This is the N-terminal domain of DNA-directed RNA polymerase. This domain has a role in interaction with regions of upstream promoter DNA and the nascent RNA chain, leading to the processivity of the enzyme [1]. In order to make mRNA transcripts the RNA polymerase undergoes a transition from the initiation phase (which only makes short fragments of RNA) to an elongation phase. This domain undergoes a structural change in the transition from initiation to elongation phase. The structural change results in abolition of the promoter binding site, creation of a channel accommodating the heteroduplex in the active site and formation of an exit tunnel which the RNA transcript passes through after peeling off the heteroduplex [2].

1: Structure of T7 RNA polymerase complexed to the transcriptional inhibitor T7 lysozyme. Jeruzalmi D, Steitz TA; EMBO J. 1998;17:4101-4113. PMID:9670025

2: Structural basis for the transition from initiation to elongation transcription in T7 RNA polymerase. Yin YW, Steitz TA; Science. 2002;298:1387-1395. PMID:12242451