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 CO, fixation of ac i72 and wild type cells in the presence of high and low 0, concentration shows that due to a lower level of carboxylation, the ac i72 cells fix CO, at half the rate of wild type cells. In ac i72,24 % of the photosynthetically fixed I4C 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 CO, fixation is half that of wild type. This toxicity to glycolate is overcome by growth under amber illumination or low 0, concentration.Glycolate was noted among the primary products of photosynthesis by Benson and Calvin [l] and has now been established as one of the main photorespiratory intermediates. However, its mechanism of synthesis has not yet been fully resolved. Several hypotheses have been proposed, one of which suggests that Rbu-l,5-P2 carboxylase acts not only as the primary enzyme in photosynthesis but also as the primary enzyme in photorespiration, and as such functions as Rbu-1,5-P2 oxygenase catalyzing the condensation of Rbu-1,5-P2 and 0, to form phosphoglycerate and phosphoglycolate [2 -51. A specific phosphoglycolate phosphatase, known to be present in the chloroplast, then catalyzes the production of glycolate from phosphoglycolate [6,7]. This scheme, which involves a competitive reaction between C 0 2 and 0, for the same enzyme [3,8], provides an explanation for the Warburg effect of 0, inhibition of photosynthesis and stimulation of photorespiration, the direct relationship between rates of photosynthesis and photorespiration, the light dependence of photo-' respiration and a site of O2 uptake in photorespiration.The oxygenase function of Rbu-1 ,5-P, carboxylase has been demonstrated in the following ways: (a) the two activities copurify to electrophoretic homogeneity [4], (b) the oxygenase activity, as measured by O2 uptake, is dependent upon Rbu-1,5-P2, 02, and enzyme [4], (c) kinetic studies show the competitive nature of O2 and C 0 2 in both carboxylase and oxygenase reactions [8], (d) the catalytic activity of both reactions resides in the larg...