Mechanisms of divergence and convergence of the human immunoglobulin alpha 1 and alpha 2 constant region gene sequences.

Nucleotide sequences of the human alpha 1 and two allelic alpha 2 immunoglobulin heavy chain constant region genes are presented. The genes contain three exons, each encoding a single constant region protein domain. The protein hinge region is encoded at the 5' end of the second exon, and the rapid evolutionary changes in length of the hinge correspond to duplications or deletions within the hinge-coding region, probably facilitated by repeats in the DNA sequence. Alignment of the alpha 1 and alpha 2 gene sequences reveals an unusual coupled deletion-duplication in the 5'-flanking region, which can be explained in terms of a slipped-strand mispairing model. Comparison of nucleotide sequences of the alpha 1 gene and two alleles of the alpha 2 gene indicates a localized transfer of genetic information from the 3' end of the alpha 1 gene to one of the alpha 2 alleles, probably by a gene conversion. At one end of the region within which conversion apparently occurred, there is a 40 bp sequence of the type that can form Z-DNA.