A mutant of Chlamydomonas reinhardtii with reduced rate of photorespiration

Photorespiration rates under air-equilibrated conditions (0.04% CO2 and 21% O-2) were measured in Chlamydomonas reinhardtii wild-type 2137, a phosphog lycolate-phosphatase-deficient (pgg1) mutant and a suppressor double mutant (7FR2N) derived from the pgp1 mutant. In both cells grown under 5% CO2 and adapted air for 24 h in the suppressor double mutant, the maximal rate of photorespiration (phosphoglycolate synthesis) was only about half of that i n either the wild type or the pgp1 mutant (18-7F) cells. In the progeny, th e reduced rate of photorespiration was accompanied by increased photosynthe tic affinity for inorganic carbon and the capacity for growth under air whe ther accompanied by the pgp1 background or not. Tetrad analyses suggested t hat these three characteristics all resulted from a nuclear single-gene mut ation at a site unlinked to the pgp1 mutation. The decrease in photorespira tion was, however, not due to an increase in the CO2/O-2 relative specifici ty of ribulose-1,5-bisphosphate carboxylase/oxygenase of 7FR2N or of any ot her suppressor double mutants tested, The relationship between the decrease in the rate of photorespiration and the CO2-concentrating mechanism is dis cussed.