Requirement for two copies of RNA polymerase alpha subunit C-terminal domain for synergistic transcription activation at complex promoters

Transcription activation by the Escherichia coli cyclic AMP receptor protein (CRP) at different promoters has been studied using RNA polymerase holoenzyme derivatives containing two full-length alpha subunits, or containing one full-length alpha subunit and one truncated alpha subunit lacking the alpha C-terminal domain (alpha CTD). At a promoter having a single DNA site for CRP, activation requires only one full-length alpha subunit. Likewise, at a promoter having a single DNA site for CRP and one adjacent UP-element subsite (high-affinity DNA site for alpha CTD), activation requires only one full-length alpha subunit. In contrast, at promoters having two DNA sites for CRP, or one DNA site for CRP and two UP-element subsites, activation requires two full-length alpha subunits. We conclude that a single copy of alpha CTD is sufficient to interact with one CRP molecule and one adjacent UP-element subsite, but two copies of alpha CTD are required to interact with two CRP molecules or with one CRP molecule and two UP-element subsites.