The structure and evolution of immunoglobulin kappa chain constant region genes in the genus Rattus

Characterization of multiple alleles of the T-cell differentiation marker ART2 (RT6) in inbred and wild rats

ART2 (RT6) belongs to the family of mono-ADP-ribosyltransferases (ARTs). ART2 is a T-cell differentiation marker expressed by the majority of mature peripheral T cells in the rat. The two known ART2 allotypes display approximately 95% amino acid identity. We sequenced the ART2 coding regions from 18 inbred rat strains and found two additional alleles, termed Art2 ( a2 ) and Art2 ( b2 ). Monoclonal antibody Gy12/61 specifically reacted with Art2 ( a2 ) but not Art2 ( a1 ) lymph node cells. Expression of ART2 allotypes in Jurkat cells confirmed this specificity. A polymerase chain reaction (PCR) assay using restriction fragment length polymorphisms is described, which allows the easy discrimination of Art2 alleles. All four laboratory rat alleles, as well as an additional sequence variant, were found amongst 18 wild rat DNA samples. PCR analysis confirmed the selective presence of a rodent identifier (ID) element in the Art2 ( a ) but not the Art2 ( b ) alleles in all rats studied. Analysis of Art2 ( a1 ) and Art2 ( b2 ) genes showed greater divergence in coding than in non-coding regions. Together with the finding of a high number of non-synonymous mutations leading mostly to non-conservative amino acid substitutions clustered on the side facing away from the cell surface, this suggests that the Art2 polymorphism has been subject to selection.

Two pseudogenes among three rat immunoglobulin lambda chain genes

In order to examine the number and organization of the immunoglobulin lambda light chain genes of the rat, we have used mouse lambda chain cDNA probes to isolate hybridizing fragments from a partial EcoRI rat liver DNA library. We have determined the partial nucleotide sequence of two such clones. One clone, containing a 5.8 kb EcoRI insert which hybridizes to both mouse C lambda 1 and C lambda 2 probes, includes an apparently expressible C lambda 2-like gene as well as a C lambda 1-like pseudogene (psi C lambda 1.1), arranged similarly to the mouse C lambda gene complexes. Sequence analysis of a second cloned EcoRI fragment (1.15 kb in length) revealed part of a second C lambda 1-like pseudogene (psi C lambda 1.2), the coding regions of both pseudogenes being interrupted by multiple frame-shifting size differences. In the case of psi C lambda 1.2, the degree of sequence identity with mouse C lambda 1 drops abruptly immediately following the termination codon, suggesting that translocation events have played a role in its generation. These two rat pseudogenes, and the mouse C lambda 4 pseudogene, clearly have been rendered unexpressible by separate evolutionary events. Comparisons between C lambda coding and non-coding regions of rats and mice indicate that some of the unusual patterns of divergence we have observed in recently diverged Ck genes may exist in C lambda genes as well.

Duplication of J kappa genes within genus Rattus

We have isolated a region containing the immunoglobulin kappa chain joining segments from a liver DNA library of the Australian rat Rattus villosissimus, and determined its nucleotide sequence. While the laboratory rat (Rattus norvegicus) had previously been shown to contain three recently duplicated copies of J kappa 2, R. villosissimus has only two. Furthermore, all three copies of J kappa 2 in R. norvegicus share an 11 bp deletion in their 5' flanking regions which is not evident in either copy of J kappa 2 in R. villosissimus. This suggests that the initial duplication events occurred separately in the two lineages, and were followed by a second duplication in R. norvegicus, all three duplications having occurred within the last 6-12 million years (although more complicated schemes involving gene conversion events cannot be excluded). These results indicate that there is a high degree of plasticity in this region of the genome, and that selective forces must exist which have maintained the number of expressible J kappa segments in humans (5) and rodents (4-6) within their narrow range.