Structural analysis of elements contributing to 5' splice site selection in plant pre-mRNA transcripts

Area of Science:

• Molecular Biology
• Plant Science
• Genetics

Background:

• Pre-mRNA splicing is a critical gene expression step.
• Splice site selection in plants is influenced by AU-rich sequences and U1 snRNA complementarity.
• Previous studies in tobacco identified positional and complementarity factors for 5' splice site selection.

Purpose of the Study:

• To identify specific nucleotide sequences that define the 5' exon-intron boundary for splice site recognition.
• To investigate the role of a 9-nucleotide element within a plant intron in 5' splice site choice.

Main Methods:

• Introduction of multiple mutations into an AU-rich block in a pea rbcS3A intron derivative.
• Transient expression of mutant transcripts in tobacco nuclei.
• Analysis of 5' splice site selection patterns in response to sequence alterations.

Main Results:

• A 9-nucleotide element, located 30-38 nucleotides downstream from the 5' splice site, significantly affects splice site choice.
• Guanosine substitutions creating an AG-rich motif (AAAGGAGAGGCAGA) reduced upstream splice site recognition and enhanced downstream splice site recognition.
• Uridine substitutions did not alter selection patterns, while cytosine substitutions marginally reduced upstream recognition.
• Mutations in an adjacent AU-rich region modulated the effects of the 9-nucleotide element but did not independently alter splice site selection.

Conclusions:

• Specific sequence elements on both sides of the exon-intron boundary are essential for defining the 5' splice site in plant transcripts.
• The identified 9-nucleotide element acts as a 5' exon-defining element, promoting downstream splice site recognition.
• AU-rich motifs contribute to upstream splice site recognition, working in concert with the exon-defining element.