Processing of intron-encoded box C/D small nucleolar RNAs lacking a 5',3'-terminal stem structure

Area of Science:

• Molecular Biology
• RNA Biology
• Genetics

Background:

• Box C/D small nucleolar RNAs (snoRNAs) are vital for precursor ribosomal RNA (rRNA) processing and 2'-O-methylation.
• In vertebrates, most box C/D snoRNAs are excised from introns via exonucleolytic pathways.
• While conserved C and D boxes and a terminal stem are known processing elements, many snoRNAs lack this terminal helix.

Purpose of the Study:

• To investigate the cis-acting elements that support the accumulation of intron-encoded box C/D snoRNAs lacking a terminal stem.
• To identify mechanisms for accurate snoRNA excision in the absence of a canonical terminal helix.

Main Methods:

• In vivo processing analysis of human U46 snoRNA.
• In vivo processing analysis of an artificial snoRNA derived from human beta-globin pre-mRNA.
• Investigated the role of internal and external stem structures in snoRNA accumulation.

Main Results:

• Demonstrated that internal and/or external stem structures support the accumulation of mammalian box C/D snoRNAs lacking a canonical terminal stem.
• Showed that these stem structures facilitate the folding of C and D boxes, promoting snoRNP protein binding.
• Identified that external intronic stems are degraded during processing, suggesting the C and D boxes confer metabolic stability to mature snoRNAs.

Conclusions:

• Internal and external stem structures are critical for the accumulation of box C/D snoRNAs lacking terminal helices.
• Base-pairing interactions within the precursor RNA stabilize the C and D boxes for protein binding.
• The C and D boxes themselves may provide metabolic stability to the mature snoRNA, even without a terminal stem.