The amino acid sequence of mouse pancreatic ribonuclease. Extremely rapid evolutionary rates of the myomorph rodent ribonucleases.

The complete amino acid sequence of mouse pancreatic ribonuclease has been 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 neraly unique substitutions were found, viz. Gly-68 leads to Arg-68, Arg-85 leads to His-85 and Ser-123 leads to Thr-123. All these three 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 ribonuclease 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 ribonuclease evolution. These extreme fluctuations in substitution rate are difficult to reconcile with the hypothesis of the evolutionary clock. The high evolution rate of rat ribonuclease is thought to be caused by positive selection, leading to new functional properties of the enzyme.