Selection on start codons in prokaryotes and potential compensatory nucleotide substitutions.

Area of Science:

• Molecular Biology
• Evolutionary Biology
• Genomics

Background:

• Start codons initiate protein synthesis in prokaryotes.
• The AUG codon is the most common start codon, but alternatives exist.
• Understanding selection pressures on start codons is crucial for deciphering gene regulation.

Purpose of the Study:

• To investigate the evolutionary forces shaping start codon usage in bacteria and archaea.
• To determine if start codons are under purifying selection and identify the strength of this selection.
• To explore the relationship between start codon selection, gene expression, and regulatory sequences.

Main Methods:

• Comparative genomics analysis of 36 bacterial and archaeal genome groups.
• Reconstruction of start codon evolution.
• Comparison of observed substitution frequencies with neutral expectations from non-coding and 4-fold degenerate sites.

Main Results:

• AUG is actively maintained as the optimal start codon by purifying selection, with less frequent than expected substitutions to GUG and UUG.
• Selection on start codons is weaker than on codons within coding sequences, even for conservative amino acid substitutions.
• The AUG to UUG switch is strongly selected against.
• Selection is most pronounced in evolutionarily conserved, highly expressed genes.
• Compensatory mutations in the Shine-Dalgarno sequence are observed following suboptimal start codon mutations.

Conclusions:

• Prokaryotic translation start signals are under weak but significant purifying selection.
• This selection aims to maximize translation initiation rates and protein production.
• Gene expression levels and evolutionary conservation influence the strength of selection on start codons.