ИСТИНА

It is known that polyribosomes translating eukaryotic mRNAs can form circular structures both in vitro and in vivo. Direct interaction between capped 5'- and polyadenilated 3'-ends of mRNA molecule is believed to be the primary cause of such polysome morphology. It was suggested that physical circularization can cause functional cyclization, i.e. the reinitiation of terminating ribosomes at the same mRNA molecule. Despite the general acceptance of the hypothesis it has been never proved experimentally mainly because of the difficulties in monitoring the behavior of individual ribosome in the course of translation process. We supposed that reinitiation of terminating ribosomes after the first round of translation should change the overall initiation rate that is to be reflected in the change of protein synthesis rate. Utilizing the technique of continuous monitoring of protein synthesis we detected the rise of initiation rate in the course of translation of cellular eukaryotic mRNAs in mammalian cell free systems. Polysome analysis proved that this acceleration reflected the increase of initiation rate. It occurred that the observed effects strongly depend on the presence of 5'- cap and 3'-poly(A) structures. Any break we induced in canonical CapeIF4E- eIF4G-PABP-poly(A) chain by specific competitors and modifiers led to the inhibition of acceleration effect testifying the importance of mRNA ends interaction. We found that the level of acceleration can be modulated by the length of 5' mRNA leader or 3' poly(A) sequence and by the distance between stop-codon and poly(A). It occurred that the optimal length of mRNA leader, in terms of the highest acceleration level, is close to the average size of mRNA 5' UTR in vertebrates. Using competitive inhibitors of initiation factors, we showed that process responsible for the increase of initiation rate is quite different from the classical initiation mechanism and do not include the 5' UTR scanning stage. All the collected data led us to the conclusion that interaction between mRNA ends has a strong impact on the overall translation efficiency likely through switching on the alternative initiation mechanism that utilizes the reinitiation of translating ribosomes.