Alternative sigma factors provide a means of regulating gene expression in response to various extracellular changes. One such class of sigma factors appears to control a variety of functions, including expression of heat-shock genes in Escherichia coli, biosynthesis of alginates and carotenoids in Pseudomonas aeruginosa and Myxococcus xanthus, respectively, iron uptake in E. coli and Pseudomonas spp., nickel and cobalt efflux in Alcaligenes europhus, plant pathogenicity in Pseudomonas syringae and synthesis of outer membrane proteins in Photobacterium sp. strain SS9. Most of these activities deal with extracytoplasmic functions, and such sigmas have been designated as ECF sigma factors. They have also been characterized in Mycobacteria as well as Gram-positive bacteria such as Streptomyces coelicolor and Bacillus subtilus and the archaea Sulpholobus acidocaldarius. ECF factors belong to a subfamily of the sigma 70 class, based on their sequence conservation and function across bacterial species. The promoter consensus sequences recognized by the ECF factors are also highly conserved. In most of the cases, the activity of these factors is modulated by a cognate inner membrane protein that has been shown, both in E. coli and in P. aeruginosa, to act as an anti-sigma activity. This inner membrane protein is presumed to serve as a sensor and signalling molecule, allowing an adaptive response to specific environmental change. Presumably, an on-and-off switch of the anti-sigma activity leads to the release of the sigma factor and thereby to the co-ordinate transcription of the specific regulon it governs.