Acid- and base-induced proteins during aerobic and anaerobic growth of Escherichia coli revealed by two-dimensional gel electrophoresis

Proteins induced by acid or base, during long-term aerobic or anaerobic gro wth in complex medium, were identified in Escherichia coli, Two-dimensional gel electrophoresis revealed pH-dependent induction of 18 proteins, nine o f which were identified by N-terminal sequencing. At pH 9, tryptophan deami nase (TnaA) was induced to a high level, becoming one of the most abundant proteins observed. TnaA may reverse alkalinization by metabolizing amino ac ids to produce acidic products. Also induced at high pH, but only in anaero biosis, was glutamate decarboxylase (GadA), The gad system (GadA/GadBC) neu tralizes acidity and enhances survival in extreme acid; its induction durin g anaerobic growth may help protect alkaline-grown cells from the acidifica tion resulting from anaerobic fermentation. To investigate possible respons es to internal acidification, cultures were grown,rn in propionate, a membr ane-permeant weak acid which acidifies the cytoplasm. YfiD, a homologue of pyruvate formate lyase, was induced to high levels at pH 4.4 and induced tw ofold more by propionate at pH 6; both of these conditions cause internal a cidification. At neutral or alkaline pH, YfiD was virtually absent. YfiD is therefore a strong candidate for response to internal acidification. Acid or propionate also increased the expression of alkyl hydroperoxide reductas e (AhpC) but only during aerobic growth. At neutral or high pH, AhpC showed no significant difference between aerobic and anaerobic growth, The increa se of AhpC in acid may help protect the cell from the greater concentration s of oxidizing intermediates at low pH, Isocitrate lyase (AceA) was induced by oxygen across the pH range but showed substantially greater induction i n acid or in base than at pH 7, Additional responses observed included the induction of MalE at high pH and induction of several enzymes of sugar meta bolism at low pH: the phosphotransferase system components ManX and PtsH an d the galactitol fermentation enzyme GatY. Overall, our results indicate co mplex relationships between pH and oxygen and a novel permeant acid-inducib le gene, YfiD.