Template-directed and template-free RNA synthesis by Q beta replicase.

In the absence of extraneously added templates, Q beta replicase produces different RNA species after long lag times spontaneously in vitro. The sequences of the spontaneous products are short (30 to 45 nucleotides) and bear little sequence relation to one another and no detectable sequence homology to Q beta virus RNA or to the host. Their replication rates are much lower than those of optimized products. Incubation without template in long closed capillaries produces after long lag times many separate RNA growth foci with heterogeneous kinetics. The template-free reaction is strongly dependent on the conditions: lowering the enzyme or the triphosphate concentrations abolishes the template-free RNA synthesis without affecting the template-dependent synthesis. An explanation of the emerging RNA species in template-free reactions by residual RNA contaminants in the incubation mixture is very unlikely in the light of the experimental evidence; however, the experimental evidence is fully compatible with a de novo mechanism (which may include instruction by non-replicatable oligonucleotides).