Significant differences between the G+C content of synonymous codons in orthologous genes and the genomic G+C content

Matthew I. Bellgard*, Takashi Gojobori

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

17 Scopus citations


The relationship between the overall G+C content of the genome (GC) and the GC content at the third codon positions (GC3) of genes, which we refer to as a GC3-plot, was examined using 15 currently available complete genome sequences. A remarkably linear relationship was found between these two quantities, confirming previous observations of a strong positive correlation in the GC3-plot. In order to conduct a more detailed analysis of the GC3-plot, we examined the GC3 content by separating orthologous codons into three categories: synonymously different codons (namely identical amino acids, IA), different amino acids (DA), and identical codons (IC), for a pairwise comparison of two closely related species. When we took pairwise species comparisons between Mycoplasma genitalium (Mg) and Mycoplasma pneumoniae (Mp) and between Mycobacterium tuberculosis (Mt) and Mycobacterium leprae (Ml) as examples, we found that for Mp and Ml, the GC3 for IA deviated the most from the linear expectation in the GC3-plot, whereas for Mg and Mt the deviation was minimal. These findings suggest that the major changes of GC content took place in Mp and Ml, but not in Mg and Mt. This analysis also enables us to predict the future direction of the evolutionary changes of the genomic GC content. (C) 1999 Elsevier Science B.V. All rights reserved.

Original languageEnglish (US)
Pages (from-to)33-37
Number of pages5
Issue number1
StatePublished - Sep 30 1999
Externally publishedYes


  • Common ancestor
  • Complete genome sequences
  • G+C content

ASJC Scopus subject areas

  • Genetics


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