Methylates the class 1 translation termination release factors RF1/PrfA and RF2/PrfB on the glutamine residue of the universally conserved GGQ motif. L-glutaminyl-[peptide chain release factor] + S-adenosyl-L-methionine = H(+) + N(5)-methyl-L-glutaminyl-[peptide chain release factor] + S-adenosyl-L-homocysteine Monomer and homodimer. The crystal structure corresponding to PDB 1SG9 shows a glutamate anhydride cross-link formed between the Glu-97 side chains from two molecules, but this has not been observed in other PrmC structures from T.maritima. This cross-link is suggested being an artifact of concentration during crystallization (PubMed:18247349). Belongs to the protein N5-glutamine methyltransferase family. PrmC subfamily.