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Model of the endonuclease active site of the influenza virus RNA polymerase. Amino acids 508, 519, and 522 derive from the PB1 subunit, while amino acids 108, 134, and 510 are from PA. Acidic amino acids in PB1 (E508, E519, and D522) and PA (D108) interact with a divalent metal ion (Mg2+) in the endonuclease active site. Two other amino acids in PA (K134 and H510) are also involved (see the text for details).
