Three-dimensional structure of a barnase-3'GMP complex at 2.2A resolution.

Barnase has been co-crystallized at neutral pH with its natural product the 3'-guanylic acid. The X-ray structure was solved by molecular replacement methods and refined to a final R-factor of 18.7%. The protein folding is essentially the same as that of the native form. The base recognition site is almost identical to that of the homologous binase-3'GMP complex, but the nucleotide is bound in a productive binding mode for a substrate with a syn glycosyl torsion angle allowing the general base Glu73 to hydrogen bond with the 2'O of the nucleotide as is assumed in the classical catalytic mechanism. The two molecules of the asymmetric unit form a dimer and the positions of the two nucleotides partially mimic the interaction of the RNA with the enzyme, one of the inhibitors being located in a secondary subsite.