RNase Sa2 (97 amino-acid residues, MW = 10 894 Da) is an extracellular ribonuclease isolated from Streptomyces, aureofaciens, strain R8/26. The enzyme hydrolyses the phosphodiester bond of RNA at the 3'-side of guanosine nucleotides. Two crystal forms of RNase Sa2/3'-GMP complex were obtained. Crystal form I was prepared by diffusion of mononucleotide into the pre-grown crystals of RNase Sa2. Crystal form II was obtained by re-crystallization of RNase Sa2 with 3'-GMP. Both crystal forms belong to space group C-2. The crystals diffracted to 2.2 and 2.25 Angstrom, respectively. Unit cell parameters for crystal form I and II are: a = 101.52, b = 65.90, c = 56:96 Angstrom, beta = 100.92degrees, and a = 108.20, b = 62.65, c = 68.30 Angstrom, beta = 129.80degrees, respectively. The V-M values are 2.86 and 2.04 Angstrom(3) Da(-1), respectively.. The crystal form I contains three and the crystal form II four molecules in the asymmetric unit. RNase Sa2/2',3'-GCPT complex was prepared by diffusion of 2',3'-GCPT into pre-grown RNase Sa2 crystals. Data from the crystal complex were collected to 2.0 Angstrom resolution. The crystals have the same symmetry and space group as the previous ones. Unit cell parameters a = 101.22, b = 66.77, c = 57.07 Angstrom, beta = 100.80degrees correspond to V-M value of 2.89 Angstrom(3) Da(-1). There are three protein molecules in the asymmetric unit. All sets of data were collected using synchrotron radiation at 100 K. Refinement of the three structures is under way. Preliminary results confirmed the presence of nucleotides in all structures.
