Common and specific amino acid residues in the prokaryotic polypeptide release factors RF1 and RF2: possible functional implications

Termination of protein synthesis is promoted in ribosomes by proper stop codon discrimination by class 1 polypeptide release factors (RFs). A large set of prokaryotic RFs differing in stop codon specificity, RF1 for UAG and UAA, and RF2 for UGA and UAA, was analyzed by means of a recently developed computational method allowing identification of the specificity-determining positions (SDPs) in families composed of proteins with similar but not identical function. Fifteen SDPs were identified within the RF1/2 superdomain II/IV known to be implicated in stop codon decoding. Three of these SDPs had particularly high scores. Five residues invariant for RF1 and RF2 [invariant amino acid residues (IRs)] were spatially clustered with the highest-scoring SDPs that in turn were located in two zones within the SDP/IR area. Zone 1 (domain II) included PxT and SPF motifs identified earlier by others as 'discriminator tripeptides'. We suggest that IRs in this zone take part in the recognition of U, the first base of all stop codons. Zone 2 (domain IV) possessed two SDPs with the highest scores not identified earlier. Presumably, they also take part in stop codon binding and discrimination. Elucidation of potential functional role(s) of the newly identified SDP/IR zones requires further experiments.