Escherichia coli responds to the accumulation of misfolded proteins by inducing the transcription of heat shock genes. Eς^E RNA polymerase controls one of the two heat shock regulons of E. coli. This regulon is activated upon accumulation of misfolded polypeptides in the double membrane envelope of E. coli. ς^E (RpoE) is a member of the extracytoplasmic function subfamily of sigma factors. Here we asked how many genes are activated by Eς^E RNA polymerase and what is the identity of these genes. Using two independent genetic approaches, 20 E. coli promoters were identified which activate reporter gene transcription in a ς^E-dependent manner. In all cases examined, a canonical ς^E binding site could be revealed upon mapping transcriptional start sites. 10 identified promoters activated the transcription of previously identified genes with four genes acting directly on the folding of E. coli envelope proteins (dsbC, fkpA, skp, andsurA). The remaining promoters transcribed genes that are presumed to encode hitherto unknown extracytoplasmic functions and were named ecf (ecfA–ecfM). Two of these ecf genes were found to be essential for E. coli growth.

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.

▪ Abstract It is now well established that protein folding requires the assistance of folding helpers in vivo. The formation or isomerization of disulfide bonds in proteins is a slow process requiring catalysis. In nascent polypeptide chains the cysteine residues are in the thiol form. The formation of the disulfide bonds usually occurs simultaneously with the folding of the polypeptide, which means in the endoplasmic reticulum of eukaryotes or in the periplasm of Gram-negative bacteria. In prokaryotes, the existence of redox proteins involved in the formation of disulfide bonds containing proteins has recently been revealed in the periplasm. The discovery of these redox proteins through various genetic approaches will be summarized, as well as the most recent insights regarding their biochemical and biological activities.