In vivo binding of trimethylpsoralen detects DNA structural alterations associated with transcribing regions in the human β-globin cluster

In order to increase our knowledge about the mechanisms that regulate expression of human β-like globin genes, we have used a novel technique to analyze the chromatin structure in living cells. This approach allowed us to detect specific DNA regions in vivo where nucleosome folding or unconstrained DNA supercoiling in erythroid cells differs from that in non-erythroid cells. In this method, we use 4,5',8-trimethylpsoralen (TMP) as a probe capable of detecting altered chromatin conformations. Our results show that TMP binds to DNA with a higher affinity over the regions in the locus that are actively expressed, including both the promoter and the transcribed region. This higher affinity detected when comparing erythroid cells with non-erythroid cells does not extend to other regions inside the β-globin cluster. Our data suggest that the observed effect is likely due to nucleosome displacement. Alternatively, it could result from localized DNA supercoiling, but not from widespread torsional stress across the entire β-like globin locus as hypothesized previously.