Sigma factors are initiation factors that promote the attachment of RNA polymerase to specific initiation sites and are then released. Positively regulates the expression of the sigY-yxlCDEFG operon upon nitrogen starvation. Also positively regulates ybgB. Positively autoregulated. Induced upon nitrogen starvation. Negatively regulated by both YxlD and YxlE. Belongs to the sigma-70 factor family. ECF subfamily.