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The photodynamic inactivation of a cell-free transcriptional system with yeast components was investigated in vitro. The photodynamic inhibition of the overall synthesis is dependent on the irradiation time and is due to the photo-oxidation of the DNA as well as of the RNA polymerase. Photodynamic treatment of the polymerase and DNA together leads to a stronger inactivation of their biological activity than photodynamic treatment of both components separately. Also the rate of RNA synthesis is more affected when the complete system is photodynamically treated shortly after beginning than before beginning of RNA synthesis. It is concluded that the polymerase-DNA complex is more sensitive to photo-oxidation than the individual components. Because the steps binding and initiation do not seem very affected photodynamically it is suggested that the Thiopyronine-sensitized inhibition of RNA synthesis is mainly due to an inhibition of elongation.

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