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The synthesis of biologically competent RNA by a purified Q,3-replicasel (an RNA-dependent RNA-polymerase induced by QO-RNA bacteriophage) has been reported in earlier publications.2 3 It was further established4 by the use of a temperature-sensitive mutant that the initiating template RNA is the instructive agent in the replicative process. Examination5 of the early events in the reaction revealed the existence of a six minute latent period which precedes the appearance of the first new infectious strands. The use of sucrose gradients' and, more informatively, electrophoresis in acrylamide gels6' I made possible a detailed analysis of the fate of the input templates during the latent period. The data showed8 that two classes of complexes containing the initiating templates materialized in the latent period. Their emergence is paralleled quantitatively by a concomitant loss of the template RNA as an infectious entity. The first complexes (HS) to appear correspond in their properties to the structures found in vivo by Francke and Hofschneider.9 The second class of complexes (FS) made their appearance one minute later and resemble the "replicative intermediate" isolated by Franklin10 from cells infected with R17. Both of these types of structures occur well before any Qf-RNA can be detected. While not as yet conclusive, the temporal order of their appearance is consistent with a mechanism which suggests that HS structures give rise to FS which, in turn, generate viral RNA. This series of events implies the existence of two activities and it was of obvious interest to see whether more direct enzymological evidence could be obtained to support this implication. As our experience with Qfl-replicase accumulated, persistent peculiarities were noted which suggested that the enzyme was separable into two components. It is the purpose of the present paper to detail the experiments which exploited these hints. The data show that the enzyme can in fact be dissociated into a light and heavy component by successive centrifugations in sucrose gradients. While neither component responds to added Qf-RNA, their combination reconstitutes a fully active enzyme system. Finally, the heavier component is associated with a remarkable ability to synthesize polyriboguanylic acid (poly G), an activity which is completely dependent on the addition of polyribocytidylic acid (poly C). Materials and Methods.-Enzyme, substrates, and assays: Purification of the Qfl-replicase followed with minor modifications the procedure of Haruna and Spiegelman.1 In the first step of the protamine fractionation, 0.011 mg (rather than 0.010) of protamine sulfate for each unit of OD260 was added. The resulting precipitate was dissolved in 10 per cent neutralized saturated ammonium sulfate in standard buffer (0.01 M tris, pH 7.4, 0.01 M MgCl2, and 0.005M mercaptoethanol), the volumes being adjusted to an OD260 of 650. After storage overnight, two volumes of standard buffer are added for each volume of 10 per cent ammonium

The dissociation of Q-beta-replicase and the relation of one of the components to a poly-C-dependent poly-G-polymerase.