Figure 7. Nc-plots of codon usage bias in Hsp60 genes from the 17 phyla. Gray scatter plots represent ENC values versus GC3 content for Hsp60 genes from 17 phyla. The black bell-shaped curves represent the expected effective number of codons (ENCexp ), i.e. predicted ENC values if codon usage bias is influenced by GC3 content (GC content at the third synonymous position of codons) in the Hsp60 gene only. Phyla were sorted by the average total GC content of Hsp60 genes.
Almost all Hsp60 genes have lower ENC values than expected, suggesting that the pressure of mutations affects codon usage59. As with the neutrality plot analysis (see above), attention should be paid to the Nc-plot for Chordata, where the ENC values are divided. Accordingly, the ENC values of the Hsp60 genes from Chordata were grouped using taxonomy (class) (Figure 8).