Bioenergetics of the Moderately Halophilic Bacterium Halobacillus halophilus: Composition and Regulation of the Respiratory Chain

In their natural environments, moderately halophilic bacteria are confronted not only with high salinities but also with low oxygen tensions due to the high salinities. The growth ofH. halophilus is strictly aerobic. To analyze the dependence of respiration on the NaCl concentration and oxygen availability of the medium, resting cell experiments were performed. The respiration rates were dependent on the NaCl concentration of the growth medium, as well as on the NaCl concentration of the assay buffer, indicating regulation on the transcriptional and the activity level. Respiration was accompanied by the generation of an electrochemical proton potential (Δμ̃H+ ) across the cytoplasmic membrane whose magnitude was dependent on the external pH. Genes encoding proteins involved in respiration and Δμ̃H+ generation, such as a noncoupled NADH dehydrogenase (NDH-2), complex II, and complex III, were identified in the genome. In addition, genes encoding five different terminal oxidases are present. Inhibitor profiling revealed the presence of NDH-2 and complex III, but the nature of the oxidases could not be resolved using this approach. Expression analysis demonstrated that all the different terminal oxidases were indeed expressed, but by far the most prominent wascta , encoding cytochromecaa 3 oxidase. The expression of all of the different oxidase genes increased at high NaCl concentrations, and the transcript levels ofcta andqox (encoding cytochromeaa 3 oxidase) also increased at low oxygen concentrations. These data culminate in a model of the composition and variation of the respiratory chain ofH. halophilus .