Cumulative frequency distribution of the 20 standard amino acids in Archaea, Bacteria, and Eukaryota. Normality tests with SAS statistic software showed that the frequencies of the 20 amino acids in the three domains followed a normal distribution (data not shown). The cumulative frequency ranged from 0 to 1. The distribution followed an S- shaped curve and the largest change in cumulative frequency occurred when the amino acid frequency was highest. The null hypothesis was that there was no difference between amino acid frequency in the proteomes and the frequency based on the universal genetic codes, namely the number of synonymous codons divided by the total number of sense codons. Difference analysis among the amino acid frequencies from different taxa was performed at the 0.05 level using Duncan's multiple-range test. The data were displayed as mean and variance. The superscripts a, b, c and d are Duncan groupings, which indicated that amino acid frequencies within the same grouping were not significantly different. The descending order of the amino acid frequencies correspond to the Duncan groupings from a to d.
Chen et al. BMC Evolutionary Biology 2013 13:219 doi:10.1186/1471-2148-13-219