Regulation of Cytochrome bd Expression in Mycobacterium smegmatis

Abstract

The metabolism and generation of energy by the majority of pathogenic bacteria in the host remains enigmatic and emerging evidence suggests that the identification of essential bacterial metabolic pathways that have no human counterparts may represent a virtually untapped source of novel drug targets for antibiotic development. The aim of this study was to determine how mycobacteria metabolize and generate energy microaerobically. To identify potential regulatory circuits that control the response to low oxygen tension, we targeted the enzyme cytochrome bd oxidase, purportedly involved in the generation of energy under these conditions, and isolated mutants that fail to activate expression of this enzyme. Instead of finding regulators of cytochrome bd oxidase, our screen revealed enzymes and regulators involved in redox homeostasis and defence against electrophiles (oxidative stress). We discovered ProR, a novel regulator of proline dehydrogenase and 1-pyrroline-5-carboxylate dehydrogenase, two enzymes involved in proline degradation. Evidence is presented that ProR is essential for growth on proline as a sole carbon source and that a proR mutant experiences DNA damage due to methylglyoxal, an endogenously produced toxic intermediate of glycolysis and glycerol metabolism. It is shown that proline confers resistance to methylglyoxal. Furthermore, we characterized the sodium/proline symporter PutP as a high-affinity transporter for proline and show that it is the major uptake system for proline in Mycobacterium smegmatis. A new model for methylglyoxal detoxification in Mycobacteria is presented.