CONSTRICTED FLUX THROUGH THE BRANCHED-CHAIN AMINO-ACID BIOSYNTHETIC ENZYME ACETOLACTATE SYNTHASE TRIGGERS ELEVATED EXPRESSION OF GENES REGULATED BY RPOS AND INTERNAL ACIDIFICATION

The first common enzyme of isoleucine and valine biosynthesis, acetola ctate synthase (ALS), is specifically inhibited by the herbicide sulfo meturon methyl (SM). To further understand the physiological consequen ces of flux alterations at this point in metabolism, Escherichia coli genes whose expression was induced by partial inhibition of ALS were s ought. Plasmid-based fusions of random E. coli DNA fragments to Photor habdus luminescens luxCDABE were screened for bioluminescent increases in actively growing liquid cultures slowed 25% by the addition of SM, From more than 8,000 transformants, 12 unique SM-inducible promoter-l ux fusions were identified, The lux reporter genes were joined to seve n uncharacterized open reading frames, f253a, f415, frvX, o513, o521, yciG, and yohF, and five known genes, inaA, ldcC, osmY, poxB, and sohA . Inactivation of the rpoS-encoded sigma factor, sigma(S), reduced bas al expression levels of six of these fusions 10- to 200-fold, These si x genes defined four new members of the sigma(S) regulon, f253a, ldcC, yciG, and yohF, and included two known members, osmY and poxB, Furthe rmore, the weak acid salicylate, which causes cytoplasmic acidificatio n, also induced increased bioluminescence from seven SM-inducible prom oter-lux fusion-containing strains, namely, those with fusions of the sigma(S)-controlled genes and inaA, The pattern of gene expression cha nges suggested that restricted ALS activity may result in intracellula r acidification and induction of the sigma(S)-dependent stress respons e.