Stop codon recognition in ciliates: Euplotes release factor does not respond to reassigned UGA codon

In eukaryotes, the polypeptide release factor 1 (eRF1) is involved in trans lation termination at all three stop codons. However, the mechanism for dec oding stop codons remains unknown. A direct interaction of eRF1 with the st op codons has been postulated. Recent studies focus on eRF1 from ciliates i n which some stop codons are reassigned to sense codons. Using an in vitro assay based on mammalian ribosomes, we show that eRF1 from the ciliate Eupl otes aediculatus responds to UAA and UAG as stop codons and lacks the capac ity to decipher the UGA codon, which encodes cysteine in this organism. Thi s result strongly suggests that in ciliates with variant genetic codes eRF1 does not recognize the reassigned codons. Recent hypotheses describing sto p codon discrimination by eRF1 are not fully consistent with the set of eRF 1 sequences available so far and require direct experimental testing.