Nido -type capping
RNAs are capped by a unique viral reaction. These viruses RdRp complex contains 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. The RNA is further N7 methylated and 2'O methylated by two other viral enzymes 
. Recent publications suggest that mRNA synthesis might be directly protein primed, thereby skipping the PRNTase step
.
(PRNTase=polyribonucleotidyl transferase, RdRp=RNA-dependent RNA polymerase, GTase=guanylyltransferase, MTase=methyltransferase, SAM= S-adenosylmethionine, SAH= S-adenosylhomocysteine)
Enzymatic reaction:
RNA:polyribonucleotidyl transferase (involves covalent link enz-Asn-RNA)
NTPase Rhea 23680
Guanylyltransferase
N7methyltransferase Rhea 65380
2'O methyltransferase : Rhea 67020 PF06460
The mechanism of RNA capping by SARS-CoV-2
Gina J. Park, Adam Osinski, Genaro Hernandez, Jennifer L. Eitson, Abir Majumdar, Marco Tonelli, Katie Henzler-Wildman, Krzysztof Pawlowski, Zhe Chen, Yang Li, John W. Schoggins, and Vincent S.
Nature. 2022; 609(7928): 793-800.
SND1 binds SARS-CoV-2 negative-sense RNA and promotes viral RNA synthesis through NSP9
Nora Schmidt, Sabina Ganskih, Yuanjie Wei, Alexander Gabel, Sebastian Zielinski, Hasmik Keshishian, Caleb A Lareau, Liv Zimmermann, Jana Makroczyova, Cadence Pearce, Karsten Krey, Thomas Hennig, Sebastian Stegmaier, Lambert Moyon, Marc Horlacher, Simone Werner, Jens Aydin, Marco Olguin-Nava, Ramya Potabattula, Anuja Kibe, Lars D?lken, Redmond P Smyth, Neva Caliskan, Annalisa Marsico, Christine Krempl, Jochen Bodem, Andreas Pichlmair, Steven A Carr, Petr Chlanda, Florian Erhard, Mathias Munschauer
Cell. 2023 Oct 26;186(22):4834-4850.e23.
