Properties of mutants of Escherichia coli lacking malic dehydrogenase and their revertants.

Mutants of Escherichia coli lacking malic dehydrogenase activity (mdh) were incapable of growth on acetate", succinate- or malate/mineral medium. Revertants of mdh strains which had regained the ability to grow on C4-dicarboxylic acids could be divided into two distinct classes. One type of revertant had regained the ability to synthesize functional malic dehydrogenase. The other type of revertant still lacked malic dehydrogenase activity but possessed a suppressor mutation which altered the regulation of the synthesis or activity of the C4-dicarboxylic acid transport system, resulting in increased C4-dicarboxylic acid transport activity. This latter class of revertants apparently synthesized oxalacetate from malate via the sequential actions of the NAD-linked malic enzyme, phosphoenolpyruvate synthetase, and phosphoenolpyruvate carboxylase. Evidence has been presented that is consistent with the hypothesis that oxalacetate is the inducer of the C4-dicarboxylic acid transport system. The inability of mutants lacking malic dehydrogenase to grow with a C4-dicarboxylic acid as the carbon source can be attributed to the difficulty such mutants have in synthesizing oxalacetate.