit σ factor from Bacillus subtilis is regulated by an integral membrane protein and acid stress

Extracytoplasmic function (ECF) σ factors constitute a major component of the physicochemical sensory apparatus in bacteria. Most ECF σ factors are co-expressed with a negative regulator called an anti-σ factor that binds to its cognate σ factor and sequesters it from productive association with core RNA polymerase (RNAP). Anti-σ factors constitute an important element of signal transduction pathways that mediate an appropriate transcriptional response to changing environmental conditions. The Bacillus subtilis genome encodes seven canonical ECF σ factors and six of these are co-expressed with experimentally verified anti-σ factors. B. subtilis also expresses an ECF-like atypical two-subunit σ factor composed of subunits SigO and RsoA that becomes active after exposure to certain cell-wall-acting antibiotics and to growth under acidic conditions. This work describes the identification and preliminary characterization of a protein (RsiO, formerly YvrL) that constitutes the anti-σ factor cognate to SigO-RsoA. Synthesis of RsiO represses SigO-RsoA-dependent transcription initiation by binding the N-terminus of SigO under neutral (pH 7) conditions. Reconstitution of the SigO-RsoA-RsiO regulatory system into a heterologous host reveals that the imposition of acid stress (pH 5.4) abolishes the ability of RsiO to repress SigO-RsoA-dependent transcription and this correlates with loss of RsiO binding affinity for SigO. A current model for RsiO function indicates that RsiO responds, either directly or indirectly, to increased extracytoplasmic hydrogen ion concentration and becomes inactivated. This results in the release of SigO into the cytoplasm, where it productively associates with RsoA and core RNAP to initiate transcription from target promoters in the cell.