Suppression of the Succinate Requirement of Lipoamide Dehydrogenase Mutants of Escherichia coli by Mutations Affecting Succinate Dehydrogenase Activity

Abstract

Reversion studies with a variety of different lipoamide dehydrogenase mutants of Escherichia coli k12 led to the discovery of a class of partial revertant which no longer required succinate for aerobic growth on glucose but retained all the other nutritional characteristics of the Lpd− phenotype. These revertants were given the phenotypic designation Lpd−Sin+ to denote their succinate independence, and mutations in the gene(s) designated sin were considered responsible for generating the succinate-independent phenotype of lpd mutants. Enzymological and genetic studies confirmed that the revertants retained their lpd mutations. Moreover, all of the 24 independent revertants tested possessed sin mutations located in the gal region of the chromosome, presumably suppressing by an intergenic and indirect mechanism. The sin mutations exhibited no allele specificity for suppression and they were recessive to the wild-type sin + allele in merodiploid strains. A common feature of the Lpd−Sin+ revertants (succinate-independent derivatives of lpd mutants) was a deficiency in succinate dehydrogenase. On their own, the sin mutations produced a phenotype analogous to that of sdh mutants. Furthermore, the probable identity of sin and sdh mutations was confirmed by reconstruction studies in which an lpd sdh double mutant was shown to exhibit an Lpd−Sin+ phenotype. It was concluded that the requirement for exogenous succinate by mutants lacking lipoamide dehydrogenase (and hence overall 2-oxoglutarate dehydrogenase complex activity) can be eliminated by inactivating succinate dehydrogenase because the latter enzyme depletes the intracellular succinate and succinylCoA pools by removing succinate faster than it can be supplied by other endogenous mechanisms. In fact, it would appear that the presence of active succinate dehydrogenase is responsible for the nutritional requirement for succinate of mutants lacking overall 2-oxoglutarate dehydrogenase complex activity during aerobic growth on glucose. A gene-dosage effect on the activities of the 2-oxoglutarate dehydrogenase complex was also demonstrated with corresponding merodiploid strains.