Evolution of eukaryotic translation elongation and termination factors: variations of evolutionary rate and genetic code deviations.

Translation is carried out by the ribosome and several associated protein factors through three consecutive steps: initiation, elongation, and termination. Termination remains the least understood of them, partly because of the nonuniversality of the factors involved. To get some insights on the evolution of eukaryotic translation termination, we have compared the phylogeny of the release factors eRF1 and eRF3 to that of the elongation factors EF-1alpha and EF-2, with special focus on ciliates. Our results show that these four translation proteins have experienced different modes of evolution. This is especially evident for the EF-1alpha, EF-2, and eRF1 ciliate sequences. Ciliates appear as monophyletic in the EF-2 phylogenetic tree but not in the EF-1alpha and eRF1 phylogenetic trees. This seems to be mainly because of phylogeny reconstruction artifacts (the long-branch attraction) produced by the acceleration of evolutionary rate of ciliate EF-1alpha and eRF1 sequences. Interaction with the highly divergent actin found in ciliates, or on the contrary, loss of interaction, could explain the acceleration of the evolutionary rate of the EF-1alpha sequences. In the case of ciliate eRF1 sequences, their unusually high evolutionary rate may be related to the deviations in the genetic code usage found in diverse ciliates. These deviations involve a relaxation (or even abolition) of the recognition of one or two stop codons by eRF1. To achieve this, structural changes in eRF1 are needed, and this may affect its evolutionary rate. Eukaryotic translation seems to have followed a mosaic evolution, with its different elements governed by different selective pressures. However, a correlation analysis shows that, beneath the disagreement shown by the different translation proteins, their concerted evolution can still be made apparent when they are compared with other proteins that are not involved in translation.