Fraction of Conserved Residues and Amino Acid Contact Density versus Sequence Length – Escherichia coli & Archaeoglobus fulgidus. Fraction of conserved residues: Red line is for Archaeoglobus fulgidus and blue line is for Escherichia coli. For each bin of 40 residues, the ratio of the total length of non-overlapping cross kingdom alignments divided by the total length of the proteins in that bin is plotted. Proteins that did not have cross kingdom matches contribute zero to the numerator but their lengths are added to the denominator. Error bars shown for Archaeoglobus fulgidus were computed as standard error of the mean and were essentially identical for Escherichia coli (not shown). Contact density: Plotted as green filled circles. For each bin of 50 residues, we plot contact density computed as described in Methods.