Different genomic organization and expression of immunoglobulin light-chain isotypes in the rainbow trout

cDNA studies have distinguished two isotypes of the rainbow trout (Oncorhyn chus mykiss) immunoglobulin (Ig) light chain (designated L1 and L2). This s tudy characterized genomic clones of these isotypes. LI genes are arranged in clusters with single copies of variable (V), joining (J), and constant ( C) segments. The transcriptional orientation of the V genes is opposite to that of the J and C segments, indicating that the V genes must be rearrange d by inversion. L2 is also organized ill clusters, consisting of two or thr ee V one J, and one C exon, all in the same transcriptional orientation. LI and L2 of rainbow trout are similar to the previously identified cod and c atfish clusters. Repeat sequences were found upstream of each J segment in the L2 genes, each of which includes a 16-bp sequence similar to the conser ved kappa sequence motifs of mammalian J kappa 1 genes. Sequence analyses s howed that the regions upstream of LI and L2 genes have several putative ci s-acting elements also present in the promoter regions of Ig genes of other organisms. Octamer motifs, a TATA box, and an E-box were found in the 5' r egion of an IgL1V gene. A kappa-Y element, a CCCT element, a TATA box, an E -box but no classical octamer were found in the 5' region of the IgL2 gene. Northern blot analyses showed that LI and L2 are expressed in spleen, head kidney, excretory kidney, thymus, and heart. The expressed ratio of L1 and L2 is estimated to 85:15% in blood and lymphoid tissues.