On a chemically defined growth medium the degree of substitution of thymidine with 5-bromodeoxyuridine (BUdR) in DNA of Tetrahymena pyriformis was controlled by the concentration of tetrahydrofiolic acid, BUdR and thymidine in the medium. A correlation between the degree of BUdR substitution in DNA and the reduction in rate of total RNA synthesis has been established. It was found that the reduction of total RNA synthesis results from inhibition of transcription of all RNA species which have been measured. However, independent of the degree of BUdR substitution in DNA, a preferential inhibition of the synthesis of 25s and 17s ribosomal RNA was found. It is concluded that the various genes may respond differently to BUdR substitution with respect to transcription.

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