RNA-directed RNA polymerase from tomato leaves. I. Purification and physical properties.

An RNA-directed RNA polymerase (RdRP, EC 2.7.7.48) from tomato leaf tissue was purified to electrophoretic homogeneity. A terminal transferase activity that was found to cofractionate with RdRP from DEAE-Sepharose and DNA-cellulose columns was removed by chromatography on a Mono Q column. The highly purified RdRP exhibits a specific activity of 500 nmol x mg-1 x 30 min-1, which corresponds to a 100,000-fold enrichment of the enzyme. In buffer containing 50% glycerol, its activity decreased by about 15%/month. RdRP activity coincided with the silver staining intensity of a single 128-kDa polypeptide when the fractions eluted from the Mono Q column were analyzed by electrophoresis in a SDS-polyacrylamide gel. Its molecular mass and its sedimentation coefficient of 6.6 S indicate that RdRP is a nearly globular molecule. The catalytic activity of RdRP is resistant to alpha-amanitin and actinomycin D. In tomato leaves systemically infected with potato spindle tuber viroid, the activity of RdRP was found to be increased about 3-fold compared with RdRP isolated from healthy leaf tissue.