Exon-intron organization of fish major histocompatibility complex class II B genes

Major histocompatibility complex (Mhc) molecules bind self and foreign peptides and present them to lymphocyte for recognition. Activation of lympocytes by Mhc-bound foreign peptides leads to specific immune response against parasites. The Mhc genes have been studied extensively in mammals and birds but much less in other vertebrate classes. In this communication we provide the first description of the exon-intron organization of class II \-chain-encoding genes from the teleostfish Aulonocara hansbaenschi, family Cichlidae. Each of the genes consists of six exons, E1 through E6, encoding the leader peptide (E1), β domain (E1+E2), β domain (E3+E4), connecting peptide (E5), transmembrane region (E5), cytoplasmic domain (E5+E6), and the 3' untranslated region (E6). The exons are separated by relatively short introns, the length of the ongest intron being 1.3 kilobase pairs. An important difference between these and all other known class II β genes is that the \2 domain-encoding exon is split by an intron 97 base pairs in length. The intron is absent in other teleost fishes suchas Brachydanio rerio. A change in the 3' splice site of intron 4 in some of the genes of A. hansbaenschi and of another cichlid fish, Cypotilapia frontosa, has produced two extra codons at the 5' end of exon 5. Comparison of the A. hansbaenschi coding sequences with those of C. frontosa has revealed a concentration of variability in exon 2 and part of exon 3. Taken together, these observations provide evidence for the existence in cichlid fishes of at least two class I β loci which are functionally equivalent to the corresponding loci in mammals. The exon-intron organization and sequence similarities indicate that the two loci arose by duplication from a common ancestral gene.