Sigma factors are initiation factors that promote the attachment of RNA polymerase to specific initiation sites and are then released. Together with its coactivator RsoA, positively regulates the expression of at least three operons, including oxdC-yvrL, sigO-rsoA and yvrJ. Required for the acid stress-dependent induction of the oxalate decarboxylase oxdC. Interacts with RNA polymerase. In the mid-exponential phase of growth under acidic conditions. Positively autoregulated, in the presence of RsoA. Down-regulated by YvrL under nonstress conditions. Belongs to the sigma-70 factor family.