Studies of the binding of actinomycin and related compounds to DNA

Cell killing by actinomycin D in relation to the growth cycle of chinese hamster cells

Using Chinese hamster cells in culture, we have measured the effectiveness of actinomycin D to suppress division as a function of the position, or age, of a cell in its growth cycle. Cells were first exposed to millimolar concentrations of hydroxyurea in order to produce a synchronized population just before the onset of DNA synthesis. Thereafter, the survival response after 30 min exposures to actinomycin D was measured. Cells become resistant as they enter the S phase and then sensitive again in the latter part of S. When they reach G(2) (or G(2)-mitosis) they are maximally resistant; at 1.0 microg/ml, for example, the survival in G(2) is 30-fold greater than it is in G(1). These results, plus measurements reported earlier on the interaction of damage in S cells due to actinomycin D and X-irradiation, suggest that the age-response pattern of the toxic effects of this drug probably reflects both the functional capacity of DNA-actinomycin complexes and the ability of this antibiotic to penetrate chromatin and bind to DNA.

Transcriptional mapping: functional organization of the ribosomal and transfer ribonucleic acid cistrons in the Bacillus subtilis genome

The transcriptional mapping method developed previously has been used to investigate the functional organization of the ribosomal ribonucleic acid (rRNA) and transfer ribonucleic acid (tRNA) cistrons in the Bacillus subtilis genome. The sensitivities of the syntheses of these RNA species to actinomycin D indicated that (i) large polycistronic groupings of the tRNA cistrons exist, but (ii) the 16S and 23S rRNA cistrons are probably transcribed as individual units, and (iii) each 5S rRNA cistron is part of an operon containing a 23S rRNA cistron.