SnoRNA guide activities: real and ambiguous

Area of Science:

• Molecular Biology
• RNA Biology
• Genetics

Background:

• Eukaryotic rRNAs and snRNAs undergo extensive post-transcriptional modifications, primarily pseudouridylation and 2'-O-methylation.
• These modifications are guided by small nucleolar (sno)RNAs and small Cajal body-specific (sca)RNAs.
• The precise mechanisms and regulation of these RNA modification pathways are not fully understood.

Purpose of the Study:

• To investigate the guide activities of snoRNAs and scaRNAs in directing RNA modifications.
• To determine if complementarity between guide RNAs and target RNAs is sufficient for modification of endogenous substrates.
• To explore the complexity and regulation of RNA modification processes in yeast and vertebrate cells.

Main Methods:

• Utilized yeast and vertebrate cell systems to test predicted guide activities of snoRNAs based on rRNA complementarity.
• Employed artificial substrate RNAs to assess modification potential.
• Investigated guide RNA activity in Xenopus cells for U2 snRNA modification.

Main Results:

• Confirmed human SNORA24 as a guide RNA for 18S rRNA pseudouridylation (18S-Ψ609), despite noncanonical base-pairing.
• Identified snoRNAs capable of modifying artificial substrates but not endogenous rRNAs.
• Observed that rRNA fragments can be modified in artificial backbones but not in endogenous rRNAs.
• Demonstrated that a scaRNA-derived guide, but not a snoRNA-derived guide, induced pseudouridylation in Xenopus U2 snRNA.

Conclusions:

• Post-transcriptional RNA modification is a highly regulated process, exceeding simple base-pairing predictions.
• The efficiency and specificity of RNA modification depend on factors beyond guide RNA-substrate complementarity.
• Unexpected regulatory mechanisms likely govern RNA modification in functionally important RNAs like rRNAs and snRNAs.