Phosphoglycolate Phosphatase-Defident Mutants of Chlamydomonas reinhardtii Capable of Growth under Air

Mutants deficient in phosphoglycolate phosphatase (PGPase) requireelevated levels of CO₂ for growth in the light and cannot growwhen photorespiration occurs. Revertants, namely, double mutantscapable of growth under air without restoration of the missingPGPase activity, might be expected to have secondary mutationsthat reduce or eliminate photorespiration. Nineteen revertantswere selected from a culture of a PGPase-deficient mutant ofChlamydomonas reinhardtii (pgp-1-18-7F) after a second mutagenesisthat involved treatment with 5-fluorodeoxyuridine and ethylmethanesulfonate. There were significant differences in thephotosynthetic affinity for CO₂ among revertant cells grownunder 5% CO₂. Eight revertants had five times higher photosyntheticaffinity for CO₂ than that of wild type 2137 cells grown under5% CO₂, resembling air-adapted wild-type cells, whereas fourrevertants had less than half the affinity for CO₂ of the wildtype. In all of the revertant cells with higher affinity grownin 5% CO₂, the rates of photosynthesis under levels of CO₂ belowthose in air were apparently higher than that of the wild type,whereas the rates under CO₂-saturating conditions were lowerthan that of wild type, indicating that the efficiency of photosynthesisunder air was significantly improved in these revertants. Inaddition, some revertants had a photosynthetic capacity anda growth rate higher than those of the wild type, without anyincreased photosynthetic affinity for CO₂. (Received July 7, 1994; Accepted November 5, 1994)