Differential dependence of reovirus-associated enzyme activities on genome RNA as determined by psoralen photosensitivity.

Irradiation with long wavelength UV light in the presence of 4'-substituted psoralens abolished the in fectivity of reovirus and reovirus subviral particles Photoreaction of intact virions, cores, or genome RNA resulted in the formation of psoralen monoadducts and cross-links, demonstrating that the viral genome RNA in situ is double-stranded. Virus-associated RNA polymerase activity was lost following psoralen photoreaction, consistent with dependence on the genome RNA as template for viral mRNA synthesis. By contrast, reovirus guanylyl transferase and methyl transferase activities that catalyze the formation of mRNA 5'-terminal cap, m GpppG, were relatively unaffected by photoreaction. Cations that inhibit psoralen binding to nucleic acids also protected against loss of RNA polymerase activity. Partially photoinactivated viral cores produced decreased amounts of full length viral mRNAs. The absence of prematurely terminated transcripts indicates that template RNA modification by psoralen adducts has an all-or-none effect on initiation of mRNA synthesis by the core-associated RNA polymerase.