Figure 12. Summary patterns of relative synonymous codon usage for Hsp60 genes being under the different mutational pressure. The average RSCU values of Hsp60 genes from phyla with the AT, AT/GC, and GC mutational pressure 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. The codons were sorted by the average RSCU value between all 17 phyla.
As can be seen, the number of high and low represented codons that correspond to phyla under the GC mutational pressure is greater than for AT and AT/GC-biased phyla. Such results for AT/GC-biased phyla can be explained by their GC content at the border between the AT and GC mutational pressure groups (Table 2). Codons encoding Ala, Arg, Gly, Ile, Leu, Ser, Thr, and Val were widely represented for almost all Hsp60 genes. For each of 17 phyla, the set of these codons is consistent with the direction of the mutational pressure (predominance of A/T- or G/C-ending codons) affecting the Hsp60 genes. CGA (R), GGG (G), and TAG (stop) codons have the lowest average RSCU values for all 17 phyla (less than 0.5) regardless of the direction of the mutational pressure.
Taking these results into account, it can be assumed that with an increase in the GC content in the Hsp60 gene due to mutations at the third synonymous codon position, the selectivity of codon usage increases.