THE ANTI-SIGMA FACTORS

A mechanism for regulating gene expression at the level of transcription utilizes an antagonist of the sigma transcription factor known as the anti-sigma (anti-σ) factor. The cytoplasmic class of anti-σ factors has been well characterized. The class includes AsiA form bacteriophage T4, which inhibits Escherichia coli σ⁷⁰; FlgM, present in both gram-positive and gram-negative bacteria, which inhibits the flagella sigma factor σ²⁸; SpoIIAB, which inhibits the sporulation-specific sigma factors, σ^F and σ^G, of Bacillus subtilis; RbsW of B. subtilis, which inhibits stress response sigma factor σ^B; and DnaK, a general regulator of the heat shock response, which in bacteria inhibits the heat shock sigma factor σ³². In addition to this class of well-characterized cytoplasmic anti-sigma factors, a new class of homologous, inner-membrane–bound anti-σ factors has recently been discovered in a variety of eubacteria. This new class of anti-σ factors regulates the expression of so-called extracytoplasmic functions, and hence is known as the ECF subfamily of anti-sigma factors. The range of cell processes regulated by anti-σ factors is highly varied and includes bacteriophage phage growth, sporulation, stress response, flagellar biosynthesis, pigment production, ion transport, and virulence.