How Dedicated Ribosomes Translate a Leaderless mRNA

Area of Science:

• Molecular Biology
• Structural Biology
• Microbiology

Background:

• Bacteriophage lambda (λ) lysogens utilize leaderless mRNA (lmRNA) for λcI repressor production, initiated at the λpRM promoter.
• Translation of lmRNA is notably enhanced in E. coli mutants lacking ribosomal protein uS2.
• Understanding the structural basis of lmRNA translation initiation is crucial, given its conservation across bacteria, archaea, and eukaryotes.

Purpose of the Study:

• To elucidate the structural mechanisms underlying enhanced translation of λcI lmRNA in uS2-deficient ribosomes.
• To provide high-resolution structural insights into the lmRNA translation initiation complex.

Main Methods:

• Cryo-electron microscopy (cryo-EM) was employed to determine the structures of wild-type and uS2-deficient (rpsB11 mutant) 70S ribosomes.
• Complexes included λcI lmRNA and initiator tRNA (fMet-tRNA^fMet).

Main Results:

• The uS2-deficient ribosome, also lacking bS21, exhibits altered anti-Shine-Dalgarno (aSD) region positioning, facilitating lmRNA exit.
• A π-stacking interaction between ribosomal base A1493 and lmRNA A(+4) may serve as a recognition signal.
• Ribosome dynamics, including increased 30S head rotation and potential charge flow, likely aid lmRNA propagation.

Conclusions:

• The absence of uS2 and bS21 structurally reconfigures the ribosome, promoting leaderless mRNA translation.
• Specific interactions and dynamic mechanisms facilitate lmRNA binding and passage through the ribosome.
• These findings provide a structural foundation for studying translation mechanisms and the co-evolution of lmRNA and ribosomes.