Complete nucleotide sequences of three VH genes in Caiman, a phylogenetically ancient reptile: evolutionary diversification in coding segments and variation in the structure and organization of recombination elements

Abstract

Complete nucleotide sequences are described for three caiman (Caiman crocodylus crocodylus) immunoglobulin VH genes (C3, E1, and G4) that hybridize with a murine VH probe. The E1 and G4 genes are physically linked (intergenic distance, approximately equal to 6.5 kilobases) in the same transcriptional orientation but are not directly contiguous with the C3 gene. When the coding segments, including both framework and complementarity-determining regions, of these genes and the murine probe sequences are compared by metric analysis, it is apparent that the caiman genes are only slightly more related to each other than to the mammalian sequence, consistent with significant preservation of nucleotide sequence over an extended period of phylogenetic time. Based on the presence of transcriptionally critical 5' sequences and the absence of terminator codons, frameshift mutations, or other recognizable alterations, the genes do not appear to be pseudogenes. The E1 gene, however, is distinguished from other VH genes because (i) the spacer region within the 3' recombination signal sequence is 12 base pairs, typical of VK genes but not of VH genes, which possess 22- to 23-base-pair spacers and (ii) a near-perfect VH recombination signal sequence is present within the intervening sequence that splits the segment encoding the leader. These studies establish VH gene multiplicity in a species that arose prior to mammalian radiation and provide a description of differences in the configuration and location of recombination elements associated with an otherwise potentially functional gene.