Fig. 26. SNS-AA contents in conserved and nonconserved regions among homologous proteins. SNS-AA contents in both conserved (open circles) and non-conserved (closed circles and open squares) regions were obtained after alignments of GC-rich Pseudomonas aeruginosa GyrA (A); and GC-rich Streptomyces coelicolor RelA/SpoT (B); with the corresponding homologous proteins from 13 other microbial taxa. These results clearly indicate that both GyrA genes and RelA/SpoT genes changed unidirectionally from GC-rich ancestral genes to AT-rich genes under AT-mutation pressure, as was expected from our hypothesis on gene evolution (Fig. 25).