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Related Concept Videos

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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.
First, the initiator tRNA must be selected from the pool of elongator tRNAs by eukaryotic initiation factor 2 (eIF2). The initiator tRNA (Met-tRNAi) has conserved sequence elements including modified bases at...
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Base complementarity between the three base pairs of mRNA codon and the tRNA anticodon is not a failsafe mechanism. Inaccuracies can range from a single mismatch to no correct base pairing at all. The free energy difference between the correct and nearly correct base pairs can be as small as 3 kcal/ mol. With complementarity being the only proofreading step, the estimated error frequency would be one wrong amino acid in every 100 amino acids incorporated. However, error frequencies observed in...
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Translation in Prokaryotes01:29

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Prokaryote translation is a complex, highly coordinated process that converts genetic information from mRNA into functional proteins. It involves three stages: initiation, elongation, and termination, each facilitated by specific molecular components.Initiation of TranslationThe process begins with the assembly of the ribosomal subunits and initiation factors on the mRNA. In bacteria, the 30S ribosomal subunit recognizes the Shine-Dalgarno sequence in the mRNA, a conserved region upstream of...
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Ribosomes01:27

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Ribosomes translate genetic information encoded by messenger RNA (mRNA) into proteins. Both prokaryotic and eukaryotic cells have ribosomes. Cells that synthesize large quantities of protein—such as secretory cells in the human pancreas—can contain millions of ribosomes.
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In eukaryotes, transcription and translation are compartmentalized; an mRNA is first synthesized in the nucleus and then selectively transported to the cytoplasm for protein synthesis. Before transport, a pre-mRNA undergoes several steps of post-transcriptional modifications including splicing, 5' capping, and the addition of a poly-adenine tail. Various proteins bind to the pre-mRNA during these modifications. The mRNA transport takes place with the help of multiple proteins playing...
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40S ribosomal subunits scan mRNA for the start codon by one-dimensional diffusion.

Hironao Wakabayashi1, Mingyi Zhu1, Elizabeth J Grayhack1

  • 1Department of Biochemistry & Biophysics at the School of Medicine and Dentistry and Center for RNA Biology, University of Rochester, Rochester, New York 14642, USA.

RNA (New York, N.Y.)
|July 31, 2025
PubMed
Summary

mRNA scanning by the 40S ribosomal subunit is not rate-limiting when the 5' untranslated region (5' UTR) lacks secondary structure. Translational helicases like eIF4A, Ded1, and Slh1 are dispensable for this process.

Keywords:
mRNA scanningone-dimensional diffusionribosometranslation initiation

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Area of Science:

  • Molecular Biology
  • Genetics
  • Biochemistry

Background:

  • Eukaryotic translation initiation involves the 40S ribosomal subunit scanning the mRNA 5' untranslated region (5' UTR) for the start codon.
  • The precise mechanism and rate-limiting steps of mRNA scanning, including the role of translocases, remain incompletely understood.

Purpose of the Study:

  • To investigate the molecular mechanism of mRNA scanning during eukaryotic translation initiation.
  • To determine the impact of 5' UTR length and secondary structure on translation efficiency.
  • To identify the roles of specific translational helicases in mRNA scanning.

Main Methods:

  • Utilized green fluorescent protein (GFP) reporter systems in Saccharomyces cerevisiae.
  • Assessed the effects of varying unstructured and structured 5' UTR lengths on protein synthesis.
  • Examined the impact of loss-of-function mutations in translational helicases (eIF4A, Ded1, Slh1) on mRNA scanning.

Main Results:

  • Variations in unstructured 5' UTR length had minimal effects on protein synthesis.
  • Structured 5' UTRs significantly inhibited translation, indicating secondary structure is a major impediment.
  • Loss-of-function mutations in eIF4A, Ded1, and Slh1 did not impair mRNA scanning.
  • One-dimensional diffusion appears to be the primary mechanism for 40S subunit movement along the 5' UTR.

Conclusions:

  • mRNA scanning is not a rate-limiting step when the 5' UTR is free of secondary structures.
  • Specific translational helicases are not essential for the mRNA scanning process.
  • The movement of the 40S ribosomal subunit during scanning is predominantly driven by one-dimensional diffusion.