In the variant rabbit strain BASILEA, immunoglobulins G were shown to contain two distinct populations of IgG molecules whose light (L) chains belonged to the known A isotype and to a new K-like type. These two L chains differed from each other by electrophoretic, chemical, and antigenic properties. The K-like L chain fraction showed (i) an acid-labile Asp-Pro bond at the end of the joining region and (ii) a tryptic peptide, whose amino acid sequence ofthe NH2-terminal 15 residues was identical to the homologous constant (C) region sequence of b9 K chain with the exception of the residue in position 70, which is asparagine in the K-like chain instead of the characteristic half-cystine residue in all L chains of KB type expressing b4, b5, b6, or b9 allotypes. Serological and chemical studies have demonstrated to date the existence of five polymorphic forms of the domestic rabbit K light (L) chain allotypes designated b4, b5, b6, b9, and b4var (1-6). Extensive chemical differences in the constant (C) regions of b4, b5, b6, and b9 molecules have been reported (7-9) and are thought to reflect the expression of the antigenic determinants of the b allotypes (10). In standard domestic rabbits, A-type L chains are the minor component of the immunoglobulin L chain pool (11). However, this situation is reversed in a variant rabbit strain designated BASILEA, which was developed and maintained by Kelus and Weiss (12). Homozygous BASILEA rabbits are characterized by the lack ofK chains bearing b4, b5, b6, or b9 allotypic markers, which are substituted by L chains of the A type bearing C7 and C21 allotypic specificities (12-14). The primary structure of the constant region ofA L chains derived from an antibody IgG raised in BASILEA rabbits was recently completed and showed extensive homology with A chains of other species (14,15

The b locus controls the synthesis of allotypic specificities of rabbit x chain (1). Five allotypes of the b series are known in the domestic rabbit: b4, b5, b6 (21 3), b9 (4), b4 v~ (5, 6); and five additional ones have been described in wild populations: b92 (7), b95 (8) b96 (9), b98 (10) and b99 (A. Benammar and P.-A. Cazenave, manuscript in preparation.)The bas gene, which behaves as an allele at the b locus, found in the rabbit colony of the Basel Institute of Immunology, has been described (11). An offspring from a mating between a male with b(4-5-6-9 +) phenotype and presumed to be homozygous b9/b9 and a female heterozygous b4/b9 expressed only the b4 allotype inherited from the mother. Subsequent genetic analysis demonstrated that the failure to make b9 allotype behaved as encoded by an allele at the b locus, and it was proposed that the bas variant arose by mutation affecting the b locus. The homozygous bas/bas Basilea rabbits compensate for their lack of b allotype-positive kappa chain by producing elevated amounts of lambda-type light chains (11,12). Alloantisera were raised by immunizing "conventional" rabbits with immunoglobulins (Ig)isolated from the sera of homozygous bas/bas rabbits (13,14). These antisera reacted with sera from rabbits homozygous or heterozygous with respect to the bas gene and with sera from some b9-positive rabbits from the Basel Institute of Immunology, but not with all other domestic rabbit sera tested, including those that are b9-positive (13,14).We have recently obtained chemical and serological evidence that anti-bas sera are directed against antigenic determinants of kappa light chains present in the sera of rabbits homozygous or heterozygous with respect to bas gene. These light chains are distinct from the b-positive kappa light chains present in the sera of conventional rabbits (14). In this paper we present serological and genetic data demonstrating that anti-bas sera are directed against an allotypic form of a kappa isotype (~2) different from the kappa isotype (~1) that bears allotypic specificities of the b series, and that the loci controlling the expression of ~ 1 and ~2 isotypes are closely linked. Materials and MethodsAnimals and Sera. The wild rabbits (O~yctolagas cuniculus) were trapped in Spain (Zaragosta), Portugal (in six different locations), Tunisia (Island of Zembra), and France (in eight different locations). The domestic rabbits were Bouscat Giant. The Basilea rabbit strain is maintained in our laboratory from animals originating in the colony maintained by Dr. A. S. Kelus, Basel Institute of Immunology. The Australian rabbit sera studied were a generous gift of Dr.

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