(a) The y components of the Z curves for B. burgdorferi, T. pallidum, C. trachomatis and C. muridarum genomes. (b) The y component of the Z curve for E. coli K-12 genome. Comparing (a) with (b) and putting emphasis on the coordinate values, it is found that the y component increases or decreases much slower along the DNA sequence for E. coli genome than that for the other four genomes. Note that the y component of the Z curve represents the plus of cumulative excess of G over C and T over A. Therefore, there are remarkable excess of G over C and T over A in the four bacterial genomes.