Figure 11. Average values of relative synonymous codon usage (RSCU) for Hsp60 genes from 17 phyla. The RSCU values for each of 17 phyla were divided into two main groups according to the type of base at the third synonymous position of codon: a - A/T-ending codons; b - G/C-ending codons. Phyla were divided into three groups by the direction of the mutational pressure (see Table 2) and sorted by the average total GC content of Hsp60 genes. The codons were sorted by the average RSCU value between 17 phyla.
The RSCU values of A/T-ending codons were higher for the Hsp60 genes under the influence of AT mutation pressure, and vice versa(Figure 11), which was to be expected72. The RSCU values greater than 1.5 and less than 0.5 correspond to high and low represented codons, respectively. Among the stop codons, TAA is the most widely represented stop codon with an average RSCU value of 1.8±0.7. In turn, TGA codon is widely used in the Hsp60 genes of Euglenozoa and Actinobacteria with the average RSCU values of 1.8±1.6 and 1.8±1.5, respectively. This may be due to the direction of the mutational pressure affecting the Hsp60 genes of these phyla, since with an increase in the GC content, the frequency of TGA codon usage increases73. This may also explain the decrease in the average RSCU values for TAA for GC-biased phyla (Figure 11).
To assess the main trends in the synonymous codon usage in Hsp60 genes from phyla with different directional mutation pressure, summary patterns of RSCU values were compiled (Figure 12).