STRUCTURE AND GENOMIC ORGANIZATION OF IMMUNOGLOBULIN LIGHT-CHAIN IN THE CHANNEL CATFISH - AN UNUSUAL GENOMIC ORGANIZATIONAL PATTERN OF SEGMENTAL GENES

Channel catfish L chain cDNA was obtained through a PCR strategy and u sed to isolate multiple L chain clones from cDNA and genomic libraries . Sequence analysis of full-length cDNA indicates that the V region is preceded by a leader peptide, and represented by framework and CDR re gions. Both VL and CL domains contain the invariant cysteines and tryp tophans as well as other phylogenetically conserved L chain residues. The sequence similarity of the catfish L chain with higher vertebrate kappa- and lambda-chains, however, does not readily allow the catfish L chain to be classified. Eight cDNA clones isolated from a cDNA libra ry were shown to represent different processed derivatives of sterile L chain transcripts. These transcripts share a similar upstream sequen ce region and extend downstream to include a CL or alternatively a JL segment in partial germ-line configuration that has been spliced into a CL. Sequence comparisons indicate that these transcripts represent t he product of different L chain loci. Genomic Southern blot analyses w ith VL and CL probes indicate that there are at least 30 VL segments a nd at least 15 CL segments. The analysis of 17 genomic L chain clones showed that each hybridized with VL-, JL-, and CL-specific probes. Cha racterization of the gene segments in three of these clones indicates a previously undescribed pattern of segmental gene organization. Gene segments are found in clusters with VL, JL, and CL segments in each cl uster. Within a cluster VL segments reside upstream of single copies o f closely linked JL and CL segments. The proximity of VL segments down stream from JL-CL segments suggests that individual clusters may be cl osely linked. The VL segments are located in opposite transcriptional polarity relative to the JL and CL gene segments, which indicates that VL segments are likely rearranged to JL-CL segments by inversion rath er than deletion events.