Genomic organization and sequences of immunoglobulin light chain genes in a primitive vertebrate suggest coevolution of immunoglobulin gene organization

The genomic organization and sequence of immunoglobulin light chain genes in Heterodontus francisci (horned shark), a phylogenetically primitive vertebrate, have been characterized. Light chain variable (V(L)) and joining (J(L)) segments are separated by 380 nucleotides and together with the single constant region exon (C(L)), occupy < 2.7 kb, the closest linkage described thus far for a rearranging gene system. The V(L) segment is flanked by a characteristic recombination signal sequence possessing a 12 nucleotide spacer; the recombination signal sequence flanking the J(L) segment is 23 nucleotides. The V(L) genes, unlike heavy chain genes, possess a typical upstream regulatory octamer as well as conserved enhancer core sequences in the intervening sequence separating J(L) and C(L). Restriction mapping and genomic Southern blotting are consistent with the presence of multiple light chain gene clusters. There appear to be considerably fewer light than heavy chain genes. Heavy and light chain clusters show no evidence of genomic linkage using field inversion gel electrophoresis. The findings of major differences in the organization and functional rearrangement properties of immunoglobulin genes in species representing different levels of vertebrate evolution, but consistent similarity in the organization of heavy and light chain genes within a species, suggests that these systems may be coevolving.