DR AND SPR SR MUTATIONS OF CHLAMYDOMONAS-REINHARDTII AFFECTING D1-PROTEIN FUNCTION AND SYNTHESIS DEFINE 2 INDEPENDENT STEPS LEADING TO CHRONIC PHOTOINHIBITION AND CONFER DIFFERENTIAL FITNESS/

The effects of introduced chloroplast gene mutations affecting D1 synt hesis, turnover and function on photosynthesis, growth and competitive ability were examined in autotrophic cultures of Chlamydomonas reinha rdtii (Chlorophyta) adapted to low or high irradiance. Few discernible effects were evident when the mutants were grown in low light (LL, 70 mu mol m(-2) s(-1)). The herbicide-resistant psbA mutation Ser(264) - -> Ala (dr) slowed electron transfer and accelerated D1 degradation in cells grown under high light (HL, 600 mu mol m(-2) s(-1)). The maximu m rate of light- and CO2-saturated photosynthesis, cell growth rate an d competitive ability in the dr mutant were reduced compared to wild t ype under HL. However, the wild-type rate of D1 synthesis in dr was ad equate to compensate for accelerated D1 degradation. 16S rRNA mutation s conferring resistance to streptomycin and spectinomycin (spr/sr) tha t altered chloroplast ribosome structure and assembly were used to inh ibit chloroplast protein synthesis. In spr/sr cells grown under HL, D1 synthesis was reduced by 40-60% compared to wild type and D1 degradat ion was accelerated, leading to a 4-fold reduction in D1 pool size. Th e reduced D1 levels were accompanied by an elevation of F-o and a decl ine in F-v/F-m, quantum yield and maximum rate of CO2-saturated photos ynthesis. Chemostat experiments showed that the growth rate and compet itive ability of spr/sr were reduced against both wild type and dr.