Summary: Reversion studies with a variety of different lipoamide dehydrogenase mutants of 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 LpdSin to denote their succinate independence, and mutations in the gene(s) designated were considered responsible for generating the succinate-independent phenotype of mutants. Enzymological and genetic studies confirmed that the revertants retained their mutations. Moreover, all of the 24 independent revertants tested possessed mutations located in the region of the chromosome, presumably suppressing by an intergenic and indirect mechanism. The mutations exhibited no allele specificity for suppression and they were recessive to the wild-type allele in merodiploid strains.

A common feature of the LpdSin revertants (succinate-independent derivatives of mutants) was a deficiency in succinate dehydrogenase. On their own, the mutations produced a phenotype analogous to that of mutants. Furthermore, the probable identity of and mutations was confirmed by reconstruction studies in which an double mutant was shown to exhibit an LpdSin 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.


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