Bacterial release factors RF1 and RF2 are methylated on the Gln residue of a universally conserved tripeptide motif GGQ, that interacts with the peptidyl transferase centre of the large ribosomal subunit, triggering hydrolysis of the ester bond in peptidyl-tRNA and releasing the newly-synthesized polypeptide from the ribosome. In vitro experiments have shown that the activity of RF2 is stimulated by Gln methylation. The viability of E. coli K12 strains depends on the integrity of the release factor methyltransferase PrmC, because K12 strains are partially deficient in RF2 activity due to the presence of a Thr residue at position 246 instead of Ala. Here, we study in vivo RF1 and RF2 activity at termination codons in competition with programmed frameshifting, and the effect of the Ala246Thr mutation. PrmC inactivation reduces the specific termination activity of RF1 and RF2(Ala246) by about 3- to 4-fold. The mutation Ala246Thr in RF2 reduces the termination activity in cells about 5-fold. After correction for the decrease in level of RF2 due to the autocontrol of RF2 synthesis, the mutation Ala246Thr reduces RF2 termination activity by about 10-fold at UGA codons and UAA codons. PrmC inactivation has no effect on cell growth in rich media, but reduces growth considerably on poor carbon sources. This suggests that the expression of some genes needed for optimal growth under such conditions can become growth limiting as a result of inefficient translation termination.