Abstract
Three structurally related pigments, rugulosin (Rug) and skyrin (Sky) which are products of Endothia parasitica, and luteoskyrin (Lut) which is a well-known mycotoxin produced by Penicillium islandicum, were tested for their inhibitory effects on rice dwarf virus (RDV) RNA transcription and E. coli DNA-dependent RNA polymerase activities, using in vitro assay systems for both enzymes. Lut inhibits only E. coli RNA polymerise but has no effect on RDV-RNA transcriptase. Rug strongly inhibits E. coli RNA polymerase but weakly RDV-RNA transcriptase. Conversely, Sky inhibits RDV-RNA transcriptase without inhibiting E. coli RNA polymerise. The Mg2+-mediated complex formation of Lut and Rug with DNA and that of Rug with RDV-RNA may account for their inhibitory effects on both enzymes, however this is not true with Sky on DNA and Sky on RDV-RNA. Sky forms complex with Mg2+ because its absorption spectrum shifts to longer wave lengths with increasing amounts of Mg2+ added but the complex does not form floccules which can be seen in Lut-Mg2+ and Rug-Mg2+ mixtures. Sky-Mg-DNA and Sky-Mg-RDV-RNA mixtures do not form such a complex which can be precipitated by centrifugation at 59, 000×g for 20min. Sephadex G-25 column chromatography of the mixtures indicates that nucleic acids are completely separated from Sky-Mg complex. When RDV-virions are pretreated with Sky and Mg2+, pelleted by centrifugation, then added to the reaction mixture, RDV-RNA transcriptase is not inhibited at all, whereas inhibited strongly by the addition of the supernatant which contains Sky. These results suggest that Sky inhibits RDV-RNA transcriptase not by its direct action to RDV-RNA present in virions but by its contact with virions at the site of transcription.
