Transcriptional and Metabolic Responses of Bacillus subtilis to the Availability of Organic Acids: Transcription Regulation Is Important but Not Sufficient To Account for Metabolic Adaptation

Abstract

The soil bacterium Bacillus subtilis can use sugars or organic acids as sources of carbon and energy. These nutrients are metabolized by glycolysis, the pentose phosphate pathway, and the Krebs citric acid cycle. While the response of B. subtilis to the availability of sugars is well understood, much less is known about the changes in metabolism if organic acids feeding into the Krebs cycle are provided. If B. subtilis is supplied with succinate and glutamate in addition to glucose, the cells readjust their metabolism as determined by transcriptome and metabolic flux analyses. The portion of glucose-6-phosphate that feeds into the pentose phosphate pathway is significantly increased in the presence of organic acids. Similarly, important changes were detected at the level of pyruvate and acetyl coenzyme A (acetyl-CoA). In the presence of organic acids, oxaloacetate formation is strongly reduced, whereas the formation of lactate is significantly increased. The alsSD operon required for acetoin formation is strongly induced in the presence of organic acids; however, no acetoin formation was observed. The recently discovered phosphorylation of acetolactate decarboxylase may provide an additional level of control of metabolism. In the presence of organic acids, both types of analyses suggest that acetyl-CoA was catabolized to acetate rather than used for feeding the Krebs cycle. Our results suggest that future work has to concentrate on the posttranslational mechanisms of metabolic regulation.Bacillus subtilis and its close relatives are among the most important industrial microorganisms. They are used for the production of antibiotics, enzymes such as proteases, amylases, lipases, or cellulases, vitamins such as riboflavin, and even insecticidal proteins. Due to this practical relevance, much research has been devoted to many aspects of these bacteria, making B. subtilis one of the best-studied living organisms.The basis for understanding the potential of B. subtilis is a firm knowledge of its metabolism. These bacteria prefer sugars and ammonia as carbon and nitrogen sources, respectively. The preferred source of carbon and energy is glucose. This sugar is taken up and concomitantly phosphorylated by the bacterial phosphoenolpyruvate:sugar phosphotransferase system. The further metabolism involves glycolysis, the pentose phosphate pathway, and the Krebs cycle, resulting in the oxidation of glucose to carbon dioxide and the generation of ATP, a proton-motive force, reducing power, and intermediates for all anabolic reactions. Similarly, other sugars and polyols can be phosphorylated and are catabolized in a similar manner (for a review, see reference 35). The utilization of organic acids requires the conversion of these acids to intermediates of the Krebs cycle, gluconeogenesis, and the pentose phosphate pathway to obtain both the energy and the metabolic precursors needed by the cell (13). The central intermediate of nitrogen metabolism is glutamate, the universal amino-group donor for the biosynthesis of amino aci...

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