Ruminant brain ribonucleases: expression and evolution

Molecular evolutionary analyses of mammalian ribonucleases have shown that gene duplication events giving rise to three paralogous genes occurred in r uminant ancestors. One of these genes encodes a ribonuclease identified in bovine brain. A peculiar feature of this enzyme and orthologous sequences i n other ruminants are C-terminal extensions consisting of 17-27 amino acid residues. Evidence was obtained by Western blot analysis for the presence o f brain-type ribonucleases in brain tissue not only of ox, but also of shee p, roe deer and chevrotain (Tragulus javanicus), a member of the earliest d iverged taxon of the ruminants. The C-terminal extension of brain-type ribo nuclease from giraffe deviates much in sequence from orthologues in other r uminants, due to a change of reading frame. However, the gene encodes a fun ctional enzyme, which could be expressed in heterologous systems. The messe nger RNA of bovine brain ribonuclease is not only expressed at a high level in brain tissue but also in lactating mammary gland. The enzyme was isolat ed and identified from this latter tissue, but was not present in bovine mi lk, although pancreatic ribonucleases A and B could be isolated from both s ources. This suggests different ways of secretion of the two enzyme types, possibly related to structural differences. The sequence of the brain-type RNase from chevrotain suggests that the C-terminal extensions of ruminant b rain-type ribonucleases originate from deletions in the ancestral DNA (incl uding a region with stop codons), followed by insertion of a 5-8-fold repea ted hexanucleotide sequence, coding for a proline-rich polypeptide. (C) 200 1 Elsevier Science B.V. All rights reserved.