Guanosine 5'-O-(3-thiotriphosphate) inhibits capping of reovirus mRNA

RNA synthesized by reovirus cores was not capped or methylated when GTP was replaced by guanosine 5'-O-(3-thiotriphosphate) [gamma-S]GTP as a reaction substrate. Full length transcripts were synthesized in the presence of [gamma-S]GTP which indicated that capping could be dissociated from the process of transcription. Exogenous [gamma-S]GTP was not cleaved by the virion-associated phosphohydrolase and the majority of transcripts contained triphosphorylated 5'-termini. [gamma-S]GTP did, however, act as a donor for the capping of RNA chains initiated with GTP. Chromatography on mercury-agarose of products initiated with [gamma-S]GTP resulted in the binding of only the small (14 S) size class of RNA, although transcripts not bound to the affinity column were also thiophosphorylated.