Summary: Gluconeogenesis in MNF3841 takes place via the derepressible enzymes phosphoenolpyruvate carboxykinase (PEPCK) and fructose bisphosphate aldolase. A Tn5-induced PEPCK deficient mutant (MNF3085) fails to grow on a wide range of simple carbon sources. PEPCK and fructose bisphosphate aldolase are rapidly derepressed when cells are transferred from a sugar to an organic acid as the sole carbon source. The addition of 0·1 mM-sucrose causes an 80% inhibition of PEPCK and fructose bisphosphate aldolase synthesis, and 0·4 mM-sucrose causes a complete inhibition of PEPCK synthesis. Pea bacteroids of MNF3841 purified on a Percoll gradient contain low levels of PEPCK and fructose bisphosphate aldolase. This bacteroid-associated PEPCK activity is clearly of bacterial not plant origin because of its nucleotide requirement, and the fact that bacteroids of MNF3085 contain no PEPCK activity at all. Although the mutant lacks PEPCK it is still able to nodulate and fix N as effectively as the parent. The capacity to synthesize sugars via gluconeogenesis is not required for an effective symbiosis. Furthermore, these data suggest that pea bacteroids receive sufficient sugar to compensate for the gluconeogenic defect in strain MNF3085, but insufficient to completely repress the synthesis of PEPCK in the wild-type.


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