Structure Bases for Nucleotide Recognition by Guanyl-Specific Ribonucleases

Abstract

Structural aspects of specificity and catalytic activity of enzymes are studied on various objects, in particular on ribonucleases. Remarkable progress has been achieved in understanding the pyrimidine specificity of RNAse A at molecular level. The tertiary structures of different microbial ribonucleases were solved at atomic resolution recently (Hill et al., 1983). These enzymes belong to the so-called guanyl-specific RNases such as the well-known RNAse T₁, which degrades single-stranded RNA to give oligonucleotides with terminal 3′ guanosine monophosphate. In many of these, the cleavage occurs as a two-step mechanism via 2′, 3′ cyclic nucleotide intermediates. In order to reveal molecular bases for substrate specificity of RNAse T₁ the structures of the RNAse T₁-2′-or 3′-GMP complex were studied at 2 Å resolution (Heinemann and Saenger, 1982; Sugio et al., 1985). Some important features of recognition patterns were obtained from these data. However, it was not possible to draw cleancut conclusions in respect to the molecular mechanism for substrate discrimination. For this reason we have undertaken a study of the crystal structure of RNAse Pb₁ (from Penicillium brevicompactum), a close analogue of RNAse T₁.