Lesions in citrate synthase that affect aerobic nitrogen fixation by Azotobacter chroococcum.
A class of Azotobacter chroococcum mutants induced by Tn1 that were defective in normal aerobic nitrogen fixation when grown on sugars (Fos-) were corrected by provision of alpha-ketoglutarate or glutamate. In a representative mutant, Fos252, rates of evolution of 14CO2 from [14C]acetate or [14C]glucose were 5% of the parental values, although uptake and incorporation were normal for both substrates. The results suggest that a lesion affects the entry of substrates into the tricarboxylic acid cycle. The activity of citrate synthase in Fos252 in vitro was 5% that of the parents. The citrate synthase (gltA) gene from Escherichia coli was cloned into broad-host-range vectors and mobilized into Fos252. The plasmids restored parental citrate synthase activities to Fos252 and complemented the inability to fix N2 in air. The data indicate that a mutation causing an intrinsic limitation in respiratory capacity abolishes normal aerobic N2 fixation, which is consistent with the hypothesis of respiratory protection for nitrogenase in Azotobacter species.
D001395:Azotobacter / Q000201:enzymology / Q000235:genetics* / Q000378:metabolism; D002950:Citrate (si)-Synthase / Q000172:deficiency / Q000235:genetics*; D002952:Citric Acid Cycle; D005816:Genetic Complementation Test; D009586:Nitrogen Fixation; D007652:Oxo-Acid-Lyases / Q000235:genetics*
D002950:Citrate (si)-Synthase; D007652:Oxo-Acid-Lyases