The use of psoralen-modified DNA to probe the mechanism of enhancer action.

Psoralen-modified DNA was used to study the SV40 enhancer-dependent transcription of the human beta-globin gene. When the enhancer is separated from the beta-globin gene by psoralen adducts on one side and plasmid vector sequences on the other, expression of the gene is strongly inhibited. When placed on the same side of the enhancer as the vector sequences, psoralen adducts have little effect on transcription unless they are located near the transcriptional start site. These results suggest that the inhibition of the transcription of a gene linked to an enhancer in a circular DNA requires the presence of blocking agents on both sides of the gene and that the vector sequences are already blocking enhancer action on one side. Psoralen monoadducts are sufficient to inhibit transcription; the formation of interstrand psoralen cross-links is unnecessary. We assess models for the enhancer mechanism in light of these results.