Molecular cloning and linkage analysis of complement C3 and C4 genes of the Japanese medaka fish

The thioester-containing complement components, C3 and C4, are believed to have arisen by gene duplication from a common ancestor, and the mammalian C 4 gene resides in the vicinity of the C2 and B genes within the major histo compatibility complex (MHC) class III region. To analyze the evolution of b oth the complement system and the MHC, we determined the complete primary s tructures of two C3 genes, termed Orla C3-1 and Orla C3-2, and one C4 gene, termed Orla C4, of a teleost, Japanese medaka fish (Oryzias latipes), by a nalyzing cDNA clones isolated from a liver library constructed using the in bred AA2 strain. The deduced basic structures of Orla C3-1, C3-2, and C4, s uch as the subunit chain structure, the thioester site, and the proteolytic activation site, are similar to their mammalian counterparts. However, the catalytic His residue which greatly increases the rate of thioester reacti on, is replaced by Ala in Orla C3-2, implying functional differentiation be tween two C3 molecules. Mapping analysis revealed a close linkage between t he C3-1 and C3-2 genes, indicating that they arose by a local duplication r ather than by a genome-wide tetrapolidization. The C4 gene belongs to a dif ferent linkage group, and no linkage was observed among the C3, C4, Bf/C2, MHC class I, and MHC class II loci. These results suggest that the MHC clas s III complement region was established in the tetrapod lineage, or lost in the teleost lineage.