Latent a1 VH germline genes in an a2a2 rabbit. Evidence for gene conversion at both the germline and somatic levels

We have previously reported the sequences of putative latent a1 cDNA derived from an alpha 2 alpha 2 rabbit. Significant similarity to nominal a1 cDNA sequences was noted, but none of the latent sequences were completely a1-like. We have now probed a genomic library, produced from the same alpha 2 alpha 2 rabbit, for evidence of germline latent a1 VH genes. Four hundred ninety-four VH+ clones were screened with oligonucleotides specific for a1 diagnostic regions of framework region 1 (FR1) and FR3. Twenty-two percent of the VH+ clones hybridized with an a1FR3 oligonucleotide probe. Two a1 FR1 probes yielded weak signals with 6% to 13% of the VH+ clones. Twenty VH genes from clones positive for one or more of the a1-specific oligonucleotide probes were sequenced, revealing 14 unique germline VH genes. All but one of these genes were 85% to 92% identical to the VH1-a1 nominal gene prototype, with sequence identity extending into the leader intron. Most genes displayed extended regions of similarity to a1 in FR1, FR3, or both and expressed 13 to 17 of the 21 allotype-associated residues, consistent with the nominal a1 sequence. The a1-like sequences were variously interspersed with short non-a1 segments, suggestive of germline gene conversion. Although none of the germline a1-like VH genes we have isolated from the alpha 2 alpha 2 rabbits are identical to the known a1 genes or protein sequences from alpha 1 alpha 1 rabbits and 8 of 14 are pseudogenes, most could make significant contributions to the synthesis of a complete nominal a1 sequence by serving as a pool of sequence donors during somatic gene conversion.