Catalysis of the reaction: S-adenosyl-L-methionine + G(5')pppR-RNA = S-adenosyl-L-homocysteine + m7G(5')pppR-RNA. m7G(5')pppR-RNA is mRNA containing an N7-methylguanine cap; R may be guanosine or adenosine.
EC:2.1.1.56; MetaCyc:MRNA-GUANINE-N7--METHYLTRANSFERASE-RXN; Reactome:R-HSA-9684016 "nsp14 acts as a cap N7 methyltransferase to modify SARS-CoV-1 mRNAs"; Reactome:R-HSA-9684017 "nsp14 acts as a cap N7 methyltransferase to modify SARS-CoV-1 gRNA (plus strand)"; Reactome:R-HSA-9684018 "nsp14 acts as a cap N7 methyltransferase to modify SARS-CoV-1 gRNA complement (minus strand)"; Reactome:R-HSA-9694476 "nsp14 acts as a cap N7 methyltransferase to modify SARS-CoV-2 mRNAs"; Reactome:R-HSA-9694492 "nsp14 acts as a cap N7 methyltransferase to modify SARS-CoV-2 gRNA complement (minus strand)"; Reactome:R-HSA-9694737 "nsp14 acts as a cap N7 methyltransferase to modify SARS-CoV-2 gRNA (plus strand)"; Reactome:R-HSA-9830848 "L protein acts as a cap N7 methyltransferase to modify RSV mRNAs"; reactome:R-HSA-77090 "Methylation of GMP-cap by RNA Methyltransferase"