ZEBRAFISH MHC CLASS-II ALPHA-CHAIN-ENCODING GENES - POLYMORPHISM, EXPRESSION, AND FUNCTION

Its small size and short generation time renders the zebrafish (Brachy danio rerio) an ideal vertebrate for immunological research involving large populations. A prerequisite for this is the identification of th e molecules critical for an immune response in this species. In earlie r studies, we cloned the zebrafish genes coding for the beta chains of the class I and class II major histocompatibility complex (Mhc) molec ules. Here, we describe the cloning of the zebrafish alpha chain-encod ing class II gene, which represents the first identification of a clas s II A gene in teleost fishes. The gene, which is less than 3 kilobase s (kb) distant from one of the beta chain-encoding genes, is approxima tely 1.2 kb long and consists of four exons interrupted by very short (<200 base pairs) introns. Its organization is similar to that of the mammalian class II A genes, but its sequence differs greatly from the sequence of the latter (36% sequence similarity). Among the most conse rved parts is the promoter region, which contains X, Y, and TATA boxes with high sequence similarity to the corresponding mammalian boxes. T he observed striking conservation of the promoter region suggests that the regulatory system of the class II genes was established more than 400 million years ago and has, principally, remained the same ever si nce. Like the DMA, but unlike all other mammalian class II A genes, th e zebrafish gene codes for two cysteine residues which might potential ly be involved in the formation of a disulfide bond in the alpha domai n. The primary transcript of the gene is 1196 nucleotides long and con tains 708 nucleotides of coding sequence. The gene is expressed in tis sues with a high content of lymphoid/myeloid cells (spleen, pronephros , hepatopancreas, and intestine). The analyzed genomic and cDNA sequen ces are probably derived from different loci (their overall sequence s imilarity in the coding region is 73% and their 3' untranslated region s are highly divergent from each other). The genes are apparently func tional. Comparison of genes from different zebrafish populations revea ls high exon 2 variability concentrated in positions coding for the pu tative peptide-binding region. Phylogenetic analysis suggests that the zebrafish class II A genes stem from a different ancestor than the ma mmalian class II A genes and the recently cloned shark class II A gene .