Molecular characterization of meiotic recombination within the major histocompatibility complex of the mouse: mapping of crossover sites within the I region.

The molecular analysis of crossing-over within the mouse major histocompatibility complex provides a useful approach for the study of the structural characteristics of meiotic recombination. In this study five intra-I-region recombinants, each derived from Ik/Ib heterozygotes, were characterized for restriction-fragment length polymorphisms (RFLPs) characteristic of the I region of the two parental strains. Southern blot analysis of intra-I recombinant strains A.TBR2, A.TBR3, A.TBR5, A.TBR13, and A.TBR17 using six I-region DNA probes revealed that the point of crossing-over in all five recombinants occurred within a 6.2-kb KpnI-EcoRI segment located within the E beta gene. The segments of DNA containing the crossover point from each of the recombinant chromosomes were cloned by screening partial genomic libraries constructed in lambda gt7 bacteriophage. Construction of partial restriction maps of the cloned segments from the parental and recombinant chromosomes permitted the boundaries of the area containing the crossover site to be narrowed to a 4.0-kb segment located almost entirely within an intron of the E beta gene. The recognition that the points of crossing-over in all five recombinants studied are clustered in a relatively small area of the I region provides further evidence for a hot spot of recombination associated with the E beta gene.