Y. Kato et al., ISOLATION OF THE XENOPUS COMPLEMENT FACTOR-B COMPLEMENTARY-DNA AND LINKAGE OF THE GENE TO THE FROG MHC, The Journal of immunology, 153(10), 1994, pp. 4546-4554
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.