MUTANTS DEFECTIVE IN THE ENERGY-CONSERVING NADH DEHYDROGENASE OF SALMONELLA-TYPHIMURIUM IDENTIFIED BY A DECREASE IN ENERGY-DEPENDENT PROTEOLYSIS AFTER CARBON STARVATION

NADH dehydrogenase is the first component of the respiratory chain. It transfers electrons from NADH to ubiquinone and concomitantly establi shes a proton motive force across the membrane. Salmonella typhimurium mutants defective in this enzyme were isolated in a screen for strain s with increased expression of beta-galactosidase from a hemA-lacZ pro tein fusion. This unexpected phenotype results from stabilization of t he hybrid protein during carbon starvation and is apparently due to an energy requirement for proteolytic attack. Sequence analysis of DNA f ragments cloned from an insertion mutant indicates that S. typhimurium has a large cluster of genes encoding the energy-conserving NADH dehy drogenase, similar to one recently described in Paracoccus denitrifica ns. These findings establish the potential for genetic analysis of a c omplex enzyme whose function, especially in proton efflux, is poorly u nderstood.