Isolation of the Xenopus complement factor B complementary DNA and linkage of the gene to the frog MHC

C factor B (Bf) is the key component of the C3 convertase of the alternative C pathway, and its gene resides in the class III region of the mammalian MHC. To elucidate the evolution of both the C system and the MHC, we isolated Bf cDNA clones from Xenopus laevis, an ectothermic vertebrate in which the MHC has been well defined at both the biochemical and functional levels. A part of the serine protease domain of the Xenopus Bf mRNA was amplified by reverse transcriptase-PCR, using degenerate primers corresponding to regions encoding the perfectly conserved amino acid sequences found in both the mouse Bf and C2 proteins. A full length Xenopus Bf cDNA clone was isolated from a Xenopus liver cDNA library. The deduced amino acid sequence of 747 residues showed the same domain structure as mammalian Bf and C2: three short consensus repeat domains, a von Willebrand domain and a serine protease domain. Xenopus Bf has 40% and 30% overall amino acid identity to mouse Bf and mouse C2, respectively. Because the amino acid identity between mouse Bf and mouse C2 is 38%, the gene duplication of Bf/C2 probably occurred before the divergence of amphibians and mammals. Southern blotting analysis of the Xenopus Bf gene showed a close linkage to the MHC, indicating that the Bf gene was linked to the class I and class II genes at the time Xenopus shared a common ancestor with mouse and man, 350 x 10(6) yr ago.