Many activator proteins generate their positive control of transcription through interactions with the COOH-terminal domain of the Escherichia coli RNA polymerase α subunit. We have examined the participation of this α- domain in transcriptional enhancement and suppression at bacteriophage T4 late promoters. Enhancement is generated by the T4 gene 45 protein, which is the DNA-tracking processivity factor of viral DNA replication; suppression of unenhanced transcription is generated by the RNA polymerase-binding coactivator T4 gene 33 protein. Enhanced and unenhanced transcription by RNA polymerase reconstituted with intact and truncated α subunits and by RNA polymerase containing ADP-ribosylated α has been compared; the internal structures of transcription complexes formed with these RNA polymerases have also been analyzed by footprinting and photocross-linking. Comparison of these structural and functional analyses suggests that enhancement of T4 late transcription by gp45 is not compatible with any significant role of the COOH-terminal domain of the RNA polymerase core a subunit in transcriptional initiation. Suppression of unenhanced T4 late transcription by the gene 33 protein also does not require the COOH-terminal domain of α.