Rapid diversification of RNase a superfamily ribonucleases from the bullfrog, Rana catesbeiana

We present sequences of five novel RNase A superfamily ribonuclease genes o f the bullfrog, Rana catesbeiana. All five genes encode ribonucleases that are similar to Onconase, a cytotoxic ribonuclease isolated from oocytes of R, pipiens. With amino acid sequence data from 14 ribonucleases from three Rana species (R, catesbeiana, R. japonica, and R. pipiens), we have constru cted bootstrap-supported phylogenetic trees that reorganize these ribonucle ases into five distinct lineages-the pancreatic ribonucleases (RNases 1), t he eosinophil-associated ribonucleases (RNases 2, 3, and 6), the ribonuclea ses 4, the angiogenins (RNases 5) and the Rana ribonucleases-with the Rana ribonucleases no more closely related to the angiogenins than they are to a ny of the other ribonuclease lineages shown. Further phylogenetic analysis suggests the division of the Rana ribonucleases into two subclusters (A and B), with positive (Darwinian) selection (d(N)/d(S) > 1.0) and an elevated rate of radical nonsynonymous substitution (d(R)) contributing to the rapid diversification of ribonucleases within each cluster. This pattern of evol ution-rapid diversification via positive selection among sequences of a mul tigene cluster-bears striking resemblance to what we have described for the eosinophil-associated ribonuclease genes of the rodent Mus musculus, a fin ding that may have implications with respect the physiologic function of th is unique family of proteins.