Acetate and formate stress: Opposite responses in the proteome of Escherichia coli

Acetate and formate are major fermentation products of Escherichia coli. Be low pH 7, the balance shifts to lactate; an oversupply of acetate or format e retards growth. E. coli W3110 was grown with aeration in potassium-modifi ed Luria broth buffered at pH 6.7 in the presence or absence of added aceta te or formate, and the protein profiles were compared by two-dimensional so dium dodecyl sulfate-polyacrylamide gel electrophoresis. Acetate increased the steady-state expression levels of 37 proteins, including periplasmic tr ansporters. for amino, acids and peptides (Ard, FRY, OppA, and ProX), metab olic enzymes (YfiD and GatY), the RpoS growth phase regulon, and the autoin ducer synthesis protein LuxS. Acetate repressed 17 proteins, among them pho sphotransferase (Pta). An ackA-pta deletion, which nearly eliminates interc onversion between acetate and acetyl-coenzyme A (acetyl-CoA),, led to, elev ated basal levels of 16 of the acetate-inducible proteins, including the Rp oS, regulon. Consistent with RpoS activation, the ackA-pta strain also show ed constitutive extreme-acid resistance., Formate, however, repressed 10 of the acetate-inducible proteins, including the RpoS regulon. Ten of the pro teins, with elevated basal levels in the ackA-pta strain were repressed by growth of the mutant with formate; thus, the formate response took preceden ce over the loss of the ackA-pta pathway. The similar effects of exogenous acetate and the ackA-pta deletion, and the opposite effect of formate, coul d have several causes; one possibility is, that the excess, buildup of acet yl-CoA upregulates stress proteins but excess formate depletes acetyl-CoA a nd downregulates these proteins.