Site-directed mutations at residue 251 of the photosystem II D1 protein of Chlamydomonas that result in a nonphotosynthetic phenotype and impair D1 synthesis and accumulation

Improved survival of very high light and oxidative stress is conferred by spontaneous gain-of-function mutations in Chlamydomonas

Investigations into high light and oxidative stress in photosynthetic organisms have focussed primarily on genetic impairment of different photoprotective functions. There are few reports of "gain-of-function" mutations that provide enhanced resistance to high light and/or oxidative stress without reduced productivity. We have isolated at least four such very high light resistant (VHL(R)) mutations in the green alga, Chlamydomonas reinhardtii, that permit near maximal growth rates at light intensities lethal to wild type. This resistance is not due to an alteration in electron transport rate or quantity and functionality of the two photosystems that could have enhanced photochemical quenching. Nor is it due to reduced excitation pressure by downregulation of the light harvesting antennae or increased nonphotochemical quenching. In fact, photosynthetic activity is unaffected in more than 30 VHL(R) isolates. Instead, the basis of the VHL(R) phenotype is a combination of traits, which appears to be dominated by enhanced capacity to tolerate reactive oxygen species generated by excess light, methylviologen, rose bengal or hydrogen peroxide. This is further evidenced in lower levels of ROS after exposure to very high light in the VHL(R)-S9 mutant. Additionally, the VHL(R) phenotype is associated with increased zeaxanthin accumulation, maintenance of fast synthesis and degradation rates of the D1 protein, and sustained balanced electron flow into and out of PSI under very high light. We conclude that the VHL(R) mutations arose from a selection pressure that favors changes to the regulatory system(s) that coordinates several photoprotective processes amongst which repair of PSII and enhanced detoxification of reactive oxygen species play seminal roles.

Chlamydomonas reinhardtii in the landscape of pigments

This review focuses on the biosynthesis of pigments in the unicellular alga Chlamydomonas reinhardtii and their physiological and regulatory functions in the context of information gathered from studies of other photosynthetic organisms. C. reinhardtii is serving as an important model organism for studies of photosynthesis and the pigments associated with the photosynthetic apparatus. Despite extensive information pertaining to the biosynthetic pathways critical for making chlorophylls and carotenoids, we are just beginning to understand the control of these pathways, the coordination between pigment and apoprotein synthesis, and the interactions between the activities of these pathways and those for other important cellular metabolites branching from these pathways. Other exciting areas relating to pigment function are also emerging: the role of intermediates of pigment biosynthesis as messengers that coordinate metabolism in the chloroplast with nuclear gene activity, and the identification of photoreceptors and their participation in critical cellular processes including phototaxis, gametogenesis, and the biogenesis of the photosynthetic machinery. These areas of research have become especially attractive for intensive development with the application of potent molecular and genomic tools currently being applied to studies of C. reinhardtii.

Transcription, translation, degradation, and circadian clock

Synthesis and degradation of mRNA together with synthesis and degradation of corresponding protein, this four-step-expression confers great fitness to all organisms. Transcription rate and mRNA stability both are essential for circadian expression of clock genes. In many cases, transcription rates and half-lives of mRNAs and corresponding proteins are not necessarily tightly linked with each other. The methods for measuring four-step-expression should be carefully selected and the experimental conditions should be strictly controlled.

Glutathione redox potential modulated by reactive oxygen species regulates translation of Rubisco large subunit in the chloroplast

Previous work showed a transient but dramatic arrest in the synthesis of Rubisco large subunit (LSU) upon transfer of Chlamydomonas reinhardtii cells from low light (LL) to high light (HL). Using dichlorofluorescin, a short-term increase in reactive oxygen species (ROS) was demonstrated, suggesting that their excessive formation could signal LSU down-regulation. A decrease in LSU synthesis occurred at LL in the presence of methyl viologen and was prevented at HL by ascorbate. Interfering with D1 function by mutations or by incubation with DCMU prevented the increase in ROS formation at HL and the concomitant down-regulation of LSU synthesis. If the electron transport was blocked further downstream, by mutation in the cytochrome b(6)/f or by incubation with 2, 5-dibromo-3-methyl-6-isopropyl-p-benzoquinone, ROS formation increased, and LSU synthesis ceased. The elevation of ROS occurred concurrently with a change in the redox state of the glutathione pool, which shifted toward its oxidized form immediately after the transfer to HL and returned to its original value after 6 h. The decrease in the reduced/oxidized glutathione ratio at HL was prevented by ascorbate and could be induced at LL by methyl viologen. We suggest that excess ROS mediate a decrease in the reduced/oxidized glutathione ratio that in turn signals the translational arrest of the rbcL transcript.

Chlamydomonas reinhardtii and photosynthesis: genetics to genomics

Genetic and physiological features of the green alga Chlamydomonas reinhardtii have provided a useful model for elucidating the function, biogenesis and regulation of the photosynthetic apparatus. Combining these characteristics with newly developed molecular technologies for engineering Chlamydomonas and the promise of global analyses of nuclear and chloroplast gene expression will add a new perspective to views on photosynthetic function and regulation.

Mobility of the primary electron-accepting plastoquinone QA of photosystem II in a Synechocystis sp. PCC 6803 strain carrying mutations in the D2 protein

Upon introduction of random mutations in a region of the psbDI gene that encodes the D2 protein in the cyanobacterium Synechocystis sp. PCC 6803, an obligate photoheterotrophic mutant was isolated that contained three mutations: V247M, A249T, and M329I. This mutant evolved oxygen in the absence of added electron acceptors, but oxygen evolution was inhibited by micromolar concentrations of several artificial quinones. Complementation analysis showed that the V247M and/or A249T mutations were responsible for this phenotype. Using fluorescence induction and decay measurements, the site of inhibition by the quinones was found to be at the level of the primary electron-accepting quinone in photosystem II, QA. Duroquinone inhibited by blocking reduction of QA, and in the presence of other quinones such as 2,5-dichloro-p-benzoquinone, 2, 5-dimethyl-p-benzoquinone, and p-benzoquinone, QA could be reduced but could not efficiently transfer an electron to QB. To distinguish the effects of the V247M and A249T mutations, single mutants were created. V247M was photoautotrophic and had an essentially normal phenotype. The A249T mutant, although photoautotrophic, was affected by artificial quinones, but less than the mutant carrying both the V247M and A249T changes. The results indicate a decreased plastoquinone affinity at the QA site in the strains carrying a A249T mutation, such that after dark-adaptation a significant percentage of the QA sites is empty or is occupied by an artificial quinone. In light, the percentage of photosystem II centers with plastoquinone bound at the QA site appears to increase, which may be due in part to an increased affinity of the semiquinone versus that of the quinone at the QA site.

Biophysical, biochemical, and physiological characterization of Chlamydomonas reinhardtii mutants with amino acid substitutions at the Ala251 residue in the D1 protein that result in varying levels of photosynthetic competence

The QB binding site of the D1 reaction center protein, located within a stromal loop between transmembrane helices IV and V formed by residues Ile219 to Leu272, is essential for photosynthetic electron transport through photosystem II (PSII). We have examined the function of the highly conserved Ala251 D1 residue in this domain in chloroplast transformants of Chlamydomonas reinhardtii and found that Arg, Asp, Gln, Glu, and His substitutions are nonphotosynthetic, whereas Cys, Ser, Pro, Gly, Ile, Val, and Leu substitutions show various alterations in D1 turnover, photosynthesis, and photoautotrophic growth. The latter mutations reduce the rate of QA to QB electron transfer, but this is not necessarily rate-limiting for photoautotrophic growth. The Cys mutant divides and evolves O2 at wild type rates, although it has slightly higher rates of D1 synthesis and turnover and reduced electron transfer between QA and QB. O2 evolution, D1 synthesis, and accumulation in the Ser, Pro, and Gly mutants in high light is reduced, but photoautotrophic growth rate is not affected. In contrast, the Ile, Val, and Leu mutants are impaired in photoautotrophic growth and photosynthesis in both low and high light and have elevated rates of D1 synthesis and degradation, but D1 accumulation is normal. While rates of synthesis/degradation of the D1 protein are not necessarily correlated with alterations in specific parameters of PSII function in these mutants, bulkiness of the substituted amino acids is highly correlated with the dissociation constant for QB in the seven mutants examined. These observations imply that the Ala251 residue plays a key role in D1 protein.