SUNY at Albany
June 19-23, 2001
Why termination tRNAs are not around? Because they are permanently incorporated in the large ribosomal RNA
Final process of a ribosomal cycle - termination, begins with recognition by the ribosome of stop codon in the mRNA. Then follows hydrolysis of an ester bond that connects the complete polypeptide chain to peptidyl tRNA, removal of the deacylated tRNA from the ribosome and dissociation of latter onto small and large subunits. Obligatory participants of the termination process are proteins - the release factors, RFs. The prokaryotic proteins RF1 and RF2 participate in recognition of stop codons: RF1 works with UAG, UAA, while RF2 traces UGA and UAA. A single eukaryotic protein, eRF1, serves all the three codons: UAG, UGA, and UAA. At present, the opinion prevails that the RFs and only RFs recognize stop codons, though another point of view on a direct pairing between stop codons and a piece of rRNA of small subunit was discussed in literature. Our phylogenetic analysis has revealed two special terminator tRNA-mimics, built integrally with a large rRNA of the big subunit. These Ter-tRNA1 (includes a hairpin 69) and Ter-tRNA2 (includes hairpin 89) have anticodons for stop codons UAG, UAA and UGA, UAA, correspondingly, to directly pair to a stop codon in mRNA. At the same time, proteins RF1 and RF2 (eRF1 for eukaryota and arhaebacteria) play a role of the assistants of the above pairing, i.e. the RFs increase accuracy and fidelity of the stop-codon recognition by Ter-tRNA, as well as assist in realizing a non-canonic A-C paring, necessary for some stop codon - stop anticodon pairs. Analysis of the sequences of the mitochondrial large rRNAs of vertebrates, which acquire new stop codons AGG and AGA, testifies to involvement of the found Ter-tRNA1 in the direct stop codon recognition: namely emergence of new stops AGG (AGA) is accompanied by appearance of the corresponding stop anticodons CCU (UCU) in the anticodon loop of Ter-tRNA1 (hairpin 69). The new stop anticodon by overlapping with the old one constitute a quadruplet for the quadruplet stop-codon UAGG (UAGA). The obtained data support the concept of "RNA world" by demonstrating how an early pre-ribosome would have recognized stop codons before release factors evolved.
Valery Ivanov,(1,2) Artemy Beniaminov,(1) Andrei Mikheyev,(2) Elvira Minyat(1)
V.A. Engelhardt Institute of Molecular Biology(1), Russian Academy of Sciences, 32
Vavilov st., 117984 Moscow, Russia
Moscow Institute of Physics and Technology(2), 141700 Moscow region, Dolgoprudny,