Kappa-chain constant-region gene sequences in genus Rattus: coding regions are diverging more rapidly than noncoding regions

We have determined the nucleotide sequence of a 1,200-base pair (bp)genomic fragment that includes the kappa-chain constant-region gene (Ckappa) from two species of native Australian rodents, Rattus leucopuscooktownensis and Rattus colletti. Comparison of these sequences with eachother and with other rodent C kappa genes shows three surprising features.First, the coding regions are diverging at a rate severalfold higher thanthat of the nearby noncoding regions. Second, replacement changes withinthe coding region are accumulating at a rate at least as great as that ofsilent changes. Third, most of the amino acid replacements are localized inone region of the C kappa domain--namely, the carboxy-terminal "bends" inthe alpha-carbon backbone. These three features have previously beendescribed from comparisons of the two allelic forms of C kappa genes in R.norvegicus. These data imply the existence of considerable evolutionaryconstraints on the noncoding regions (based on as yet undeterminedfunctions) or powerful positive selection to diversify a portion of theconstant-region domain (whose physiological significance is not known).These surprising features of C kappa evolution appear to be characteristiconly of closely related C kappa genes, since comparison of rodent withhuman sequences shows the expected greater conservation of coding regions,as well as a predominance of silent nucleotide substitutions within thecoding regions.