Sigma factors are initiation factors that promote the attachment of RNA polymerase to specific initiation sites and are then released. This sigma factor contributes to both stress response and virulence gene expression. Negatively regulated by the anti-sigma-I factor RsgI. Interacts with RsgI. Part of the sigI-rsgI operon, which is induced by heat shock. Expressed from both the sigma I and sigma A promoters. Transcription from the sigma A promoter is likely more significant to virulence regulation. Disruption results in increased sensitivity to heat shock. Loss of SigI results in a coordinated decrease in expression of all three toxin subunits. Belongs to the sigma-70 factor family. SigI subfamily.