Evolution of the eukaryotic translation termination system: Origins of release factors

Accurate translation termination is essential for cell viability, In eukary otes, this process is strictly maintained by two proteins, eukaryotic relea se factor 1 (eRF1), which recognizes all stop codons and hydrolyzes pcptidy l-tRNA, and eukaryotic release factor 3 (eRF3), which is an elongation fact or 1 alpha (EF-1 alpha) homolog stimulating eRF1 activity. To retrace the e volution of this core system, we cloned and sequenced the eRF3 genes from T richomonas vaginalis (Parabasalia) and Giardia lamblia (Diplomonada), which are generally thought to be "early-diverging eukaryotes," as well as those from two ciliates (Oxytricha trifallax and Euplotes aediculatus). We also determined the sequence of the eRF1 gene for G. lamblia. Surprisingly, the G. lamblia eRF3 appears to have only one domain, corresponding to EF-1 alph a, while other eRF3s (including the T. vaginalis protein) have an additiona l N-terminal domain, of 66-411 amino acids. Considering this novel eRF3 str ucture and our extensive phylogenetic analyses, we suggest that (1) the cur rent translation termination system in eukaryotes evolved from the archaea- like version, (2) eRF3 was introduced into the system prior to the divergen ce of extant eukaryotes, including G. lamblia, and (3) G. lamblia might be the first eukaryotic branch among the organisms considered.