Most E. coli promoters interact with σ⁷⁰-RNA polymerase, the major initiating form of the bacterial enzyme. The transcription of certain groups of genes, however, is initiated by E. coli RNA polymerases containing one of several alternative sigma factors that recognize different consensus promoter sequences than σ⁷⁰ does (Table 9-1). These alternative σ-factors are required for the transcription of sets of genes with related functions, such as those involved in the response to heat shock or nutrient deprivation, motility, or sporulation in gram-positive eubacteria. In E. coli, there are 6 alternative σ-factors in addition to the major “housekeeping” σ-factor, σ⁷⁰. The genome of the gram-positive, sporulating bacterium Streptomyces coelicolor encodes 63 σ-factors, the current record, based on sequence analysis of hundreds of eubacterial genomes. Most are structurally and functionally related to σ⁷⁰. Transcription initiation by RNA polymerases containing σ⁷⁰-like factors is regulated by repressors and activators that bind to DNA near the region where the polymerase binds. But one class, represented in E. coli by σ⁵⁴, is unrelated to σ⁷⁰ and functions differently.