mRNA leader length and initiation codon context determine alternative AUG selection for the yeast gene MOD5

Area of Science:

• Molecular Biology
• Yeast Genetics
• Gene Expression Regulation

Background:

• The MOD5 gene in Saccharomyces cerevisiae encodes a tRNA-modifying enzyme with two isozymes.
• These enzymes are crucial for modifying both cytoplasmic and mitochondrial tRNAs.
• The MOD5 gene utilizes two in-frame AUG start codons, leading to two protein forms with distinct cellular localizations.

Purpose of the Study:

• To investigate the regulatory mechanisms controlling translation initiation site selection in the MOD5 gene.
• To understand how different protein isoforms are generated from a single transcript.
• To identify factors influencing the differential localization of MOD5 protein products.

Main Methods:

• Analysis of MOD5 mRNA transcripts and their leader sequences.
• Experimental manipulation of 5' untranslated regions (UTRs) by incorporating ADH1 leader sequences.
• Site-directed mutagenesis of nucleotides surrounding the primary AUG start codon.

Main Results:

• Extending the 5' end of MOD5 mRNA with ADH1 leader sequences altered the AUG start-site selection.
• Modifications to the nucleotides flanking the first AUG codon also influenced translation initiation.
• These findings indicate that leader sequence characteristics and local nucleotide context play significant roles in controlling translation initiation.

Conclusions:

• The MOD5 gene employs alternative translation initiation mechanisms to produce functionally distinct proteins.
• Leader sequence length/structure and the sequence context of the initiation codon are key determinants of AUG selection.
• Ribosomal bypass of suboptimal AUG codons contributes to the differential expression of MOD5 protein isoforms, impacting their cellular functions.