THE ANTI-SIGMA FACTORS

A mechanism for regulating gene expression at the level of transcripti on utilizes an antagonist of the sigma transcription factor known as t he anti-sigma (anti-sigma) factor. The cytoplasmic class of anti-sigma factors has been well characterized. The class includes AsiA form bac teriophage T4, which inhibits Escherichia coil sigma(70); FlgM, presen t in both gram-positive and gram-negative bacteria, which inhibits the flagella sigma factor sigma(28); SpoIIAB, which inhibits the sporulat ion-specific sigma factors, sigma(F) and sigma(G), of Bacillus subtili s; RbsW of B. subtilis, which inhibits stress response sigma factor si gma(B); and DnaK, a general regulator of the heat shock response, whic h in bacteria inhibits the heat shock sigma factor sigma(32). In addit ion to this class of well-characterized cytoplasmic anti-sigma factors , a new class of homologous, inner-membrane-bound anti-sigma factors h as recently been discovered in a variety of eubacteria. This new class of anti-sigma factors regulates the expression of so-called extracyto plasmic functions, and hence is known as the ECF subfamily of anti-sig ma factors. The range of cell processes regulated by anti-sigma factor s is highly varied and includes bacteriophage phage growth, sporulatio n, stress response, flagellar biosynthesis, pigment production, ion tr ansport, and virulence.