Selection of AUG initiation codons differs in plants and animals

Area of Science:

• Molecular Biology
• Genetics
• Biochemistry

Background:

• Protein synthesis initiation is a critical regulatory step in gene expression.
• The sequence context surrounding the AUG start codon plays a role in translation initiation efficiency.

Purpose of the Study:

• To investigate the impact of the nucleotide at position -3 relative to the AUG start codon on protein synthesis initiation.
• To compare translation initiation mechanisms in different in vitro systems and across species.

Main Methods:

• Synthetic messenger RNAs (mRNAs) with variations at nucleotide -3 were created using SP6 promoter-directed cDNA transcription.
• In vitro translation assays were performed using reticulocyte lysate and wheat germ systems.
• Analysis of plant and animal mRNA sequences to determine consensus sequences around AUG initiation codons.

Main Results:

• A hierarchy of translational efficiency was observed in the reticulocyte lysate system, with nucleotide A at position -3 yielding the highest efficiency (100%), followed by G (85%), U (61%), and C (38%).
• No significant differences in translational efficiency were observed in the wheat germ system.
• Differential mRNA degradation or polypeptide chain elongation were ruled out as explanations for the observed differences in the reticulocyte lysate.
• mRNA competition assays exacerbated efficiency differences in reticulocyte lysate but had no effect in wheat germ.
• Consensus sequences around AUG initiation codons differed between plants (AACAAUGGC) and animals (CACCAUG).

Conclusions:

• The nucleotide at position -3 significantly influences translation initiation efficiency in a reticulocyte lysate system.
• Plant and animal translation initiation factors exhibit distinct preferences for start codon context, suggesting divergent mechanisms for selecting AUG initiation codons.