Lethal and temperature-sensitive mutations and their suppressors identify an essential structural element in U2 small...

Area of Science:

• Molecular Biology
• RNA Structure
• Eukaryotic Gene Expression

Background:

• U2 small nuclear RNA (snRNA) is vital for the spliceosome, the molecular machine responsible for RNA splicing in eukaryotes.
• Phylogenetic analysis suggests multiple secondary structures for the 5' region of U2 snRNA, necessitating experimental validation.
• Understanding U2 snRNA structure is key to elucidating the mechanisms of pre-mRNA splicing.

Purpose of the Study:

• To determine the functional secondary structure of the 5' half of yeast U2 snRNA.
• To identify essential structural elements required for U2 snRNA function in splicing.
• To correlate in vivo RNA structure with genetic data.

Main Methods:

• Site-directed mutagenesis of the yeast U2 snRNA gene (>35 mutations).
• Construction of compensatory mutations to assess base-pairing requirements.
• In vivo chemical structure probing of U2 RNA.
• Analysis of splicing phenotypes, including temperature and salt sensitivity, and precursor accumulation.

Main Results:

• An essential stem-loop structure adjacent to the branchpoint interaction region was identified.
• A conserved complementarity to the loop upstream of the Sm site and another stem-loop were found to be dispensable.
• Non-Watson-Crick base pairings in the essential stem caused temperature-sensitive splicing defects and precursor accumulation.
• In vivo structure probing confirmed genetic findings, indicating the essential stem-loop is conserved.

Conclusions:

• The study identifies a critical stem-loop structure in U2 snRNA essential for splicing in yeast.
• This structure may not bind U2 snRNP proteins directly but could mediate interactions with other splicing factors.
• The findings provide insights into the functional architecture of the spliceosome and RNA structure-function relationships.