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

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 .