FIG. 17. Biosynthetic pathways of homoserine, threonine, and methionine, showing sites of gene action (124, 208, 209, 351, 352, 518, 547, 965). For conversion of threonine to isoleucine, see Fig. 15. H4PteGlu, Tetrahydrofolate. It is not clear whether the polyglutamylation step controlled by met-6 occurs only at the stage shown.