Probing the legitimate initiation of RNA synthesis by Qβ replicase with oligonucleotide primers.

Oligoribonucleotides complementary to the template 3' terminus were tested for their ability to initiate RNA synthesis on legitimate templates capable of exponential amplification by Qβ replicase. Oligonucleotides shorter than the distance to the nearest predicted template hairpin proved able to serve as primers, with the optimal length varying for different templates, suggesting that during initiation the template retains its native fold incorporating the 3' terminus. The priming activity of an oligonucleotide is greatly enhanced by its 5'-triphosphate group, the effect being strongly dependent on Mg ions. This indicates that, unlike other studied RNA polymerases, Qβ replicase binds the 5'-triphosphate of the initiating nucleotide GTP, and this binding is needed for the replication of legitimate templates.