FIG 9.

Growth of the indicated strains on solid media with different nutritional additions or subtractions. See Materials and Methods for more details. AA, mixture of all 20 amino acids. Omitted amino acids are indicated separately after a minus sign. The mixture without isoleucine did not support growth of any of the strains, presumably because of the toxicity of valine in the absence of isoleucine (68, 69), and the results are not shown. The first four rows represent controls. The third line (red) shows that with the synthetic mixture (−Hx+AA), the optA1 gpt strain was not able to grow at a low dilution. The rows in blue are the cases where growth of this strain at a low dilution was enabled by the absence of histidine (His), the absence of serine (Ser), or the addition of hypoxanthine (Hx) or thiamine (Thi). Not shown here are the +Hx−AA and −Hx−AA conditions for the optA1 gpt strain, which showed normal growth (no growth inhibition) (also see reference 20).