The Amino Acid Sequence of Mouse Pancreatic Ribonuclease

Abstract

The complete amino acid sequence of mouse pancreatic RNase was determined by analysis of tryptic, chymotryptic, thermolytic and CNBr peptides and by automatic sequence analysis of the intact protein. The sequence of mouse RNase differs in 20-30% of the positions from other RNase sequences. Three unique or nearly unique substitutions were found, viz. Gly-68 .fwdarw. Arg-68, Arg-85 .fwdarw. His-85 and Ser-123 .fwdarw. Thr-123. All these 3 residues might be involved in interactions with substrate molecules. A most parsimonious tree of the myomorph rodent RNase shows that after the divergence of rat and mouse, the RNase of rat accumulated substitutions at a rate 2.5-4.3 times as high as the rates in other branches of the tree and 23 times as high as the average rate in the Bovidae RNase evolution. These extreme fluctuations in substitution rate are difficult to reconcile with the hypothesis of the evolutionary clock. The high evolution rate of rat RNase is thought to be caused by positive selection, leading to new functional properties of the enzyme.