3-DIMENSIONAL STRUCTURE OF THE TERNARY COMPLEX BETWEEN RIBONUCLEASE-T1, GUANOSINE-3',5'-BISPHOSPHATE AND INORGANIC-PHOSPHATE AT 0.19 NM RESOLUTION

The ternary complex formed between RNase T1, guanosine 3',5'-bisphosphate (3',5'-pGp) and P(i) crystallizes in the cubic space group 123 with a = 8.706(l) nm. In a previous publication [Lenz, A., Heinemann, U., Maslowska, M. & Saenger, W. (1991) Acta Crystallogr. B47,521 - 527], the structure of the complex (in which P(i) was not located) was described at a resolution of 0.32 nm. This is now extended to 0.19 nm with newly grown, larger crystals. Refinement with restrained least-squares converged at R = 17.8% for 8027 reflections with \F(o)\ greater-than-or-equal-to 1sigma(\F(o)\); the final model comprises 120 water molecules. 3',5'-pGp is bound to RNase T1 in the anti form, with guanine in the specific recognition site; the 3'-phosphate protrudes into the solvent, and the 5'-phosphate hydrogen bonds with Lys41 O and Asn43 N4. A tetrahedral anion assigned as P(i) occupies the catalytic site and hydrogen bonds to the side chains of Tyr38, Glu58, Arg77 and His92. The overall polypeptide fold of RNase T1 in the cubic space group does not differ significantly from that in the orthorhombic space group P2(1)2(1)2(1) except for changes less-than-or-equal-to 0.2 nm in loop regions 69 - 72 and 95 - 98.