Complex allotypes of the rabbit immunoglobulin kappa light chains are encoded by structural alleles

We have isolated the rabbit immunoglobulin b9 Ck light chain gene and compared its nucleotide sequence with the b4, b4var , b5 and bas Ck sequences. In spite of the high number of substitutions found between the different rabbit Ck coding regions, only very few changes are silent. Furthermore, the nucleotide changes are clustered in segments which correlate with the bends and helical regions found in the tertiary structure of the Ck domain of the protein. The flanking regions present a higher degree of conservation than the coding regions. The two genomic EcoRI fragments hybridizing to a b4cDNA probe have been correlated with the two distinct loci, Ck1 and Ck2 : one encodes for the nominal b9 Ck allotype and the other contains the information for the bas Ck region. The b allotypes are true alleles which could have evolved by intergenic conversion.