A divergent non-classical class I gene conserved in salmonids

Complementary DNA for two class I genes of the rainbow trout, Oncorhynchus mykiss, were characterized, MhcOnmy-UBA*01 is similar to Onmy-UA-C32 and th e classical major histocompatibility complex class I genes of other fish sp ecies, whereas Onmy-UAA*01 is divergent from all class I genes so far chara cterized. Onmy-UAA*01 is expressed at lower levels than Onmy-UBA*01. Althou gh Onmy-UAA*01 exhibits restriction fragment length polymorphism on Souther n blotting, the encoded protein is highly conserved. Two allotypes, which d iffer only by substitution at amino acid position 223 of the alpha(3) domai n, have been defined. Onmy-UAA*01 has an exon-intron organization like othe r class I genes and contains a Tc1-like transposon element in intron III. O rthologues of Onmy-UAA*01 have been characterized in four other species of salmonid, Between four species of Oncorhynchus, UAA*01 proteins differ by o nly 2-6 amino acids, whereas comparison of Oncorhynchus with Salmo trutta ( brown trout) reveals 14-16 amino acid differences. The Onmy-UAA*01 gene has properties indicative of a particularly divergent non-classical class I ge ne.