sigmah

SigmaH, written as σH, is an alternative sigma factor found in bacteria, most notably in Bacillus subtilis and related Bacillales. Like other sigma factors, σH binds to the core RNA polymerase and directs transcription to a defined subset of promoters, enabling rapid reprogramming of gene expression in response to environmental and physiological cues.

Function: σH is associated with the transition from exponential growth to stationary phase and with certain stress responses. In Bacillus subtilis, the σH regulon comprises genes implicated in DNA repair, protein quality control, and stress tolerance, and it also participates in developmental programs such as competence and early sporulation steps. The exact composition of the σH regulon varies among strains and environmental conditions.

Regulation: σH activity is controlled at multiple levels. Transcription of sigH is responsive to growth phase and stress signals; the σH protein is subject to regulated proteolysis, and its activity can be modulated by anti-sigma factors or other regulatory proteins. These mechanisms ensure that σH-driven transcription is upregulated under nutrient limitation or stress but restrained during rapid growth.

Distribution and evolution: Homologs of σH are found across Gram-positive bacteria, with variations in regulons and regulatory networks. In some species, σH interacts with a broader set of promoters or is integrated with other stress-response pathways.