Evidence for a pyrimidine‐nucleotide‐specific initiation site (the i site) on Escherichia coli RNA polymerase

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Abstract
Escherichia coli RNA polymerase has two sites, the i and i + 1, for the binding of the first two substrates. The i site is template- and Mg(2+)-independent and purine-nucleotide-specific, whereas the i + 1 site is template- and Mg(2+)-dependent and shows no nucleotide preference. The specificity of the i site for purine nucleotides is well in accord with the fact that most promoters initiate with a purine nucleotide. But there are a few promoters that initiate with a pyrimidine nucleotide. Dinucleotide synthesis at these promoters is completely inhibited by rifampicin. Earlier studies have failed to identify an i site for pyrimidine nucleotides. In this paper, using a fluorescent analog of UTP, namely uridine 5'-[gamma-(5-sulfonic acid)naphthylamidate]-triphosphate, abbreviated as UTP[AmNS], we are able to show its binding to RNA polymerase, with a Kd of 0.8 microM, in the absence of Mg2+ and template. This suggests the presence of an i pyrimidine nucleotide site. The fact that UTP-[AmNS] is capable of initiating RNA synthesis from the i site is further evidenced by the abortive transcription analyses at the lac promoter. Fluorescence titration studies performed in the presence and absence of purine initiator molecules indicate that this site is different from the i purine site. Scatchard analysis of the above data indicates the presence of a single binding site for UTP[AmNS] in the absence of Mg2+. Moreover UTP[AmNS] binds to the core enzyme with a Kd of 3.0 microM implying that, unlike the i purine nucleotide site, the sigma protein confers a tighter binding of UTP-[AmNS] to the low-Kd site. Forster's energy transfer measurements using UTP[AmNS] as the donor and rifampicin as the acceptor have been used for estimation of the distance of the i pyrimidine nucleotide site from the rifampicin site. From these measurements, we infer that there is no direct interference of rifampicin with the first phosphodiester bond between two pyrimidine nucleotides.