Molecular mapping of murine I region recombinants. III. Crossing over at two discrete sites within the beta 1-beta 2 intron of the E beta gene.

The murine MHC provides a unique genetic system for studying meiotic recombination. A large number of murine H-2 recombinants cross over within a stretch of the E beta gene referred to as the E beta hot spot. The crossing over of eight such recombinants, derived from the s and k haplotypes, was studied at the nucleotide level. A 3-kb stretch of DNA, 3' to the beta 1 exon of the E beta gene, was sequenced after amplification of the genomic DNA from B10.S (one of the parental strains) by polymerase chain reaction. A number of sequence variations were identified with respect to B10.A (the other parental strain). Examination of these sequence variations by RFLP, simple sequence length polymorphism, as well as direct sequencing after polymerase chain reaction-amplification of genomic DNA from the recombinants led to unambiguous identification of the cross-over sites. Although all eight recombinants crossed over within the beta 1-beta 2 intron, two discrete nonoverlapping sites were involved. Five of the recombinants B10.BASR1, B10.ASR1, B10.ASR12, B10.HTT, and B10.S(9R) crossed over within a maximum of 395 bp of DNA 3' to the beta 1 exon. The remaining three recombinants B10.ASR7, B10.ASR11, and B10.S(8R) crossed over within 950 bp of DNA, adjacent to the cross-over site noted above. Each of these stretches of DNA was completely identical in the two parental haplotypes precluding further dissection of the cross-over sites. These cross-over sites are within those reported for the b and k recombination.