Photoreaction of furocoumarins with DNA is similarly inhibited by minor and major groove-interacting ligands.

It was found that methyl green, a major groove binding ligand and the minor groove binding ligands, netropsin and 2,7-di-tert-butylproflavine inhibit, to a similar extend a monoadduct forming benzopsoralen and monoadduct and diadduct forming derivatives of psoralen (8-methoxypsoralen and 3,4'-dimethyl-8-methoxypsoralen). Caffeine exhibits an inhibitory effect on furocoumarin photobinding to DNA at 10(3) fold higher concentration. Together with the previously published results it is concluded that both occupancy of the major and minor groove as well as intercalation hinder photobinding of furocoumarins to DNA.

Thyroliberin and other neurohormones have no detectable direct effect on RNA synthesis in isolated nuclei.

Effect of thyroliberin, somatostatin, vasopressin and luliberin on RNA synthesis in isolated calf pituitary, brain cortex, rat liver and rat brain nuclei was assayed. None of the examined neuropeptides affected RNA synthesis.

Effect of intercalating and groove-binding ligands on formation of covalent complexes between nitracrine (Ledakrin, C-283) or 8-methoxypsoralen and DNA.

The effect of ethidium bromide, actinomycin D, distamycin A and netropsin on covalent binding of nitracrine (1-nitro-9-(3,3-N,N-dimethylaminopropylamino)acridine, Ledakrin, C-283) and 8-methoxypsoralen to DNA was examined. The competition was assayed either directly with [3H]- and [14C]nitracrine or indirectly by estimation of transcriptional template activity of nitracrine-DNA and 8-methoxypsoralen-DNA complexes formed in the presence of the ligands. A higher protective effect of ethidium bromide and distamycin on the photo-binding of 8-methoxypsoralen than on the dithiothreitol-dependent attachement of nitracrine to DNA assayed at 0.15 M KCl or NaCl was observed. The non-intercalating antibiotics showed lower competitive effect on 8-methoxypsoralen binding than ethidium bromide. Actinomycin D showed relatively low competition for both drugs with DNA. In contrast to the reaction of 8-methoxypsoralen, the decrease of nitracrine binding in the presence of competing ligands considerably depends on ionic strength. Particularly high inhibition of the adduct formation in the presence of ethidium at 1 M KCl was shown, while the amount of nitracrine bound in the presence of distamycin increases at elevated ionic strength. The results may indicate steric demands of the reaction between nitracrine and DNA.