Regulatory Loop between Redox Sensing of the NADH/NAD + Ratio by Rex (YdiH) and Oxidation of NADH by NADH Dehydrogenase Ndh in Bacillus subtilis

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Abstract
NADH dehydrogenase is a key component of the respiratory chain. It catalyzes the oxidation of NADH by transferring electrons to ubiquinone and establishes a proton motive force across the cell membrane. The yjlD (renamed ndh) gene of Bacillus subtilis is predicted to encode an enzyme similar to the NADH dehydrogenase II of Escherichia coli, encoded by the ndh gene. We have shown that the yjlC-ndh operon is negatively regulated by YdiH (renamed Rex), a homolog of Rex in Streptomyces coelicolor, and a redox-sensing transcriptional regulator that responds to the NADH/NAD+ ratio. The ndh gene regulates expression of the yjlC-ndh operon, as indicated by the fact that mutation in ndh causes a higher NADH/NAD+ ratio. An in vitro study showed that Rex binds to the downstream region of the yjlC-ndh promoter and that NAD+ enhances the binding of Rex to the putative Rex-binding sites in the yjlC-ndh operon as well as in the cydABCD operon. These results indicated that Rex and Ndh together form a regulatory loop which functions to prevent a large fluctuation in the NADH/NAD+ ratio in B. subtilis.