STRUCTURE OF THE CRYSTALLINE COMPLEX OF DEOXYCYTIDYLYL-3',5'-GUANOSINE (3',5'-DCPDG) COCRYSTALLIZED WITH RIBONUCLEASE AT 1.9-ANGSTROM RESOLUTION

X-ray structure of bovine ribonuclease A cocrystallized with the dinuc leotide deoxycytidylyl-3',5'-guanosine has been determined at 1.9 Angs trom resolution and refined by restrained least squares to R = 0.218 f or 7807 reflections. The structure established that the recently obser ved retrobound mode of attachment of substrate analogues cytidylyl-2', 5'-guanosine and deoxycytidylyl-3',5'-guanosine found in soaked RNase A crystals is also present in the cocrystallized complex. Retrobinding is thus unlikely to be the result of restrictions imposed by the crys talline environment as the ligands soak into the lattice but rather a phenomenon specific to small nucleotides containing guanine.