The sigma factors of Mycobacterium tuberculosis: regulation of the regulators.

One of the important determinants of virulence of Mycobacterium tuberculosis is adaptation to adverse conditions encountered in the host cells. The ability of Mycobacterium to successfully adapt to stress conditions is brought about by the expression of specific regulons effected by a repertoire of sigma factors. The induction and availability of sigma factors in response to specific stimuli is governed by a complex regulatory network comprising a number of proteins, including sigma factors themselves. A serine-threonine protein kinase-mediated signaling pathway adds another dimension to the mycobacterial sigma factor regulatory network. This review highlights the recent advances in understanding mycobacterial sigma factors, their regulation and contribution to bacterial pathogenesis.