Faustovirus Capping Enzyme (FCE) catalyzes the addition of N7-methylguanosine cap (m7G) to the 5 prime end of triphosphorylated and diphosphorylated transcripts producing Cap-0 RNA. FCE is a single-subunit enzyme that combines the three activities necessary to produce the Cap-0 structure- triphosphatase guanylyltransferase and (guanine-N7)-methyltransferase. Installation of Cap-0 is a key step in eukaryotic mRNA maturation along with 2-O-methylation at position 1 (Cap-1) and polyadenosine (poly(A)) tailing. The Cap-0 structure also promotes RNA stability and prevents inadvertent activation of innate immune responses triggered by triphosphorylated RNA. FCE retains significant capping activity at low temperatures and tolerates reaction temperatures up to 55C. In many cases 1 uL of FCE (25 units) can cap over 100 ug of RNA in 1 hour at 37C. GTP and S-adenosylmethionine (SAM) are required for capping activity and are included with the enzyme.