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. 2023 Oct 16;11(10):1600. doi: 10.3390/vaccines11101600

Figure 4.

Figure 4

Scheme of cap-dependent and cap-independent initiation of mRNA translation. Cap-dependent translation initiation is the classical way to start the assembly of the eukaryotic initiator complex. The complex includes methionyl initiator tRNA (Met-tRNAi), eukaryotic initiation factor 1 (eIF1), eIF1A, eIF2, eIF3, eIF4A, eIF4B, eIF4G, eIF4E, eIF5, and poly(A)-binding protein (PABP). Class I–IV IRES initiates mRNA translation in a cap-independent way and is typical for viruses. The initiator complex of mRNA translation containing Class I IRES includes all the factors characteristic of the cap-dependent pathway except eIF4E, which recognizes the cap, but adds IRES trans-activating factors (ITAFs). Class II IRES includes all the initiator factors characteristic of Class I IRES, but the initiation itself occurs directly at the AUG codon. IRES class III, similar to IRES class II, attracts translation initiation factors directly to the AUG codon, but only Met-tRNAi, eIF2, and eIF3 perform initiation. Class IV IRES engages the 40S subunit independently without the eIF and AUG codons, promoting translation initiation by structurally mimicking Met-tRNAi.