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Paramecium bursaria Chlorella virus PBCV-1 mRNA guanylyl transferase (capping enzyme) has been complexed with an mRNA cap analogue G[5']ppp[5']G and crystallized. The crystals belong to space group C2221 with unit cell dimensions a = 78.4 A, b = 164.1 A, c = 103.3 A, and diffraction data to 3.1 A has been collected by using synchrotron radiation. The structure has been solved by molecular replacement by using each of the two domains in the previously determined structure of the enzyme in complex with GTP. The conformation is open with respect to the active site cleft, and all contacts between enzyme and ligand are mediated by domain 1. One of the guanine bases is bound in the same pocket that is utilized by GTP. The conformation of the ligand positions the beta phosphate and the active site lysine on opposite sides of the alpha phosphate. This geometry is optimal for nucleophilic substitution reactions and has previously been found for GTP in the closed conformational form of the capping enzyme, where the lysine can be guanylylated upon treatment with excess manganese(II) ions. The remainder of the cap analogue runs along the conserved active site Lys82 Thr83 Asp84 Gly85 Ile86 Arg87 motif, and the second guanine, corresponding to the 5' RNA base, is stacked against the hydrophobic Ile86. The ligand displays approximate 2-fold symmetry with intramolecular hydrogen bonding between the 2' and 3' hydroxyls of the two ribose rings.