A barrier-type insulator forms a boundary between active and inactive chromatin at the murine TCRβ locus.

In CD4(-)CD8(-) double-negative thymocytes, the murine Tcrb locus is composed of alternating blocks of active and inactive chromatin containing Tcrb gene segments and trypsinogen genes, respectively. Although chromatin structure is appreciated to be critical for regulated recombination and expression of Tcrb gene segments, the molecular mechanisms that maintain the integrity of these differentially regulated Tcrb locus chromatin domains are not understood. We localized a boundary between active and inactive chromatin by mapping chromatin modifications across the interval extending from Prss2 (the most 3' trypsinogen gene) to D(β)1. This boundary, located 6 kb upstream of D(β)1, is characterized by a transition from repressive (histone H3 lysine 9 dimethylation [H3K9me2]) to active (histone H3 acetylation [H3ac]) chromatin and is marked by a peak of histone H3 lysine 4 dimethylation (H3K4me2) that colocalizes with a retroviral long terminal repeat (LTR). Histone H3 lysine 4 dimethylation is retained and histone H3 lysine 9 dimethylation fails to spread past the LTR even on alleles lacking the Tcrb enhancer (E(β)) suggesting that these features may be determined by the local DNA sequence. Notably, we found that LTR-containing DNA functions as a barrier-type insulator that can protect a transgene from negative chromosomal position effects. We propose that, in vivo, the LTR blocks the spread of heterochromatin, and thereby helps to maintain the integrity of the E(β)-regulated chromatin domain. We also identified low-abundance, E(β)-dependent transcripts that initiate at the border of the LTR and an adjacent long interspersed element. We speculate that this transcription, which extends across D(β), J(β) and C(β) gene segments, may play an additional role promoting initial opening of the E(β)-regulated chromatin domain.