Printed in Great Britain INHIBITION OF rRNA SYNTHESIS FOLLOWING INCORPORATION OF 5-BROMODEOXYURIDINE INTO DNA OF

Tetrahymena pyriformis was grown on chemically defined medium in the presence of 5-bromodeoxyuridine (BUdR). Under these growth conditions more than 60 % of the thy-midine sites in DNA were substituted with BUdR. It was found that RNA synthesis was strongly inhibited by the presence of BUdR in DNA. To assure that incorporation of BUdR into DNA was a prerequisite of the effect observed, BUdR was added to synchronously dividing cells. BUdR had no effect on the cells when present outside the period of nuclear DNA replication, whereas RNA synthesis was strongly inhibited as soon as the genes coding for ribosomal RNA had replicated in the presence of BUdR