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The hypothesis that protein synthesis in bacteria is mediated by an unstable, rapidly turning over RNA fraction, designated messenger RNA (mRNA),' is well supported by evidence from a variety of experiments. When either phage-in-fected2-4 or normal5' 6 bacterial cells are given short pulses of radioactive RNA precursors, the radioactivity is incorporated into an unstable RNA having a base composition similar to that of DNA and able to form molecular hybrids with it.' This RNA fraction is distinguishable from the ribosomal and transfer RNA by its sedimentation behavior and by its ability to stimulate protein synthesis in cell-free ribosomal systems.8 9 Experimental evidence as to the fate of mRNA molecules has been meager however and is subject to various interpretations. 10 Furthermore, the number of times a given mRNA molecule functions during the formation of protein has not been determined. The elucidation of both of these problems depends upon preventing the reutilization of any breakdown products of mRNA either to form more messenger or to form stable RNA. A substance which might be suitable for this purpose is actinomycin D, an inhibitor of RNA synthesis." Actinomycin D is known to attach to DNA'2 and to prevent RNA synthesis in Staphylococcus, Neurospora,'3 mammalian cells, and in cell-free systems."' 14 27 It allows formation of RNA viruses in mammalian cells, but not of DNA viruses. 15, 27 It seems likely, therefore, that it inhibits specifically DNA-dependent RNA formation.'4' 27

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