THE GENOMIC ORGANIZATION OF IMMUNOGLOBULIN-VH GENES IN XENOPUS-LAEVIS SHOWS EVIDENCE FOR INTERSPERSION OF FAMILIES

The complete genomic sequences of Xenopus laevis immunoglobulin heavy chain variable region (V(H)) genes comprising families IV-XI are reported. Using V(H) family-specific probes, linkage relationships for Xenopus V(H)I-V(H)XI have been determined. With the possible exceptions of V(H)III and V(H)VII, Xenopus V(H) genes appear to be interspersed. When from two to five V(H) segments are identified in individual clones, the elements are found to be in the same relative transcriptional orientation. Although the relationships of promoter sequences, including the regulatory octamer, resemble those seen in other vertebrate V(H) genes, several Xenopus V(H) families are associated with additional 5' octamer sequences and octamer-like motifs. The similarities between the genomic organization of V(H) genes in Xenopus and higher vertebrates, indicate an early phylogenetic emergence of the mammalian-type of gene organization and regulation.