Fig. 25. Deduced change of SNS-encodeing amino acid (SNS-AA) contents in conserved and non-conserved regions of homologous proteins occurring in a process of gene evolution. (A) Change of SNS-AA contents (shaded boxes) is deduced according to our evolution models of genes (Fig. 24) and proteins (Fig. 19), in which genes have been evolved uni-directionally from GC-rich to AT-rich genes. In this case, SNS-AA contents in regions conserved among homologous proteins keep them high during the gene evolution, while SNS-AA contents in nonconserved regions of proteins encoded by AT-rich genes gradually decrease as the GC content of a gene decreases. (B) Change of SNS-AA contents is deduced according to a model, in which genes have been evolved uni-directionally but inversely from AT-rich to GC-rich genes. In this case, SNS-AA contents (shaded boxes) in regions conserved among homologous proteins remains low during the gene evolution, while SNS-AA contents in nonconserved regions of proteins encoded by GC-rich genes gradually increase as the GC content of a gene increases.