Phage Qβ Replicase: Cell‐Free Synthesis of the Phage‐Specific Subunit and Its Assembly with Host Subunits to Form Active Enzyme

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Abstract
Cell-free translation of Qbeta RNA and subsequent partial purification of the enzyme resulted in replicase activity. From 0.5 to 1.5% of all R chains synthesised were found in the 7-S replicase complex. The presence in the 7-S complex of the host subunits of authentic replicase, i (= S1) and EF-Ts, was shown by the effect of antisera directed against ribosomal protein S1 and EF-Ts, respectively. Furthermore, the presence of EF-Ts was demonstrated by thermal denaturation of in vitro replicase made by a cell extract from an Escherichia coli mutant with a thermolabile EF-Ts. In vitro replicase did not assemble spontaneously during protein synthesis but was formed upon subsequent purification. Assembly could be induced by ammonium sulphate precipitation (60% saturation) alone. It is concluded that the functional phage-coded subunit synthesised in vitro recognises i and the EF-Tu - EF-Ts complex among a mixture of host proteins.