Templated nucleoside triphosphate binding to a noncatalytic site on RNA polymerase regulates transcription

The regulation of RNA synthesis by RNA polymerase (RNAP) is essential for proper gene expression. Crystal structures of RNAP reveal two channels: the main channel that contains the downstream DNA and a secondary channel that leads directly to the catalytic site. Although nucleoside triphosphates (NTPs) have been seen only in the catalytic site and the secondary channel in these structures, several models of transcription elongation, based on biochemical studies, propose that template-dependent binding of NTPs in the main channel regulates RNA synthesis. These models, however, remain controversial. We used transient state kinetics and a mutant of RNAP to investigate the role of the main channel in regulating nucleotide incorporation. Our data indicate that a NTP specific for the i + 2 template position can bind to a noncatalytic site and increase the rate of RNA synthesis and that the NTP bound to this site can be shuttled directly into the catalytic site. We also identify fork loop 2, which lies across from the downstream DNA, as a functional component of this site. Taken together, our data support the existence of a noncatalytic template-specific NTP binding site in the main channel that is involved in the regulation of nucleotide incorporation. NTP binding to this site could promote high-fidelity processive synthesis under a variety of environmental conditions and allow DNA sequence-mediated regulatory signals to be communicated to the active site.