Under stress conditions, alternative o factors to o 70 with different promoter specificities are induced, resulting in the expression of specialty regulons (a system in which two or more structural genes are subject to coordinated regulation by a common regulator molecule) in response. to a variety of stresses. In this manner, gene expression is modified by different sigma factors. In E. coli and other enteric bacteria, o S (RpoS) is the master regulator of the general stress response (Small and others 1994; Hengge-Aronis 1996). The sigma factors of E. coli are listed in Table 1.