Control of V(D)J recombinational accessibility of the D beta 1 gene segment at the TCR beta locus by a germline promoter

The germline promoter region upstream of the D beta 1 gene segment in the m urine TCR beta locus was deleted to assess its role in controlling V(D)J re combination. Associated with diminished D beta 1 region germline transcript ion, rearrangement of the D beta 1 but not the D beta 2 gene segment was re duced 10- to 20-fold. A corresponding reduction in RAG-mediated cleavage at the D beta 1 and J beta 1 signal sequences was apparent only when purified CD4(-)CD8(-) thymocytes were analyzed because, as we demonstrate, cleavage at these gene segments also occurred in CD4(+)CD8(+) thymocytes. These fin dings suggest that germline promoters regulate localized accessibility of g ene segments for recombination and that in CD4(+)CD8(+) thymocytes TCR beta allelic exclusion does not result from inaccessibility of D beta gene segm ents.