Concerted evolution of ruminant stomach lysozymes. Characterization of lysozyme cDNA clones from sheep and deer

Contradictory evolutionary histories of ruminant lysozymes have been predicted by analysis of genomic blots (Irwin, D.M., Sidow, A., White, R., and Wilson, A.C. (1989) in The Immune Response to Structurally Defined Proteins: The Lysozyme Model (Smith-Gill, S.J., and Sercarz, E.E., eds) pp. 73-85, Adenine Press, Guilderland, NY) and sequences of cow stomach lysozyme cDNAs (Irwin, D.M., and Wilson, A.C. (1989) J. Biol. Chem. 264, 11387-11393). Genomic blots indicate that the amplification of the lysozyme gene family occurred 40-50 million years ago, while the cDNA sequences imply that the stomach genes began diverging from one another after the splitting of the deer and cow lineages, 25 million years ago. To resolve this contradiction, we characterized 111 stomach lysozyme cDNAs from two additional ruminant species: domestic sheep and axis deer. The cDNA sequences of the coding region of mature lysozyme together with the 3'-untranslated region were obtained from abomasum (true stomach) mRNA with the use of the polymerase chain reaction. The two primers for amplifying the cDNA were a lysozyme-specific primer, encoding a conserved sequence at the amino terminus of mature stomach lysozyme, and oligo(dT) as a general mRNA primer. Comparison of the cDNA sequences from these species to one another and to those of the cow revealed that different parts of the ruminant stomach lysozyme genes have had different evolutionary histories. The 3'-untranslated region has evolved in a divergent fashion since the original duplications 40-50 million years ago, supporting the genomic blot interpretation; by contrast, the coding region has evolved in a concerted fashion, that is, the multiple sequences within a species have evolved in unison. The 3'-untranslated portion of the lysozyme genes appears to have escaped from concerted evolution due to inability to initiate concerted evolution, rather than due to reduced sequence similarity. The process of concerted evolution in stomach lysozymes may have had roles both in adapting lysozyme to the stomach environment in early ruminants as well as in retarding amino acid sequence evolution in the well adapted lysozyme of modern ruminants.