A genomic gene encoding the b5 rabbit immunoglobulin kappa constant region: implications for latent allotype phenomenon.

Abstract

We previously reported that domestic rabbits of five immunoglobulin K allotype strains (b4', b4, b5, b6, and b9) harbor at least two DNA sequences that hybridize strongly to K constant region probes in Southern blots. One of these sequences ("type A") has been identified as encoding the constant region of the K2 isotype, an immunoglobulin chain that most rabbits express only at low levels, if at all. We identified the second sequence-for rabbits of the b4 allotype-as encoding the nominal b4 K chain (or K1 isotype), but for rabbits of other allotypes no definite identification for this "type B" sequence could be made. Here we suggest that the type B sequence in rabbits of the other domestic allotypes also encodes the nominal K1 immunoglobulin chain. We show this directly for the b5 allotype; a type B sequence cloned from b5 DNA has been found to contain an apparently functional gene encoding the b5 constant region sequence. Indirect arguments suggest the corresponding conclusion for the b'V, b6, and b9 allotypes. We have considered the implications of these results for the phenomenon of "latent allotype" expression.The immunoglobulin K gene system of rabbits is more complex than the well-studied homologous loci of mouse and man in that rabbits demonstrate several allotypic variants of "b series" K chains (known as b4v, b4, b5, b6, and b9 in domestic rabbits) as well as an additional K isotype (known as K2 or "bas") that is generally expressed at very low levels. The inheritance of b series allotype expression is not completely understood (reviewed in ref. 1). Although initial studies indicated that allotypes were controlled by codominantly expressed allelic structural genes, this model was challenged by serologic detection in some rabbits of "latent" allotypes-i.e., K chains unpredicted by pedigree of the rabbits. The phenomenon of latent allotypes suggested that rabbits might harbor structural genes for more K chains than are normally expressed. The usual inheritance of "nominal" allotype expression might then result from an inherited regulatory mechanism that would select the allotype to be expressed; latent allotype expression could result from "leakage" of the regulatory mechanism.We have been pursuing the molecular basis for latent allotype expression by investigating the K chain gene system of rabbits. We recently cloned the nominal b4 germline constant K (CK) gene (designated Klb4) from a b4 rabbit and showed by Southern blot analyses that rabbits of five different b series pedigrees each harbor two sequences strongly homologous to the cloned b4 CK gene (2); the two sequences were arbitrarily designated type A and type B. Subsequent work from our laboratory (3) and from Heidmann and Rougeon (4) identified the type A sequence of a b4 rabbit as encoding the K2 isotype; we refer to this gene as K2b4. By inference, the type A sequences in b4v, b5, b6, and b9 rabbits probably also represent K2 isotype genes.The identity of the type B sequences has been clearly established only in b4 rabbits, where the B...

Nonrandom allele associations between unlinked protein loci: are the polymorphisms of the immunoglobulin constant regions adaptive?

“…The b4 and b5 genomic sequences differ in their coding regions (330 base pairs) by 39 base-pair substitutions at amino acid replacement sites for only four at synonymous sites, while 96% ofthe base pairs ofthe 3'-untranslated region are conserved (7)(8)(9). A somewhat similar situation exists for Ig C-region genes in other species (33,34).…”

“…The different b-locus allotypes (b4, b5, b6, and b9) are associated with multiple amino acid differences throughout the C region of the K L chain. Although alleles can differ by up to 33% in amino acid sequence (6, 7), studies at the genome level have confirmed them as true alleles at a single gene locus (8,9). A most interesting finding was that the nucleotide sequences of b-locus alleles differ almost exclusively by base-pair substitutions causing amino acid replacements (7-9).…”

Characterization and DNA sequence of the b6w2 allotype of the rabbit immunoglobulin kappa 1 light chain (b locus)

“…The N9 sequence was found to be identical to either the b95 sequence or to the sequence representing a nominal b6 allotype except for six nucleotide substitutions. Two of these substitutions convert an Eco RI Akimenko et al 1984Emorine et al 1983Emorine et al 1984Dreher et al 1990 Ayadi and site into a Bam HI site. The restriction fragment analysis confirmed that such a site is indeed present, not only in PCR products obtained with DNA from rabbit N9, but also in products derived from each of four other rabbits typed as b6w2 (see Table 1).…”

Multiple-alphabet amino acid sequence comparisons of the immunoglobulin kappa-chain constant domain.

“…There exists a high degree of C, polymorphism in rabbit, with allotypes b4, b5, b9, and bas-N4 common in domesticated populations. These allotypes were distinguished originally by serologic methods and recently by detection of multiple amino acid substitutions (15,16). The rabbit K complex embodies two isotypes.…”

“…1). The rabbit C, allotypes (b4, b5, b9, bas) feature an extraordinary degree of amino acid replacement (15,16). In fact, >40% of all nucleotide substitutions between these C, sequences are nonsynonymous.…”