Ig heavy chain genes and their locus in grass carp Ctenopharyngodon idella

The cDNA and genomic sequences of IgD and IgZ were characterized in grass carp Ctenopharyngodon idella in the present study, and with the identification of a BAC clone covering ζ, μ, and δ genes, the IgH locus containing these Ig genes and other V, D, J genes was also illustrated in this fish. Secretory and membrane-bound IgZ were identified, with two transmembrane exons spliced within the CH4 exon, as reported in IgM of mammals and IgZ in other teleost fish. The first and second constant domains of IgZ shows more than 90% nucleotide identity with respective domains of grass carp IgM. The IgD has a structure of δ1–(δ2–δ3–δ4)2–δ5–δ6–δ7–TM–UTR, with the repeat of δ2–δ3–δ4; but intron was not found between the two repeat, i.e. between the first δ2–δ3–δ4 (δ2.1–δ3.1–δ4.1) and the second δ2–δ3–δ4 (δ2.2–δ3.2–δ4.2), and the intron between δ3.1 and δ4.1 was much shorter than the intron between δ3.2 and δ4.2. The genomic organization of the IgH locus has a pattern of Vn–Dn–Jn–Cζ–Dn–Jn–Cμ–Cδ, as reported in other teleost fish. Thirteen VH, fourteen D, and twelve JH genes were observed in this locus, with the similarity of three D segments and four JH segments being the same in the upstream of Cζ and Cμ. The transcriptional enhancer located at the μ–δ intergenic region was also analyzed and it seems possible that this enhancer is functional as verified in zebrafish and channel catfish.