Clustered and interspersed gene families in the mouse immunoglobulin x locus
Although numerous solitary germ-line Vx genes and two small Vx contiguously cloned gene regions (contigs) are known, no attempts to systematically elucidate the structure of the x locus of the mouse have been reported so far. As a first step to this aim we screened a cosmid library of C57BW6J mouse DNA with 18 probes that are more or less specific for the different Vx gene families. Ninetyone Vx gene-containing cosmid clones were characterized by detailed restriction mapping and hybridizations. Several contigs were constructed from overlapping clones. The contigs and the still unlinked cosmid clones cover 1.6 Mb. Many of the cosmid clones were localized on chromosome 6 where the x locus is known to reside; no evidence for the existence of dispersed Vx genes (orphons) was obtained. Eighty-five strong hybridization signals were assigned to distinct Vx gene families, while for 11 weak signals the assignment was less definite. As to the distribution of gene families within the locus the following situation emerged: there are both, groups of genes which belong to one Vx gene family ("clusters") and groups in which genes of different families are interspersed. The interspersion of gene families seems to be more pronounced than has been assumed so far. Additional Vx genes which are known to exist will have to be isolated from other gene libraries of the same mouse Igx haplotype.