Photosensitization of SV 40 DNA mediated by promazine derivatives and 4'-hydroxymethyl-4,5',8-trimethylpsoralen. Inhibition of the in vitro transcription.

In vitro transcription by E. coli RNA polymerase was carried out on SV40 DNA photoreacted with various promazine derivatives. Inhibition of the template activity was recorded with increasing irradiation times in the presence of promazine derivatives. Promazine covalent adducts on guanine did not terminate RNA synthesis and seemed to be bypassed by the enzyme. HMT (4'-hydroxymethyl-4,5',8-trimethylpsoralen) photoreaction with DNA was carried out under two conditions: irradiation with lambda greater than 395 nm favouring monoadduction on pyrimidine residues and irradiation at 360 nm inducing a maximum of interstrand diadducts. Both adducts were able to terminate RNA synthesis on the phototreated SV40 DNA and using the O-methyl-nucleotide sequencing procedure, the termination sites were precisely mapped. Monoadducts on the coding strand and cross-links induced termination two bases away from the covalent adduct, but monoadducts on the noncoding strand did not half RNA polymerase.