Newly made viral RNA in lysates of Escherichia coli infected with phage R 17 was studied as a model messenger RNA. The viral RNA that was associated with groups of 1, 2, 3 and 4 ribosomes banded in caesium chloride at densities of approximately 1.68, 1.65, 1.645 and 1.635 g cm-3 respectively and the single ribosomes of uninfected cells at 1.62. The density of infected ribosomes was restored to 1.62 by treatment with RNase. These values are consistent with the presence of a single molecule of viral RNA in each group.
In uninfected bacteria the mean density of the smaller polysomes exceeded that of actinomycin-treated or RNase-treated ribosomes by about 0.005 g cm-3. Similarly, pulse-labelled RNA associated with single ribosomes or pairs banded at a density only marginally higher than that of the bulk of the single ribosomes. If any RNA was attached to the ribosomes it probably amounted to not more than one fifth of the size of the phage RNA for each ribosome pair.