Both endonucleolytic and exonucleolytic cleavage mediate ITS1 removal during human ribosomal RNA processing

Area of Science:

• Molecular Biology
• Cell Biology
• Genetics

Background:

• Ribosome production is vital for cell function and is often dysregulated in diseases like cancer and ribosomopathies.
• Human precursor ribosomal RNA (pre-rRNA) processing is a complex pathway with significant gaps in understanding, particularly the removal of internal transcribed spacer 1 (ITS1).

Purpose of the Study:

• To elucidate the molecular mechanisms of human pre-rRNA processing, focusing on the ITS1 removal step.
• To identify the key enzymes and factors involved in ITS1 processing and compare them to other organisms.

Main Methods:

• Utilized RNA interference (RNAi) to knockdown specific enzymes.
• Employed biochemical assays to analyze pre-rRNA processing intermediates.
• Compared human processing pathways with those in budding yeast.

Main Results:

• Human ITS1 processing is initiated by endonuclease cleavage dependent on large subunit biogenesis factors.
• RRP6 and the exosome complex were identified as key 3' to 5' exonucleases responsible for ITS1 removal.
• Knockdown of the endoribonuclease MRP did not significantly impair ITS1 processing.
• Human ITS1 processing exhibits distinct features compared to budding yeast, despite conserved ribosome synthesis factors.

Conclusions:

• The study uncovers a novel pathway for human ITS1 processing involving RRP6 and the exosome.
• Findings provide critical insights into the molecular basis of ribosomopathies and ribosome production defects.
• Highlights species-specific differences in fundamental pre-rRNA processing pathways.