A Photorespiratory Mutant of Chlamydomonas reinhardtii.

A mutant strain of Chlamydomonas reinhardtii, designated 18-7F, has been isolated and characterized. 18-7F requires a high CO(2) concentration for photoautrophic growth in spite of the apparent induction of a functional CO(2) concentrating mechanism in air-adapted cells. In 2% O(2) the photosynthetic characteristics of 18-7F and wild type are similar. In 21% O(2), photosynthetic O(2) evolution is severely inhibited in the mutant by preillumination in limiting CO(2), although the apparent photosynthetic affinity for inorganic carbon is similar in preilluminated cells and in cells incubated in the dark prior to O(2) evolution measurements. Net CO(2) uptake is also inhibited when the cells are exposed to air (21% O(2), 0.035% CO(2), balance N(2)) for longer than a few minutes. [(14)C]Phosphoglycolate accumulates within 5 minutes of photosynthetic (14)CO(2) fixation in cells of 18-7F. Phosphoglycolate does not accumulate in wild type. Phosphoglycolate phosphatase activity in extracts from air-adapted cells of 18-7F is 10 to 20% of that in wild-type Chlamydomonas. The activity of phosphoglycolate phosphatase in heterozygous diploids is intermediate between that of homozygous mutant and wild-type diploids. It was concluded that the high-CO(2) requiring phenotype in 18-7F results from a phosphoglycolate phosphatase deficiency. Genetic analyses indicated that this deficiency results from a single-gene, nuclear mutation. We have named the locus pgp-1.