Reversible inhibition of gene expression by a psoralen functionalized triple helix forming oligonucleotide in intact cells.

Triple helix formation of nucleic acids is the most rational approach to designing site-specific transcription inhibitors. To increase their efficiency, reactive moieties such as psoralen or ethenocytosine have been introduced on the third strand. In transfected cells, these compounds induce a site-specific covalent binding of the third strand to the targeted sequence and efficiently block RNA polymerases. However, the stability of this transcription inhibition has never been checked. We have designed a plasmid containing a triple helix binding site in the coding region of the beta-galactosidase reporter gene and a polymerase chain reaction assay to follow quantitatively the cross-link of a psoralen-derivatized third strand in transfected cells. This assay has revealed that the cross-link was removed within a few hours, leading only to a transitory inhibition of gene expression. Control experiments in DNA repair-deficient cells suggest the implication of repair enzymes in this process.