Cloning of shark RAG2 and characterization of the RAG1/RAG2 gene locus

The recombination-activating genes (RAG) encode a site-specific recombinase that is centrally responsible for the rearrangement of genomic V(D)J exons necessary to form functional immunoglobulin and T-cell receptor genes. To help elucidate the origins of the RAG genes, we have cloned the RAG2 gene from the sandbar shark (Carcharhinus plumbeus) and characterized the entire RAG1/RAG2 gene locus. The shark RAG2 protein consists of 520 amino acids, is approximately 50% identical with RAG2 proteins from other vertebrates, and contains the same three domains identified in mammalian RAG2. Residues critical for RAG2 function are conserved in the shark sequence. In common with other vertebrate species, the shark RAG2 coding region lacks introns and is closely linked in opposite orientation to the RAG1 gene. The intergenic region is 9.4 kb, which is considerably larger than of teleosts (2-3 kb) and is comparable to that of tetrapods. This length is partially explained by the presence of several SINE and LINE fragments. The ancestors of the sharks were apparently the first vertebrates in phylogeny to have RAG genes, and our results confirm that the RAG genes have been highly conserved during evolution both in terms of sequence and gene organization.