[First class transcription termination factors--functional analogs of aminoacyl tRNA]

Reviewed and discussed are the recent data demonstrating profound functional similarity between class-1 translation termination factors (RF1 and RF2 in prokaryotes, aRF1 and eRF1 in Archaea and eukaryotes, respectively) and aminoacyl-tRNA as regards their roles in the course of translation on the ribosome. Functional analogy of these two components of the cell protein-synthesizing machinery was suggested long ago; however, numerous experimental proofs have been obtained only recently. This similarity implies that decoding of the genetic information by the ribosomal machine is performed similarly at all stages of translation, though tRNA plays the main role at initiation and elongation, while the protein is most important for termination. Earlier it was found that nucleic acids (ribozymes) can operate like the protein enzymes, and now we have got evidence for the reverse: a protein (translation termination factor) can act like a nucleic acid (tRNA). Thus one can speak of "exchange" of molecular functions between proteins and nucleic acids. Therefore, the profound chemical difference between proteins and nucleic acids is not an insuperable barrier to their mutual functional replacement in certain situations.