Distinct control of the frequency and allelic exclusion of the V beta generearrangement at the TCR beta locus

Ag receptor gene loci contain many V gene segments, each of which is recomb ined and expressed at a different frequency and is subject to allelic exclu sion. To probe the parameters that mediate the different levels of regulati on of V gene rearrangement, a V beta gene segment together with 3.6-kb 5 ' and 0.7-kb 3 ' flanking sequences was inserted 6.8 kb upstream of the D bet a1 gene segment in the murine TCR beta locus. Despite its proximity to the DIS gene segments and the E beta enhancer, the inserted V beta segment unde rwent VDJ recombination at the same frequency as the natural copy located 4 70 kb upstream. However, the inserted V beta segment was no longer under al lelic exclusion control as it recombined at a similar frequency in the pres ence of a TCR beta transgene. These results suggest that while the inserted fragment contains the necessary cis-regulatory elements for determining th e frequency of V beta rearrangement, additional cis-regulatory elements are required for mediating V beta allelic exclusion. Interestingly, most of th e inserted V beta rearrangements were not transcribed and expressed in the presence of a TCR beta transgene, suggesting that TCR beta allelic exclusio n can also be achieved by blocking the transcription of the rearranged gene segments. These findings provide strong evidence for distinct control of t he frequency and allelic exclusion of V beta gene rearrangement.