Spheroplasts of Escherichia coli infected with RNA phage R17 were treated with actinomycin D to inhibit the synthesis of host RNA and protein. Ribosomes occurred predominantly as monomers, dimers and trimers which were active in viral protein synthesis. Partly finished strands of viral RNA in lysates sedimented at a rate characteristic of groups of 6-12 ribosomes and their buoyant density suggested that they were in fact attached to ribosomes. Newly completed strands were associated with single (70-S) ribosomes, forming a 75-S complex, and with ribosome dimers and trimers. Some were probably loosely bound to 30-S ribosomal subunits.
Newly made RNA began to accumulate in mature virions after a lag of about 5 min and rose to 16% of the total labelled viral RNA by the end of the growth cycle, either with continuous labelling or after a 5-min pulse followed by a non-radioactive chase. It is suggested that most, if not all, new viral RNA forms a rather stable association with ribosomes and that a small proportion of this RNA may subsequently pass into progeny virus.