Model for transcription activation by ^meAda at the ada and aidB promoters. The upper panel illustrates the specific interactions established between RNA polymerase and promoter DNA in the absence of ^meAda; the RNA polymerase-promoter complex results from protein-DNA interactions between αCTD and the UP elements. The lower panel shows the RNA polymerase-promoter-^meAda ternary complex. ^meAda binds to its DNA site via its NTD and stimulates transcription initiation (black arrow) via protein-protein interaction between its own CTD and the CTD of ς⁷⁰. Additional evidence suggests that RNA polymerase may also make DNA contacts farther upstream, most likely by bending the DNA. However, the nature of these contacts and their possible functions remain to be determined.

Model for transcription activation by ^meAda at ada and alkA. At ada, RNA polymerase docks to the promoter region by interactions between the α subunit and DNA, and ^meAda-ς interaction triggers transcription initiation. At alkA, ^meAda interacts with both the α and ς subunits, recruiting RNA polymerase to the promoter. Since the CTD of Ada is dispensable for activation at alkA, the NTD is shown to be involved in both ^meAda-α and ^meAda-ς contacts.