The complete nucleotide sequence of mouse immunoglobin gamma 2a gene and evolution of heavy chain genes: further evidence for intervening sequence-mediated domain transfer.

We have determined the complete nucleotide sequence (1990 base pairs) of mouse immunoglobulin gamma 2a gene, and compared it with the sequences of other gamma subclass genes so far sequenced, i.e. gamma 1 and gamma 2b genes. Divergence of the nucleotide sequence between a compared pair of the gamma genes varies extensively among different segments of the gene. For example, comparison of the gamma 2a and gamma 2b genes has revealed a remarkable homology in a long continuous segment (about 900 bases) that covers from the 3' portion of the first intervening sequence to the third intervening sequence. However, there is no particular segment of the gamma gene that is conserved universally among the three gamma genes. These findings suggest that, during their evolution, segments of the gamma genes had been scrambled between different subclass genes through recombinations within intervening sequences, thus providing further evidence for the intervening sequence-mediated domain transfer hypothesis. We have discussed several possible phylogenic trees which can explain the difference of divergence in various segments of the gamma genes.