Heavy chain joining region segments of the channel catfish. Genomic organization and phylogenetic implications.

The JH locus of the channel catfish has been characterized to determine the organization and structural diversity of JH segments. These analyses indicate that there are a total of nine JH segments tightly clustered within a region spanning about 2.2 kb. The JH locus is closely linked to the CH 1 domain of the expressed catfish H chain; the distance between the CH proximal JH segment (JH9) and the CH 1 domain is about 1.8 kb. Each JH segment has an upstream recombination sequence, which includes a T-rich nonamer, a 22- to 24-bp spacer, and a phylogenetically conserved heptamer. Each JH segment also has an open reading frame that encodes the conserved framework region 4 tryptophan (Trp-103) and terminates with a RNA donor splice site. The catfish JH locus contains an internal repetitive sequence region characterized by a short (183-188 bp) repeat that occurs sequentially five times. Strong sequence homology as well as the unified length of the repeated sequences indicate that JH segments JH3-JH7 probably arose as the result of a series of homologous but unequal crossover events. Sequence alignments of the duplicated JH segments indicates that there is diversity within the 5-11 nucleotides located immediately downstream from the heptamer, an observation which indicates that closely related JH segments can serve to enhance CDR3 diversity in the expressed H chain. Comparisons of the genomic JH sequences with different cDNA clones indicate that each JH segment is probably functional and that junctional diversity serves an important role in the generation of CDR3 diversity. In addition, single base differences observed in comparisons of JH-encoded regions indicate that there is probably somatic mutation or allelic variation of genomic JH segments. These studies suggest that the characteristic structure and organizational pattern of JH segments in higher vertebrates may have evolved early in vertebrate phylogeny at the level of the bony fish.