Nucleic acid composition, codon usage, and the rate of synonymous substitution in protein-coding genes

Area of Science:

• Molecular Evolution
• Genomics
• Bioinformatics

Background:

• Synonymous substitution rates vary significantly across genes.
• Unequal codon usage is a common phenomenon in many organisms.
• Understanding factors influencing substitution rates is crucial for evolutionary studies.

Purpose of the Study:

• To investigate the correlation between nucleotide composition and synonymous substitution rates.
• To determine the role of selection in shaping codon usage and evolutionary rates.
• To explore the relationship between synonymous and nonsynonymous substitution rates.

Main Methods:

• Comparative analysis of 42 protein-coding gene pairs from rat and human.
• Statistical correlation analysis between nucleotide frequencies and synonymous substitution rates.
• Assessment of mutation bias and selection pressures.

Main Results:

• A positive correlation was observed between AT content and synonymous substitution rates.
• A negative correlation was found between GC content and synonymous substitution rates.
• Selection against A and T in synonymous positions explains observed variations, not mutation bias.
• Nucleotide composition in synonymous positions explains most rate variation.
• The nucleotide at the second codon position influences synonymous substitution rates, linking it to nonsynonymous rates.

Conclusions:

• Gene nucleotide composition, particularly AT richness, is a major determinant of synonymous substitution rates.
• Selection acts on synonymous sites, driving codon usage bias and influencing evolutionary divergence.
• Protein conservation is linked to slower synonymous evolution and biased codon usage.