Figure 3.

The asymmetric relationship between asd and thrB, two proteins conserved between E. coli (green) and S. cerevisiae (blue), is reflected in their asymmetric phylogenetic distributions. The activity of asd does not depend on thrB while the activity of thrB does depend on asd. Although in most cases both enzymes are present or absent together (243), thrB is more frequently absent while asd is present (129) than vice versa (1). The exception to the pattern comes from Thermofilum pendens, a species that has lost a large number of amino acid biosynthetic pathways, and imports most of its amino acids [14]. Note that a second asymmetric reaction pair between asd and the initial enzyme in the lysine synthesis pathway, present in E. coli, is not conserved in S. cerevisiae.