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Department of Biology, McMaster University, 1280 Main Street West, Hamilton, Ontario, Canada L8S 4K1
ABSTRACT
The formation of phosphoenolpyruvate (PEP) is a major step in the gluconeogenic pathway in which tricarboxylic acid (TCA) cycle intermediates are converted to hexose sugars. In Rhizobium (now Sinorhizobium) meliloti this step is catalysed by the enzyme PEP carboxykinase (PCK) which converts oxaloacetate to PEP. R. meliloti Pck-mutants grow very poorly with TCA cycle intermediates as the sole source of carbon. Here, the isolation and mapping of suppressor mutations which allow Pck-mutants to grow on succinate and other TCA cycle intermediates is reported. Tn5 insertions which abolished the suppressor phenotype and mapped to the suppressor locus were located within the pod gene encoding pyruvate orthophosphate dikinase (PPDK). Strains carrying suppressor mutations had increased PPDK activity compared to the wild-type. The suppressor phenotype was dependent on the combined activities of malic enzyme and PPDK, which thus represent an alternative route for the formation of PEP in R. meliloti. PPDK activity was not required for symbiotic N2 fixation.
Author for correspondence: Turlough M. Finan. Tel: +1 905 525 9140 ext. 12932. Fax: + 1 905 522 6066. e-mail: finan@mcmaster.ca
Present address: Department of Natural Resource Science, McGill University, Macdonald Campus, 21, 111 Lakeshore, Ste-Anne-de-Bellevue. Québec, Canada H9X 3V9.
Present address: Laboratoire de Biologie Végétale et Microbiologie URA CNRS 1114, Université de Nice Sophia Antipolis, Parc Vairose, 06108 Nice Cedex, France.
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