ABSTRACT: Gene expression profiles of Escherichia coli K-12 W3110 were compared as a function of steady-state external pH. Cultures were grown with aeration to an optical density at 600 nm of 0.3 in potassium-modified Luria-Bertani medium buffered at pH 5.0, 7.0, and 8.7. For each of the three pH conditions, cDNA from RNA of five independent cultures was hybridized to Affymetrix E. coli arrays. Analysis of variance with a significance level of 0.001 resulted in 98% power to detect genes showing a twofold difference in expression. Normalized expression indices were calculated for each gene and intergenic region (IG). Differential expression among the three pH classes was observed for 763 genes and 353 IGs. Hierarchical clustering yielded six well-defined clusters of pH profiles, designated Acid High (highest expression at pH 5.0), Acid Low (lowest expression at pH 5.0), Base High (highest at pH 8.7), Base Low (lowest at pH 8.7), Neutral High (highest at pH 7.0, lower in acid or base), and Neutral Low (lowest at pH 7.0, higher at both pH extremes). Flagellar and chemotaxis genes were repressed at pH 8.7 (Base Low cluster), where the cell's transmembrane proton potential is diminished by the maintenance of an inverted pH gradient. High pH also repressed the proton pumps cytochrome o (cyo) and NADH dehydrogenases I and II. By contrast, the proton-importing ATP synthase F1Fo and the microaerophilic cytochrome d (cyd), which minimizes proton export, were induced at pH 8.7. These observations are consistent with a model in which high pH represses synthesis of flagella, which expend proton motive force, while stepping up electron transport and ATPase components that keep protons inside the cell. Acid-induced genes, on the other hand, were coinduced by conditions associated with increased metabolic rate, such as oxidative stress. All six pH-dependent clusters included envelope and periplasmic proteins, which directly experience external pH. Overall, this study showed that (i) low pH accelerates acid consumption and proton export, while coinducing oxidative stress and heat shock regulons; (ii) high pH accelerates proton import, while repressing the energy-expensive flagellar and chemotaxis regulons; and (iii) pH differentially regulates a large number of periplasmic and envelope proteins. Experiment Overall Design: Gene expression profiles of Escherichia coli K-12 W3110 were compared as a function of steady-state external pH. Overnight cultures were diluted 1:1000 in potassium-modified Luria-Bertani medium (LBK) buffered with 50 mM HOMOPIPES at pH 5.0, pH 7.0, and pH 8.7. Bacteria were cultured in baffled flasks (less than 10% volume filled) with rotation at 240 rpm, incubated at 37°C to an optical density at 600 nm of 0.3. For each of the three pH conditions, RNA was isolated from five independent cultures. Labeled cDNA was hybridized to Affymetrix antisense arrays according to standard procedures. To analyze the expression levels, Dchip software was used to generate model-based expression indices normalized to sample pH 7 replicate 1. ANOVA was used to identify genes with sitnificant expression differences among the three pH classes (p = 0.001). For genes showing significant differences, the Log2 expression ratios were determined for each pair of pH classes, and significance was determined by Tukey's test (p = 0.001).
