The differential pattern of tissue-specific expression of ruminant pancreatic type ribonucleases may help to understand the evolutionary history of their genes

Molecular evolutionary analyses of mammalian ribonucleases have shown that gene duplication events giving three paralogous genes occurred in ruminant ancestors. The enzymes of the bovine species encoded by these genes, isolat ed from pancreas, brain and seminal vesicles, present similar enzymological properties but distinct structural features. In other ruminant species, ge nomic sequences orthologous to the bovine genes of pancreas and brain ribon ucleases encode active enzymes. In mammalian species other than ruminant ar tiodactyls, only one gene encoding ribonuclease of the pancreatic type is g enerally present. In this work, we describe a differential pattern of trans criptional expression of the pancreas and brain ribonuclease genes in the o x species and report transcription of the human ribonuclease gene in brain as well as in pancreas and in mammary gland. We also report the molecular c loning of the gene encoding the bovine seminal ribonuclease in which the st ructural organization already described for the two paralogous genes is con served. The seminal RNAase is exclusively expressed in seminal vesicles of Bos taurus, whereas in other ruminant species, the orthologous sequence is a pseudogene. Previous studies from a number of research groups demonstrate d that, unlike other mammalian ribonucleases, the seminal enzyme is a coval ent dimer, and its unique quaternary structure correlates with special biol ogical activities. The major determinant of dimer formation, i.e. the prese nce of two adjacent cysteine residues, is absent in the pseudogenes. We adv ance the hypothesis that the differentiation of distinct expression pattern s could represent an important evolutionary determinant for the genes encod ing pancreas and brain ribonucleases in ruminants, whereas the differentiat ion of a quaternary structure endowed with new biological functions could b e the main determinant for the evolutionary success of the seminal gene in the bovine species. (C) 1999 Elsevier Science B.V. All rights reserved.