The N-terminal two-thirds of the alpha subunit of Escherichia coli RNA polymerase plays an essential role in the initiation of subunit assembly, by gathering two large subunits, beta and beta', together into a core-enzyme complex. One group of RNA polymerase mutants deficient in response to transcription activation carries mutations in the C-terminal region of the alpha subunit, indicating that the C-terminal region of the alpha subunit is involved in protein-protein contact in positive control of transcription. A set of activators (class I transcription factors) which make contact with this contact site I region on RNA polymerase alpha subunit bind in most cases to DNA upstream of the promoter -35 signal. Genetic fine mapping indicates that a cluster of subsites exists in the contact site I region, each interacting with a set of the class I factors and each consisting of a structure formed by only 5-10 amino acid residues.