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Figure 7.

Figure 7.

Nucleotide addition cycle. (A) Details of the PB1 catalytic site showing key residues associated with the conserved polymerase motifs A–F and the two catalytic magnesium ions A and B (gray spheres). The primer/product RNA is in blue and the template strand in yellow. An incoming NTP, here a nonhydrolyzable UTP analog (UpNHpp), is shown in the precatalytic state. (B) First precatalytic step in the nucleotide addition cycle with incoming NTP stabilized by stacking on motif B Met409 (same structure as in A). (C) Postcatalytic, pretranslocation step in which the nucleotide has been added onto the product releasing pyrophosphate (PPi). (D) Translocation step, in which the template–product duplex has moved forward one position, bringing the next unpaired template base opposite the empty NTP binding site. Translocation may be assisted by flipping of the motif B loop into an alternative configuration.