One codon – two amino acids.

Abstract

In ciliated protozoa Euplotes crassus, cysteine (Cys) is encoded by three codons, UGA, UGU and UGC. UGA is a stop signal in the universal genetic code, and this codon can also code for the 21st amino acid, selenocysteine (Sec), in all three domains of life. Metabolic labeling of E. crassus with 75Se revealed specific incorporation of radioactive Se into several proteins. We found that in this organism the UGA codon specifies insertion of both Sec and Cys, and that the dual use of this codon can occur within a single gene. We studied a Euplotes protein, thioredoxin reductase 1 (eTR1), which has seven in‐frame UGA codons. Our experiments demonstrated that Sec was only inserted into the classical Sec site in eTR1, whereas other UGA positions were not served by SECIS for Sec insertion and instead supported termination of translation in mammalian cells. LC‐MS/MS sequencing revealed eTR1 peptides containing Cys in positions 63, 68, 208, and 270, which are encoded by UGA codons, whereas peptides containing Sec at these positions were not detected. Thus, our data show that E. crassus utilizes UGA for insertion of both Cys and Sec, establishing it as the first known organism that utilizes one codon to code unambiguously for two different amino acids.