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Related Experiment Videos

snoRNA nuclear import and potential for cotranscriptional function in pre-rRNA processing.

B A Peculis1

  • 1Genetics and Biochemistry Branch, National Institute of Diabetes, Digestive and Kidney Diseases, National Institutes of Health, Bethesda, Maryland 20892-1766, USA. bp51h@nih.gov

RNA (New York, N.Y.)
|March 10, 2001
PubMed
Summary

Small nucleolar RNAs (snoRNAs) are crucial for rRNA maturation. U8 and U22 snoRNAs are essential for processing precursor rRNA into mature forms, with U8 and U22 needed during transcription.

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Area of Science:

  • Molecular Biology
  • Genetics
  • Cell Biology

Background:

  • Small nucleolar RNAs (snoRNAs) play critical roles in processing precursor ribosomal RNA (pre-rRNA) into mature forms (18S, 5.8S, and 28S rRNA).
  • The precise mechanisms by which snoRNAs mediate these essential cleavage events remain largely unknown, with possibilities including direct cleavage or facilitating ribosome assembly.
  • Specific snoRNAs, such as U22 and U8, have been identified as essential for the accumulation of mature 18S rRNA and both 5.8S and 28S rRNA, respectively.

Purpose of the Study:

  • To investigate the kinetics of pre-rRNA processing in Xenopus oocytes to elucidate the mechanisms of snoRNA-mediated processing.
  • To determine the functional requirements and nuclear import dynamics of essential snoRNAs (U8 and U22) during rRNA biogenesis.

Main Methods:

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  • Utilized Xenopus oocytes as a model system to study pre-rRNA processing kinetics.
  • Performed snoRNA-depletion and subsequent rescue experiments by injecting in vitro-synthesized snoRNAs into oocytes.
  • Employed autoradiography to assess snoRNA nuclear import rates and efficiency, using U1 small nuclear RNA (snRNA) as a control.
  • Applied transcriptional inhibitors to differentiate between transcription-dependent and independent processing steps.

    • Main Results:

    • SnoRNA-mediated pre-rRNA processing could be restored in depleted oocytes via cytoplasmic injection of synthesized snoRNAs, although at a slower rate than in untreated oocytes.
    • Nuclear import of U8 and U22 snoRNAs was less efficient and slower compared to U1 snRNA.
    • Functional rescue of rRNA processing occurred hours before snoRNAs were detectable in the nucleus, indicating high sensitivity to low snoRNA concentrations.
    • Rescue experiments failed with pre-accumulated precursors when transcription was inhibited, suggesting U8 and U22 are required during pre-rRNA transcription.

    Conclusions:

    • U8 and U22 snoRNAs are essential for efficient pre-rRNA processing and subsequent accumulation of mature rRNAs.
    • These snoRNAs are required during the transcription of pre-rRNA, highlighting a co-transcriptional role in ribosome biogenesis.
    • The functional requirement for very low nuclear concentrations of snoRNAs underscores their potent catalytic or assembly-facilitating roles in rRNA maturation.