Characterization of the Oxygenase Activity in a Mutant of Chlamydomonas reinhardi Exhibiting Altered Ribulosebisphosphate Carboxylase

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Abstract
A previously described Mendelian mutant of Chlamydomonas reinhardi, ac i72, exhibiting altered ribulosebisphosphate carboxylase activity and unable to grow on minimal medium is examined for changes in ribulosebisphosphate oxygenase activity. The ribulosebisphosphate oxygenase activity of the enzyme purified from both wild type and ac i72 is compared over a pH range from 7.0 to 9.5. Both enzymes exhibit maximum activity at pH 9.0. However, the ac i72 enzyme is twice as active as the wild type enzyme at a physiological pH of 7.0. The studies in vivo of the products of CO2 fixation of ac i72 and wild type cells in the presence of high and low O2 concentration shows that due to a lower level of carboxylation, the ac i72 cells fix CO2 at half the rate of wild type cells. In ac i72, 24% of the photosynthetically fixed 14C is channelled into the water-soluble fraction as opposed to 6% in wild type. Thin-layer chromatography of the watersoluble fraction showed extensive accumulation of components of the glycolate pathway in ac i72 as compared to wild type. This indicates that the oxygenase activity of the enzyme prevails in ac i72 in vivo. Since a high concentration of glycolate is toxic to cells of C. reinhardi, the high oxygenase activity of ac i72 provides an explanation for the inability of ac i72 to grow phototrophically even though its rate of CO2 fixation is half that of wild type. This toxicity to glycolate is overcome by growth under amber illumination or low O2 concentration.