Translational activators align mRNAs at the small mitoribosomal subunit for translation initiation

Area of Science:

• Molecular Biology
• Cell Biology
• Biochemistry

Background:

• Mitochondrial gene expression is crucial for cellular energy production via oxidative phosphorylation.
• Mitochondrial ribosomes (mitoribosomes) translate mitochondrial mRNAs, but their regulation is not fully understood.
• Nuclear-encoded translational activators (TAs) in yeast regulate specific mRNA translation through unknown mechanisms.

Purpose of the Study:

• To elucidate the mechanism by which RNA-binding pentatricopeptide repeat domain TAs regulate mitoribosome function.
• To understand how TAs achieve transcript-specific translation initiation in mitochondria.

Main Methods:

• Selective mitoribosome profiling to identify ribosome footprints on specific mRNAs.
• Cryo-electron microscopy (cryo-EM) to determine the structural basis of TA-mitoribosome interactions.
• Analysis of stalled mitoribosome complexes in the post-initiation, pre-elongation state.

Main Results:

• TAs demonstrate high selectivity for mitoribosomes initiating translation on their target mRNAs.
• TAs recruit mitoribosomes to positions near the start codon on the mRNA.
• Cryo-EM structures reveal TAs binding to both the mRNA's 5' untranslated region and the mitoribosome, aligning the mRNA for initiation.

Conclusions:

• TAs provide a mechanistic basis for transcript-specific translation initiation in mitochondria.
• This mechanism allows precise positioning of mitoribosomes without relying on general sequence elements.
• The findings clarify how mitochondria achieve regulated gene expression for essential functions.