5'-Terminal capping of RNA by guanylyltransferase from HeLa cell nuclei.

Abstract

A soluble extract prepared from HeLa cell nuclei was shown to catalyze the 5''-terminal modification of RNA and synthetic polyribonucleotides to form m7G(5'')pppA- and m7G(5'')pppG- structures referred to as caps. The reaction involves the transfer of a GMP moiety from GTP to the 5'' end of an RNA molecule containing at least 2 terminal phosphates. Neither the .beta. nor the .gamma. phosphates of GTP are transferred and polynucleotides with no 5''-terminal phosphate or only 1 are not acceptors. In the absence of methyl donor, G(5'')pppA- and G(5'')pppG- structures were synthesized, indicating that methylation is not required for guanylylation. Cap formation was considered to occur by the following mechanism: .**GRAPHIC**. in which AdoMet is S-adenosylmethionine, AdoHcy is S-adenosylhomocysteine, and (p)ppN-represents either the original 5'' end of an RNA molecule or an internal site to which 1 or more phosphates were added after processing.