Complete coding sequence of rainbow trout Mhc II beta chain.

Several cDNA clones comprising the entire coding sequence of the rainbow trout (Oncorhynchus mykiss) major histocompatibility comlex (Mhc) class II B gene have been isolated from different sources. A single B gene appears to be transcribed in the rainbow trout and it encodes a 247 amino acid long polypeptide, which is of similar size to mammalian, avian, and amphibian and other teleost beta chains. The amino acid sequence identity to mammalian, amphibian, and avian class II beta chains is only about 30%. Despite the low similarity, a striking pattern of conservation is observed, both in the putative peptide-binding domain and in the Ig-like domain. Most of the conserved residues are located in the Ig-like domain and in the transmembrane segment. The majority of polymorphic residues reside in the beta 1 domain, with the greatest variability found in the amino-terminal half of the domain. The sequence data are compatible with a rather limited polymorphism of a single, expressed Mhc class II B gene.