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Translational Regulation01:29

Translational Regulation

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Translational regulation in prokaryotes ensures efficient protein synthesis by controlling ribosome access to mRNA. This regulation is mediated by secondary RNA structures, including translational riboswitches, RNA thermometers, and small RNAs (sRNAs), which respond to intracellular and environmental signals to modulate gene expression.Translational RiboswitchesRiboswitches in the leader region of mRNAs can regulate translation by altering the accessibility of the Shine-Dalgarno (SD) sequence,...
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The gene expression in cells is regulated at different stages: (i) transcription, (ii) RNA processing, (iii) RNA localization, and (iv) translation. Transcriptional regulation is mediated by regulatory proteins such as transcription factors, activators, or repressors—these control gene expression by initiating or inhibiting the transcription of genes. Once a precursor or pre-mRNA is produced, it undergoes post-transcriptional modification, including 5' capping, splicing, and the...
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Gene expression can be regulated at almost every step from gene to protein. Transcription is the step that is most commonly regulated. This involves the binding of proteins to short regulatory sequences on the DNA. This association can either promote or inhibit the transcription of a gene associated with the respective sequence.
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During most eukaryotic translation processes, the small 40S ribosome subunit scans an mRNA from its 5' end until it encounters the first start AUG codon. The large 60S ribosomal subunit then joins the smaller one to initiate protein synthesis. The location of the translation initiation is largely determined by the nucleotides near the start codon as there may be multiple translation initiation sites present on the mRNA.  Marilyn Kozak discovered that the sequence RCCAUGG (where R...
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Initiating translation is complex because it involves multiple molecules. Initiator tRNA, ribosomal subunits, and eukaryotic initiation factors (eIFs) are all required to assemble on the initiation codon of mRNA. This process consists of several steps that are mediated by different eIFs.
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Structural basis for non-AUG translation regulation by 5MPs.

Ximena Zottig1,2, Chun-Ying Huang1, Zahra Seraj1

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Summary

eIF5-mimic proteins (5MPs) control protein production by limiting non-canonical start codon use. This study reveals 5MPs stabilize ribosome complexes, promoting mRNA scanning and preventing initiation at incorrect start sites.

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Area of Science:

  • Molecular Biology
  • Structural Biology
  • Genetics

Background:

  • Cellular proteome regulation involves translation initiation at AUG or non-canonical (non-AUG) start codons.
  • Non-AUG initiation is crucial during stress and linked to diseases like cancer.
  • eIF5-mimic proteins (5MPs) limit non-AUG start codon usage, reprogramming proteoform expression from mRNAs with alternative start sites.

Purpose of the Study:

  • To elucidate the mechanism by which 5MPs induce translational reprogramming.
  • To understand how 5MPs restrict non-AUG start codon usage.

Main Methods:

  • In extracto cryo-electron microscopy (cryo-EM)
  • Biochemical assays
  • Analysis of 5MP-bound 48S pre-initiation complexes (PICs) from native cell extracts.

Main Results:

  • Translational repression by 5MP is sequence context-dependent near start codons.
  • Cryo-EM structures show 5MP binding at the A site of the small ribosomal subunit, stabilizing an open-head PIC conformation.
  • 5MP's N-terminal region blocks the A site, while the C-terminal domain interacts with eIF2β and initiator tRNA outside the P site (Pout).

Conclusions:

  • 5MP directly biases initiating 48S complexes toward an open conformation.
  • This bias promotes mRNA scanning and inhibits initiation at suboptimal start codons.
  • The findings provide a structural mechanism for 5MP-mediated translational control.