The X-Ray Analysis of Ribonuclease Sa

The tertiary structure of ribonuclease Sa from Streptomyces aureofaciens as well as the structure of its complex with 3’guanylic acid (3’GMP) have been determined by X-ray analysis. The electron density that was finally interpreted was calculated with phases obtained by multiple isomorphous replacement technique and solvent flattening at a resolution of 2.5 A. Refinement against 1.8 A data yields R factor of 0.172 and 0.184 for the native and complex structures, respectively. The three dimensional structure of the enzyme is similar to that of other microbial ribonucleases and, as expected from the amino acid sequences, is closest to RNase St. For comparison sequences of ribonucleases T1 (Takahashi, 1980), C2 (Bezborodova, 1983), Pchl (Shlyapnikov, 1986), Pbl (Shlyapnikov, 1984), Ba (Hartley, 1972), Bi (Aphanasenko, 1980), St (Yoshida, 1976), and Sa (Shlyapnikov, 1986) are given in Table 1.