The roles of nitrate,photosynthesis and protein turnover in the formation of nitrate reductase in the nit A mutant of Chlamydomonas

The appearance of nitrate reductase activity in derepressed cultures of the Nit A mutant of Chlamydomonas reinhardtii required concomitant photosynthetic CO₂ fixation and was inhibited when protein turnover was prevented. Provided leupeptin was included in the extraction buffer, a single species of nitrate reductase (molecular mass, m = 390 kDa) was extracted from Nit A cultures incubated in nitrate medium for 4 h. Cultures of the mutant incubated in nitrate-free medium contained a number of nitrate reductase species (m = 52–500 kDa). This evidence suggests that nitrate plays a role in the stabilisation of the structure of the mutant nitrate reductase. Only one species of nitrate reductase (m = 188 kDa) was extracted from wild type cultures grown with nitrate.