Q beta replicase template specificity: different templates require different GTP concentrations for initiation.

Abstract

Q.beta. replicase is notable for its high degree of template specificity. It was shown to transcribe Q.beta. RNA and synthetic polymers containing cytidylate. Other natural RNA are not transcribed unless Mn2+ ions are present. The enzyme initiates all RNA synthesis with GTP. Q.beta. replicase can transcribe heterologous natural RNA species in theabsence of Mn2+ if sufficient GTP is present. Each RNA tested requires a different GTP concentration for initiation. The site for the initiating nucleoside triphosphate on Q.beta. replicase is strongly influenced by the template. The high degree of template specificity is a consequence of the fact that different templates induce initiation sites with varying affinities for GTP. Two lines of evidence support this idea. Mn2+ which reduce template specificity, reduce the concentration of GTP required for initiation. High ionic strength, which decreases transcription of all templates except Q.beta. RNA, increases the GTP requirement. The possibility is considered that variable promoter or ribosome binding site strengths could result from a mechanism similar to that proposed here.