The crystal structures of ribonuclease from Streptomyces aureofaciens (RNase Sa)
and its complex with 3'-guanylic acid (guanosine 3'-monophosphate, 3'-GMP) have
been determined by the method of isomorphous replacement. The atomic parameters
have been refined by restrained least-squares minimization using data in the
resolution range 10.0-1.8 A. All protein atoms and more than 230 water atoms in
the two crystal structures have been refined to crystallographic R factors of
0.172 and 0.175 respectively. The estimated r.m.s. error in the atomic positions
ranges from 0.2 A for well-defined atoms to about 0.5 A for more poorly defined
atoms. There are two enzyme molecules in the asymmetric unit, built
independently, and referred to as molecules A and B. The value of the average B
factor for protein atoms in both structures is about 19 A2 and for water
molecules about 35 A2. Electron density for the substrate analogue 3'-GMP was
found only at the active site of molecule A. The density was very clear and the
positions of all 3'-GMP atoms were refined with precision comparable to that of
the protein.
