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. Author manuscript; available in PMC: 2020 Sep 20.
Published in final edited form as: J Mol Biol. 2019 May 31;431(20):3975–4006. doi: 10.1016/j.jmb.2019.05.042

Fig. 1.

Fig. 1.

The nucleotide addition and proofreading cycle. In the TEC, the RNA:DNA hybrid is separated from the downstream DNA, a duplex of the template (t; black) and non-template (nt; blue) DNA strands, by a 90° bend near the active site (indicated by the position of the catalytic MG1 ion) and the β’ bridge helix (BH; orange). The substrate NTP complexed with the low-affinity MG2 ion binds to the post-translocated TEC (❶) to form an inactive, preinsertion intermediate (❷). Upon folding of the tip of the β’ trigger loop (TL; cyan) into the trigger helices (TH), the NTP is repositioned to form the catalytically-competent insertion TEC, in which the active site is closed (❸). During catalysis, a cognate NMP is added to the RNA 3’ end (❹). The release of pyrophosphate (PPi) and the reverse TH→TL transition reset the cycle, with the transcript extended by one nucleotide. If an incorrect NMP is added to the growing chain (❺), RNAP backtracks by one nucleotide and removes two nucleotides from the mismatched 3’ terminus to restore the 3’ end in the active site.