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 alter native 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 v ertebrate in which the MHC has been well defined at both the biochemic al and functional levels. A part of the serine protease domain of the Xenopus Bf mRNA was amplified by reverse transcriptase-PCR, using dege nerate primers corresponding to regions encoding the perfectly conserv ed 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 cD NA library. The deduced amino acid sequence of 747 residues showed the same domain structure as mammalian Bf and C2: three short consensus r epeat domains, a von Willebrand domain and a serine protease domain. X enopus 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 occurre d before the divergence of amphibians and mammals. Southern blotting a nalysis of the Xenopus Bf gene showed a close linkage to the MHC, indi cating that the Bf gene was linked to the class I and class II genes a t the time Xenopus shared a common ancestor with mouse and man, 350 X 10(6) yr ago.