Provided herein is a method for capping RNA in vitro. In some embodiments the capping reaction may be done using an RNA guanylyltransferase capping enzyme from a Tupanviruses virus, and in particular a capping enzyme from Tupanvirus soda lake virus or Tupanvirus deep ocean virus, or a fragment or variant thereof. Compositions and kits for capping RNA in vitro are also provided. Also included are methods, compositions and kits for the manufacturer and capping of therapeutic RNA oligonucleotides, such as RNA-based vaccines and therapeutics.