A TRANSCRIPTION INHIBITOR SPECIFIC FOR UNWOUND DNA IN RNA-POLYMERASE PROMOTER OPEN COMPLEXES

The kinetically competent open complexes formed at prokaryotic and euk aryotic transcription start sites are efficiently nicked by the chemic al nuclease activity of the 2:1 1,10-phenanthroline.copper(I) complex [(OP)2Cu+] and hydrogen peroxide. This reaction specificity has been a ttributed to the creation of a binding site(s) for redox-active tetrah edral (OP)2Cu+ when RNA polymerases form productive complexes with pro moters. This proposal has been confirmed for the Escherichia coli lac UV-5 promoter by the demonstration that the 2:1 2,9-dimethyl-1,10-phen anthroline.copper(I) complex [(Me2OP)2Cu+], a redox-inactive isostere of (OP)2Cu+, protects the transcription start site from scission by th e chemical nuclease activity. (Me2OP)2Cu+ is also an effective inhibit or of transcription. The inhibition of transcription and the protectio n from scission of the open complex by (OP)2Cu+ exhibit the same depen dence on the concentration of (Me2OP)2Cu+. This redox- and exchange-st able species is a previously undescribed transcription inhibitor that binds to a site generated by the interaction of RNA polymerase with th e promoter. Unlike the intercalating agent proflavine, which is also a n effective transcription inhibitor, it does not displace the enzyme f rom the promoter. The ability of (Me2OP)2Cu+ to inhibit transcription may be partially responsible for its potent cytotoxicity.