Étude des échanges de CO 2 des cellules entières et des activités photochimiques des fragments de chloroplastes de trois mutants non photosynthétiques de Chlamydomonas reinhardti

Abstract

Studies of the CO 2 exchanges of whole cells and of the photochemical activities of chloroplast fragments, with three non-photosynthetic mutants of Chlamydomonas reinhardti Investigations were performed in order to determine the different kinds of electron transport, which are carried out by three non-photosynthetic mutants ( Fl 5, Fl 9, Fl 15) of Chlamydomonas reinhardti. These mutants had been previously selected for their fluorescence and O 2 evolution anomalies, which indicate a blocking of the internal electron transport chain close to Photoreaction II. Aerobic and anaerobic (with H 2 or N 2) CO 2 exchanges of whole cells were measured with a pCO 2 electrode. Photochemical activities of chloroplast fragments with various exogenous electron carriers were measured spectrophotometrically. The three mutants were unable to fix CO 2 in the light, in air and in the presence of H 2 with 3- p-chlorophenyl-1,1-dimethylurea (CMU) only; in addition, they could not perform photoreduction of NADP + with water. However, Fl 9 and Fl 15 were able to carry out photoreduction of NADP + with 2,6-dichlorophenolindophenol (DCIP)-ascorbate, photooxidation of exogenous cytochrome c, and CO 2 fixation in the light in the presence of H 2, CMU and phenazine methosulfate. Fl 5 did not perform any of these reactions. With DCIP and plastocyanin the Hill activities of the three mutants were very weak, compared to those of the wild type; with K 3Fe(CN) 6, the activities were relatively more important, particularly for Fl 5. These results indicate a non-functioning of System I in Fl 5, on the one hand, and a blocking of the electron transport chain between the two photoreactions (both of which are functional) in Fl 9 and in Fl 15, on the other. An interpretation is attempted, taking into consideration the analytical results obtained elsewhere (lack of P700 for Fl 5, deficiencies in bound cytochromes c-553 and b-563 for Fl 9 and Fl 15). Some points, concerning the nature of the defect(s) in each mutant, the reaction sites of the various cytochromes, and the possibility of different pathways for the photosynthetic electron transports, in vivo and in vitro, are discussed.