CHARACTERIZATION OF THE CAROTENOIDLESS STRAIN OF SCENEDESMUS-OBLIQUUS, MUTANT C-6E, A LIVING PHOTOSYSTEM-I MODEL

When grown heterotrophically in the dark. on enriched culture medium, the pigment-deficient strain of Scenedesmus obliquus, mutant C-6E, is uniquely characterized by a complete deficiency in carotenoids and chl orophyll b while retaining a low level of chlorophyll a which is exclu sively utilized in photosystem I-type reactions. The strain lacks phot osystem II activity but exhibits all PS-I reactions tested, including P700 redox reactions, photoreduction of CO2 with hydrogen as electron donor, and O-2 uptake following methyl viologen reduction. The mutant contains 10 times more P700 per chlorophyll than the wild type and dev elops the pigment-protein complex of PS-I, CP-I. The action spectrum f or methyl viologen reduction compares favorable to the low temperature absorption spectrum of whole cells. Both the chlorophyll fluorescence excitation and emission spectra of pigment-protein complexes derived from apoproteins. The absence of carotenoids appears to prevent the de velopment of the normal variety of pigment-protein complexes and the a ccumulation of Chl b. This inability is also expressed by the presence of only cells of C-6E show patterns typical of PS-I. The strain lacks the LHCs and CP-II as well as their respective single stranded thylak oid membranes in the chloroplast of C-6E. When heterotrophically grown cells of this mutant are exposed to white light of 8 or 22 W m(-2), 5 0 % of its chlorophyll is lost by photooxidation within 4 or 1.5 hours , respectively.