Abstract
Ag receptor genes are assembled through somatic rearrangements of V, D, and J gene segments. This process is directed in part by transcriptional enhancers and promoters positioned within each gene locus. Whereas enhancers coordinate reorganization of large chromatin stretches, promoters are predicted to facilitate the accessibility of proximal downstream gene segments. In TCR beta locus, rearrangement initiates at two D-J cassettes, each of which exhibits transcriptional activity coincident with DJ rearrangement in CD4/CD8 double-negative pro-T cells. Consistent with a model of promoter-facilitated recombination, assembly of the DJbeta1 cassette is dependent on a Dbeta1 promoter (PDbeta1) positioned immediately 5' of the D. Assembly of DJbeta2 proceeds independent from that of DJbeta1, albeit with less efficiency. To gain insight into the mechanisms that selectively alter D usage, we have defined transcriptional regulation at Dbeta2. We find that both DJbeta cassettes generate germline messages in murine CD44+CD25- double-negative 1 cells. However, transcription of unrearranged DJbeta2 initiates at multiple sites 400-550 bp downstream of the Dbeta2. Unexpectedly, loci from which germline promoter activity has been deleted by DJ rearrangement redirect transcription to sites immediately 5' of the new DJbeta2 joint. Our analyses suggest that 3'-PDbeta2 activity is largely controlled by NF-kappaB RelA, whereas 5'-PDbeta2 activity directs germline transcription of DJbeta2 joints from initiator elements 76 bp upstream of the Dbeta2 5' recombination signal sequence. The unique organization and timing of Dbeta2 promoter activity are consistent with a model in which promoter placement selectively regulates the rearrangement potential of Dbeta2 during TCR beta locus assembly.