Faustovirus Capping Enzyme (FCE) catalyzes the addition of N7-methylguanosine cap (m7G) to the 5' 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.

• Improved capping efficiency relative to vaccinia capping enzyme
• Improved capping on difficult substrates
• Alternative to vaccinia capping system
• Broad temperature range
• Includes GTP and SAM

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 55 °C. In many cases, 1 µl of FCE (25 units) can cap over 100 µg of RNA in 1 hour at 37 °C. GTP and S-adenosylmethionine (SAM) are required for capping activity and are included with the enzyme.