Mononega -type capping

RNAs are capped by a unique viral reaction at the onset of viral transcription. These viruses encode an RNA:GDP polyribonucleotidyltransferase (PRNTase) that forms a covalent bond to the 5' end of the viral RNA through a monophosphate group. The enzyme then transfers the 5'-monophosphate RNA to GDP and forms the GpppRNA structure, which is further 2'O methylated and N7 methylated. These activities are encoded in the viral RdRp protein .

(NTPase=nucleoside-triphosphatase, PRNTase=polyribonucleotidyl transferase, RdRp=RNA-dependent RNA polymerase, GTase=guanylyltransferase, MTase=methyltransferase, SAM= S-adenosylmethionine, SAH= S-adenosylhomocysteine)

Enzymatic reactions:
TPase (involves covalent link enz-His-RNA)
(RNA:GDP-polyribonucleotidyltransferase, + GTP transfer): Rhea 65436
2'O methylase : Rhea 67020
N7methyltransferase Rhea 65380