Chymotrypsin-derived cores, but not virions, catalyze the transfer of methyl groups from S-adenosyl methionine (SAM) to nascent mRNA synthesized in vitro by the core polymerase. The reaction requires Mg++, and is dependent on the presence of all 4 ribonucleoside triphosphates (rNTPs). Methylation proceeds optimally at 51°C. All ten species of mRNA become methylated during transcription and it is estimated that one methyl residue is incorporated per RNA chain. Experiments designed to determine the location of the methylated nucleotide clearly demonstrate that methylation occurs exclusively at the 5′ ends of nascent mRNA.