Bi-directional ribosome scanning controls the stringency of start codon selection

The fidelity of start codon recognition by ribosomes is paramount during protein synthesis. The current knowledge of eukaryotic translation initiation implies unidirectional 5MODIFIER LETTER PRIME -> 3MODIFIER LETTER PRIME migration of the pre-initiation complex (PIC) along the 5MODIFIER LETTER PRIME UTR. In probing translation initiation from ultra-short 5MODIFIER LETTER PRIME UTR, we report that an AUG triplet near the 5MODIFIER LETTER PRIME end can be selected via PIC backsliding. Bi-directional ribosome scanning is supported by competitive selection of closely spaced AUG codons and recognition of two initiation sites flanking an internal ribosome entry site. Transcriptome-wide PIC profiling reveals footprints with an oscillation pattern near the 5MODIFIER LETTER PRIME end and start codons. Depleting the RNA helicase eIF4A leads to reduced PIC oscillations and impaired selection of 5MODIFIER LETTER PRIME end start codons. Enhancing the ATPase activity of eIF4A promotes nonlinear PIC scanning and stimulates upstream translation initiation. The helicase-mediated PIC conformational switch may provide an operational mechanism that unifies ribosome recruitment, scanning, and start codon selection. Start codon selection is commonly thought to occur through the unidirectional scanning of the mRNA by the 40 S ribosome. Here the authors provide evidence that the pre-initiation complex can backslide on the mRNA to initiate translation at upstream AUG codons.

Automated summary

The fidelity of start codon recognition by ribosomes is crucial for protein synthesis. Recent studies suggest that ribosomes can select upstream AUG codons via backsliding on the mRNA, with the RNA helicase eIF4A playing a key role in this process.