Sigma factors are initiation factors that promote the attachment of RNA polymerase (RNAP) to specific initiation sites and are then released. May be involved in the regulation of iron metabolism. Associates with RNAP core during early growth phases, association decreases as cells age (PubMed:21710567). Interacts transiently with the RNAP core. Induced 3-fold by the beta-lactam antibiotic cefuroxime. No effect on antibiotic resistance; double sigM-sigX mutants have 130-fold increases sensitivity to the beta-lactam antibiotic cefuroxime (PubMed:22211522). Association with RNAP core increases during many stresses (at protein level) (PubMed:21710567). Belongs to the sigma-70 factor family. ECF subfamily.