Capping of tobacco mosaic virus RNA: Analysis of viral-coded guanylyltransferase-like activity

The 5′ end of tobacco mosaic virus (TMV) genomic RNA is capped with 7-methylguanosine. A virus-coded polypeptide with guanylyltransferase activity has been investigated. This enzyme is responsible for forming the 5′ → 5′ linkage of guanosine 5′-monophosphate to the 5′-diphosphate of an acceptor RNA, thereby forming the cap. A critical step in the mechanism for cap formation in the eukaryotic nucleus is for guanylyltransferase to bind covalently to guanosine 5′-monophosphate with the hydrolysis of pyrophosphate when guanosine 5′-triphosphate is the substrate. The TMV 126-kilodalton protein, which is most probably a component of the TMV replicase, was found to have this activity. The mechanism of this reaction has been characterized biochemically.