Carbon assimilation by Pseudomonas oxalaticus (OX1). 3. Oxalate utilization during growth on oxalate

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Abstract
The incorporation of carbon atoms from [Cl4]oxalate, [C14]formate or [C14]bicarbonate into the aqueous ethanol-soluble fraction of Pseudomonas oxalaticus (OX1) grown on oxalate has been studied. Only isotope from [C14]oxalate appeared in appreciable quantities in the aqueous ethanol-soluble fraction of the cells during 10 min. incubation periods. This shows that CO2 fixation processes, similar to those found to occur during growth of the organism on formate, do not play a major part during growth on oxalate. This conclusion is supported by the findings that carboxydismutase and phosphoribulokinase are synthesized during growth on formate but not (except under special conditions) during growth on oxalate. Carboxydismutase synthesis has been observed in cells growing on oxalate only if they have previously been growing on formate. Such cells continue to synthesize carboxydismutase during the first generation of growth on oxalate subsequent to their adaptation from growth on formate. Addition of formate to the oxalate growth medium leads to resumed enzyme synthesis. No enzyme is formed if the cells have not been grown previously on formate. The radioactivity incorporated at the earliest times (3 sec.) from [C14]oxalate was present mainly in glycine (65%) and 3-phosphoglyceric acid (25%). After incubation for 45 sec., radioactivity also appeared in alanine, glutamate, malate and phosphates of glucose, fructose and sedoheptulose. The data show that the isotope enters glycine much more rapidly than it enters phosphoglycerate. Cell-free extracts catalyse the formation of glyceric acid and CO2 by a diphospho-pyridine nucleotide-linked reductive condensation of glyoxylate, or oxalate, in the presence of acetyl-coenzyme A. It is suggested that during growth on oxalate, the oxalate, possibly in the form of oxalyl-coenzyme A, is reduced to glyoxylate, which is taken to be the precursor of glycine and phosphoglycerate. The phosphoglycerate formed then enters established routes of biosynthesis.