RIBONUCLEASE FROM STREPTOMYCES-AUREOFACIENS AT ATOMIC-RESOLUTION

Crystals of ribonuclease from Streptomyces aureofaciens diffract to at omic resolution at room temperature. Using synchrotron radiation and a n imaging-plate scanner, X-ray data have been recorded to 1.20 Angstro m resolution from a crystal of native enzyme and to 1.15 Angstrom from a cry stal of a complex with guanosine-2'-monophosphate. Refinement w ith anisotropic atomic temperature factors resulted in increased accur acy of the structure. The R factors for the two structures are 10.6 an d 10.9%. The estimated r.m.s. error in the coordinates is 0.05 Angstro m, less than half that obtained in the previous analysis at 1.7 Angstr om resolution. For the well ordered part of the main chain the error f alls to below 0.02 Angstrom as estimated from inversion of the least-s quares matrix. The two independent molecules in the asymmetric unit al lowed detailed analysis of peptide planarity and some torsion angles. The high accuracy of the analysis revealed density for a partially occ upied anion in the nucleotide binding site of molecule A in the native structure which was not seen at lower resolution. The anisotropic mod el allowed correction of the identity of the residue at position 72 fr om cysteine to threonine. Cys72 SG had been modelled in previous analy ses with two conformations. The solvent structure was modelled by mean s of an automated procedure employing a set of objective criteria. The solvent structure for models refined using different programs with is otropic and anisotropic description of thermal motion is compared.