Recombination-activating gene 1 and 2 (RAG1 and RAG2) in flounder (Paralichthys olivaceus)

During the development of B and T lymphocytes, Ig and TCR variable region genes are assembled from germline V, D, and J gene segments by a site-specific recombination reaction known as V(D)J recombination. The process of somatic V(D)J recombination, mediated by the recombination-activating gene (RAG) products, is the most significant characteristic of adaptive immunity in jawed vertebrates. Flounder (Paralichthys olivaceus) RAG1 and RAG2 were isolated by Genome Walker and RT-PCR, and their expression patterns were analysed by RT-PCR and in situ hybridization on sections. RAG1 spans over 7.0 kb, containing 4 exons and 3 introns, and the full-length ORF is 3207 bp, encoding a peptide of 1068 amino acids. The first exon lies in the 5'-UTR, which is an alternative exon. RAG2 full-length ORF is 1062 bp, encodes a peptide of 533 amino acids, and lacks introns in the coding region. In 6-month old flounders, the expression of RAG1 and RAG2 was essentially restricted to the pronephros (head kidney) and mesonephros (truck kidney). Additionally, both of them were mainly expressed in the thymus. These results revealed that the thymus and kidney most likely serve as the primary lymphoid tissues in the flounder.