Modification of the 5' End of mRNA Leader Sequence Alters the Set of Initiation Factors Essential for Initiation of Translation

The abundance of noncanonical mechanisms of eukaryotic initiation of translation indicates their involvement in the regulation of protein synthesis during key events in a cell life. One of the well-known examples of a noncanonical cap-independent process is the initiation of translation of mRNA with the 5'-untranslated (leader) region of the messenger encoding for the photoprotein obelin (the obelin leader). In the present work, mRNA with the obelin leader was modified by adding 45 deoxycytidyl nucleotides and a fluorescent label to its 5'end. Formation of the 48S ribosomal initiation complexes at the start codon of the modified mRNA was studied using primer extension inhibition (toeprinting). In contrast to mRNA with the intact obelin leader, translation initiation of which strictly requires the eIF4F factor, initiation on the modified mRNA can take place in the absence of this factor, although with less efficiency. The finding thus indicates the unknown function of the eIF4F factor in the first step(s) of mRNA recognition by ribosomal subunits.