Azotobacter RNA Polymerase Transitions and the Release of Sigma

A key insight into the mechanism of RNA synthesis at the level of recognition of the DNA promoter sites (which determines initiation and asymmetry of transcription) has evolved from the findings of Burgess, Travers, Dunn, and Bautz (1969). On adsorbing E. coli RNA polymerase on phosphocellulose, the enzyme dissociated into two components: a minimal enzyme (catalytic unit = E) possessing a limited capacity to transcribe certain restrictive DNA templates (e.g., T4 DNA) and a factor subunit (σ). Addition of the σ factor subunit to the RNA polymerase catalytic unit restored the ability to transcribe T4 DNA. RNA polymerase lacking the σ subunit is considered the catalytic unit since it will synthesize complementary ribopolymers with non-restrictive templates [d(A-T), poly U, calf thymus DNA]. A complementary finding was made with the Azotobacter vinelandii RNA polymerase with the demonstration that the σ subunit (formerly called γ) is released coincident with the onset of...