Unveiling translocation intermediates of RNA polymerase.

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Translocation of RNA polymerase (RNAP) is a robust target for regulation of gene expression in prokaryotes and eukaryotes (1⇓–3). During elongation, RNAP frequently encounters a broad range of DNA/RNA conformations (RNA hairpins, curved and cruciform DNA), DNA-binding proteins, DNA lesions, and misincorporation events at the 3′ ends of the RNA. These encounters impede forward translocation, leading to RNAP pausing (3). There are many protein factors that strengthen or weaken pausing by targeting translocation, such as archaeal/eukaryotic Spt5 and bacterial NusG/RfaH (4) or N/Nun proteins of lambdoid phages (3). In metazoa, RNAP II pausing in promoter-proximal regions plays a role in having polymerases in place for a rapid transcription response to environmental stimuli, such as heat-shock or cell differentiation, and maintaining a basal level of gene expression (5, 6). Translocation blocks also initiate RNAP backtracking (7). Backtracking events disengage the 3′ RNA end from the RNAP catalytic site, which stabilizes pausing; this type of event broadly controls gene transcription in eukaryotes (8). Forward translocation of RNAP along DNA has long been regarded as the movement of the RNA–DNA hybrid through the catalytic cleft, which vacates the active center, termed i+1 site, for an NTP binding. In this sense, the mechanism of translocation has been primarily focused on the hybrid movement, with only limited emphasis on the DNA sequences surrounding the hybrid (1, 2, 9). However, several reports indicate that the DNA sequences immediately upstream and downstream from the hybrid regulate translocation (7, 10⇓–12). In PNAS, Silva et al. (13) identify an important component of the translocation mechanism using millisecond molecular dynamics (MD) simulation of translocation of yeast RNAP II: Translocation of the hybrid occurs before entry of the template DNA base to the i+1 site, where it can pair …