Multiple ribosomal RNA cleavage pathways in mammalian cells

Area of Science:

• Molecular Biology
• Genetics
• Cell Biology

Background:

• Ribosomal RNA (rRNA) synthesis involves the transcription of large precursor molecules (pre-rRNA) that are subsequently processed into mature rRNAs.
• Understanding pre-rRNA processing pathways is crucial for comprehending ribosome biogenesis and cellular function.

Purpose of the Study:

• To investigate the sequence content and processing pathways of mouse L cell pre-rRNA.
• To identify novel pre-rRNA intermediates and elucidate their role in rRNA maturation.

Main Methods:

• RNA gel transfer and blot hybridization using various restriction fragments complementary to 45S pre-rRNA.
• Agarose gel electrophoresis for nuclear RNA separation.
• Hybridization with probes to detect specific pre-rRNA species.

Main Results:

• Detection of abundant 34S pre-rRNA and less abundant 37S, 26S, and 17S pre-rRNAs in mouse L cells.
• Identification of a new 12S precursor to 5.8S rRNA common to all pathways.
• Confirmation of previously identified pre-rRNAs (45S, 41S, 32S, 20S) and their structures.
• Absence of 36S pre-rRNA, suggesting a shift from a previously described mouse L cell pathway.
• Localization of the 5.8S rRNA sequence within the internal transcribed spacer.

Conclusions:

• Mouse L cells utilize at least two novel pre-rRNA cleavage pathways, in addition to the known 20S pathway.
• These novel pathways, characterized by 34S and 26S pre-rRNAs, are also found in HeLa cells, suggesting they are conserved in mammalian cells.
• Cleavage sites in these pathways are conserved and located at or near the termini of mature rRNAs, with variations in the temporal order of cleavage.