Secretory ribonucleases in the primitive ruminant chevrotain (Tragulus javanicus).
Phylogenetic analyses of secretory ribonucleases or RNases 1 have shown that gene duplication events, giving rise to three paralogous genes (pancreatic, seminal and brain RNase), occurred during the evolution of ancestral ruminants. A higher number of paralogous sequences are present in chevrotain (Tragulus javanicus), the earliest diverged taxon within the ruminants. Two pancreatic RNase sequences were identified, one encoding the pancreatic enzyme, the other encoding a pseudogene. The identity of the pancreatic enzyme was confirmed by isolation of the protein and N-terminal sequence analysis. It is the most acidic pancreatic ribonuclease identified so far. Formation of the mature enzyme requires cleavage by signal peptidase of a peptide bond between two glutamic acid residues. The seminal-type RNase gene shows features of a pseudogene, like orthologous genes in other ruminants investigated with the exception of the bovine species. The brain-type RNase gene of chevrotain is expressed in brain tissue. A hybrid gene with a pancreatic-type N-terminal and a brain-type C-terminal sequence has been identified but nothing is known about its expression. Phylogenetic analysis of RNase 1 sequences of six ruminant, three other artiodactyl and two whale species support previous findings that two gene duplications occurred in a ruminant ancestor. Three distinct groups of pancreatic, seminal-type and brain-type RNases have been identified and within each group the chevrotain sequence it the first to diverge. In taxa with duplications of the RNase gene (ruminants and camels) the gene evolved at twice as fast than in taxa in which only one gene could be demonstrated; in ruminants there was an approximately fourfold increase directly after the duplications and then a slowing in evolutionary rate.