Anthramycin and actinomycin D, two different types of DNAbinding antibiotics, were compared for their effects on RNA synthesis in suspension cultures of mouse leukemia L1210 cells. RNA was labeled with radioactive uridine and selective effects on the synthesis of different classes of RNA were studied by sucrose gradient sedimentation of RNA purified from whole cells or from nucleolar and nucleoplasmic fractions.
Two major differences were noted.
(1) Whereas actinomycin produced two phases in the inhibition of uridine incorporation, the rapid phase being complete within a few minutes, anthramycin produced only a slow progressive inhibition. (2) Whereas actinomycin selectively inhibits nucleolar 45s RNA synthesis, anthramycin inhibits this RNA equally to the inhibition of the same size RNA in the nucleoplasm.
Both antibiotics caused a shift towards lower molecular weight (slower sedimentation) i n the distribution of nucleoplasmic RNA molecules synthesized in the presence of drug. When the two antibiotics were compared at concentrations producing equal extents of inhibition of nucleoplasmic RNA synthesis, anthramycin produced the greater shift. The shift in sedimentation was not due to a slowing of RNA chain growth rate, since the change in sedimentation persisted when uridine incorporation time was increased so as to compensate for the reduction in RNA synthesis rate.
The selective inhibition of nucleolar RNA synthesis by actinomycin could be due to possible differences in the properties of the different RNA polymerases,