ИСТИНА

The two major and principally different mechanisms of translation initiation in eukaryotes are cap-dependent scanning and internal ribosome entry. The latter one requires specific structures known as internal ribosome entry sites (IRES). IRESes were initially found in the 5’ untranslated regions (5’ UTR) of some cytoplasmic viral mRNAs. Later, the presence of IRES elements were also suggested for some cellular mRNAs. However, the existence of “cellular IRESes” is still questionable. The conventional approach to identify IRES-elements is the method of dicisrtonic constructs. The only criterion used so far is the comparison of various dicistronic constructs, i.e. second cistron translation driven by the sequence tested with one driven by known (previously characterized) IRES element. This criterion is rather subjective. Morover, in natural conditions all cellular mRNAs are monocistronic and possess m7G-cap, and theoretically are able to use the conventional cap-dependent way. Here we suggest new criteria to evaluate an actual mechanism of mRNA translation: 1) comparison of translation driven by the 5’ UTRs in monocistronic context vs. dicistronic one; 2) comparison of capped monocistronic RNA translation vs. uncapped transcript. We applied these criteria to several human mRNAs with complex 5’UTRs previously reported to contain IRES elements: c-myc, Apaf-1, L1 and Hsp70. For all of them the translation was much more efficient for monocistronic mRNAs. Moreover, it was strongly dependent on the presence of the 5’-cap. These data lead us to conclude that mRNAs with long structured 5’UTRs are able to translate efficiently by the cap-dependent mechanism, at least under normal conditions.