STRUCTURE AND GENOMIC ORGANIZATION OF A 2ND CLASS OF IMMUNOGLOBULIN LIGHT-CHAIN GENES IN THE CHANNEL CATFISH

Earlier studies distinguished two classes of catfish light (L) chain ( designated F and G). The cDNA structure and genomic organization of G L chain gene clusters has also been characterized previously. In this study, full length cDNA encoding F L chain was derived using PCR strat egies based on the determined amino-terminal protein sequence. The enc oded V region is readily delineated into framework regions (FR) and co mplementarity-determining regions (CDR). Multiple sequence alignments indicate that the F V-L is closely related to kappa gene families. The F C-L cannot be generally classified but it is structurally distinct from the C-L regions of G: the amino acid sequence similarity is <35%. cDNA sequences representing processed sterile F transcripts of differ ent loci were identified. Each sequence begins within the J(L) recombi nation signal sequence and extends downstream through the J(L)-C-L seg ments. Genomic blots hybridized with C-L probes indicate that there ar e at least 50 different C-L segments. Based upon V-L hybridization stu dies, different families of V-L segments appear to be associated with closely related F C-L segments. In characterized genomic clones, F gen e segments are arranged in closely linked clusters with single copies of V-L,J(L), and C-L segments within each cluster. The V-L segments ar e located in opposite transcriptional polarity relative to the J(L) an d C-L segments, which indicates that V-L segments rearrange by inversi on. These combined studies establish that two structurally distinct cl asses of L chains are present in teleost fish and that both of the L c hain classes evolved within a common organizational pattern of cluster ed segmental genes.