Regulation of protein synthesis directed by coliphage MS2 RNA: II. In vitro repression by phage coat protein

Abstract

In vitro protein synthesis directed by bacteriophage MS2 RNA is inhibited by prior incubation of the RNA with MS2 coat protein. This inhibition involves exclusively non-coat proteins as shown by the selective decrease in incorporation of histidine, which is absent in phage coat protein, and by the results of acrylamide gel fractionation of the cell-free protein products. The specificity of the inhibitory effect is shown by the failure of MS2 coat protein to inhibit protein synthesis directed by coliphage Qβ RNA, by the lack of activity of Qβ coat protein with MS2 RNA, and by the indiscriminate inhibition of phage protein synthesis by the polycation polylysine. Coat protein inhibition was correlated with the formation of a complex between MS2 RNA and a small number of MS2 coat protein molecules (complex I of Sugiyama, Hebert & Hartman, 1967). Qβ RNA failed to form such a complex with MS2 coat protein, although it did form a complex with a high protein to RNA ratio. On the basis of these observations we suggest that coat protein inhibition results from the binding of a small number of protein molecules to the phage RNA at one or more sites outside the coat protein gene and prevents the translation of the other phage genes.