Very high light resistant mutants of Chlamydomonas reinhardtii with impaired PSII function: D1 protein dynamics, functional PSII and PSI centers and intersystem electron transport capacities

Abstract

Substitution of the Ala251 residue in the D1 reaction center protein of PSII with Leu resulted in a herbicide resistant phenotype with strongly decreased quantum efficiency, maximum rates of photosynthetic O2 evolution and photoautotrophic growth. Pool size of mutant D1 protein was similar to wildtype, even though D1 turnover was 25% faster in high light (HL, 600 µmol photons m-2 s-1), because synthesis was equally accelerated (Lardans et al., 1998, J. Biol. Chem. 272: 11082-11091). We have found that faster D1 turnover and impairment of PSII function persisted in very high light resistant (VHLR) mutants derived from L* when grown in HL and was further enhanced in very high light (VHL, 1500 µmol photons m-2 s-1). The D1 substitution decreased Fv/Fm, slowed forward electron transfer to QB 5 fold and eliminated detectable lateral electron transfer between PSII centers. In general, whole chain electron transport and redox kinetics at PSI were unaffected by the PSII mutation, but delivery of electrons from PSII and the intersystem carriers was less efficient than in wildtype. The A251L substitution diminished the amounts of functional PSII and PSI, determined from O2 flash yield and 800-830 nm absorbance changes, only when grown under low light (LL, 70 µmol photons m-2 s-1). Clearly, maximum PSII function is not a prerequisite for VHL resistance.