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

cDNA studies have distinguished two isotypes of the rainbow trout (Oncorhynchus mykiss) immunoglobulin (Ig) light chain (designated L1 and L2). This study characterized genomic clones of these isotypes. L1 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 rearranged by inversion. L2 is also organized in clusters, consisting of two or three V, one J, and one C exon, all in the same transcriptional orientation. L1 and L2 of rainbow trout are similar to the previously identified cod and catfish 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 conserved kappa sequence motifs of mammalian Jkappa1 genes. Sequence analyses showed that the regions upstream of L1 and L2 genes have several putative cis-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' region 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 L1 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.