RNA-dependent RNA polymerases of plants

Comparative studies on ribonucleic acid dependent RNA polymerases in cucumber mosaic virus infected cucumber and tobacco and uninfected tobacco plants

RNA-dependent RNA polymerases have been isolated in almost pure form from cucumber mosaic virus (CMV) infected cucumber cotyledons and from tobacco leaves and were compared with the less pure enzyme from uninfected tobacco. The purified polymerase from cucumber shows on sodium dodecyl sulfate gels two peptide chains of about 100 and 112 kdaltons. The enzyme from tobacco shows a close doublet of about 125 kdaltons, which is also present in the less pure preparation from healthy tobacco. While both the cucumber and tobacco enzymes can use many polynucleotides and RNAs as templates, considerable quantitative differences exist, poly(C) being by far the most effective template for the cucumber enzyme but of low activity with the tobacco enzyme and poly(UG) being highly active with the latter but not the former. Poly(A) and poly(G) are inactive. Different viral RNAs, including CMV RNA, show smaller differences. The sedimentation rates of the enzyme from both sources are the same as that of gamma-globulin. A uridine 5'-triphosphate (UTP) terminal transferase also present in both plants sediments much more slowly and can be completely removed from the RNA polymerases. However, slight nucleolytic activity remains associated with the purified polymerases and appears to be proportional to the polymerase activity. The conclusion derived from these data is that the RNA-dependent RNA polymerases of different plants differ and are not detectably affected by virus infection in qualitative terms while being produced in greatly increased amounts upon some virus infections. Similar conclusions were previously reached with less purified enzyme preparations from tobacco as compared to cowpea, infected with different viruses, if any.

In vitro transcription of viroid RNA into full-length copies by RNA-dependent RNA polymerase from healthy tomato leaf tissue

RNA-dependent RNA polymerase from healthy tomato plant tissue accepts potato spindle tuber viroid (PSTV) RNA as a template for the in vitro synthesis of full-length RNA copies of the PSTV genome. Viroid transcription requires the presence of Mn2+ and /or Mg2+ ions and is not inhibited by concentrations of 10(-5) M alpha-amanitin. This is the first report of a well-defined product synthesized in vitro by an RNA-dependent RNA polymerase from healthy plants.

Heterogeneous chromatographic behaviour or soluble RNA replicase from healthy and virus-infected tobacco leaves: an improvement of the purification methodology

During purification of soluble RNA replicase from healthy and alfalfa mosaic virus-infected tobacco leaves, crude extracts were chromatographed on DEAE-Sephadex A-25, and three RNA-dependent RNA polymerase activities were obtained; one of these was previously characterized as a true RNA replicase, the remaining two were not studied (Chifflot et al., 1980, Virology 100, 91). We now demonstrate that all of these activities correspond to the same RNA replicase, complexed or not to cellular or viral RNA. Upon chromatography on DEAE-Sephadex A-25, the free replicase did not bind to the gel at low ionic strength, whereas the RNA-replicase complexes were bound to the gel through the medium of RNA. Increasing the ionic strength allowed the dissociation of the complexes and elution of the replicase only. From this observation, a new and rapid purification procedure combining all these activities and yielding large amounts of replicase was developed; the first main step of purification was chromatography on Blue-Sepharose CL6B under conditions conductive to the dissociation of RNA-replicase complexes, and thus maximal adsorption of the replicase. The enzyme was then eluted by increasing the ionic strength and was further purified on coupled DEAE-Sephadex A-25 and phosphocellulose columns. The DEA-Sephadex A-25 was used to bind the remaining RNA, while the replicase passed through, and bound to the cationic ion exchanger. The final replicase preparations which were obtained were very stable and had a purification factor of 1100-1400. The recovery averaged 70% and specific activities were much higher than those already described for similar enzymes from healthy or virus-infected tobacco leaves.