Genes identified by MGK as essential for cell growth in the absence of aromatic amino acids. (a) Biosynthetic pathway of aromatic amino acids in E. coli. Shown in bold are the 13 genes whose inactivation is expected to cause aromatic amino acid auxotrophy. Genes aroF, aroH, aroG, aroK, and aroL are involved in parallel biochemical routes and their disruption should not cause auxotrophy. Underlined in red are genes identified by MGK with the defined library, and in blue with the random library. (b) Growth of select mutants in defined medium lacking aromatic amino acids. The behavior of aroB, aroC, aroD, aroE, epd, pheA, pdxA, tktA, trpA, trpB, trpC, trpD, trpE, tyrA, and tyrB mutants identified by MGK screen were essentially indistinguishable from aroA. Growth of ygdD mutant was similar to rpe mutant. Supplementing the medium with aromatic amino acids restored growth of all mutants to wild-type level. Supplementing the medium with vitamin B6 restores growth of epd and pdxA mutants (data not shown). MGK, Monitoring of Gene Knockouts; OD, optical density; wt, wild-type Escherichia coli.