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 Was identified as a high-confidence drug target. Belongs to the protein N5-glutamine methyltransferase family. PrmC subfamily. Extended N-terminus.