The primary structure of D1 near the QB pocket influences oxygen evolution

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.

Modification of the photosystem II acceptor side function in a D1 mutant (arginine-269-glycine) of Chlamydomonas reinhardti

Bicarbonate anions have a strong positive influence on the electron and proton transfers in photosystem II (PS II). It has been suggested that bicarbonate binds to the non-heme iron and the QB binding niche of the PS II reaction center. To investigate the potential amino acid binding environment of bicarbonate, an arginine residue (R269) of the D1 protein of PS II of Chlamydomonas reinhardtii was mutated into a glycine; our characterization of the resultant mutant (D1-R269G) shows that both the TyrD+ and QA- Fe2+ EPR signals are substantially reduced and assembly of the tetranuclear Mn is lost (R.S. Hutchison, J. Xiong, R.T. Sayre, Govindjee, Biochim. Biophys. Acta 1277 (1996) 83-92). In order to understand the molecular implications of this mutation on the electron acceptor side of PS II, we used chlorophyll (Chl) a fluorescence as a probe of PS II structure and function, and herbicide binding as a probe for changes in the QB binding niche of PS II. Chl fluorescence measurements with the heterotrophically grown D1-R269G mutant cells (or thylakoids), as compared to that of the wild type, show that: rate of electron transfer from QA to the plastoquinone pool, measured by flash-induced Chl a fluorescence decay kinetics, is reduced by - 17 fold; the minimum Chl a fluorescence yield when all QA- is oxidized, is elevated by 2 fold; the level of stable charge separation as inferred from variable Chl fluorescence is reduced by 44%; binary oscillation pattern of variable Chl a fluorescence obtained after a series of light flashes is absent, indicative of the loss of functioning of the two-electron gate on the PS II acceptor side; 77 K PS II Chl a fluorescence emission bands (F685 and F695) are reduced by 20-30% (assuming no change in the PS I emission band). Thermoluminescence data with thylakoids show the absence of the S2QA- and S2QB- bands in the mutant. Herbicide 14C-terbutryn binding measurements, also with thylakoids, show that the QB niche of the mutant is significantly modified, at least 7-8 fold increased terbutryn dissociation constant is shown (220 nM in the mutant versus 29 nM in the wild type); the PS II sensitivity to bicarbonate-reversible formate inhibition is reduced by 5 fold in the mutant, although the formate/bicarbonate binding site still exists in the mutant. This suggests that D1-R269 must play some role in the binding niche of bicarbonate. On the basis of the above observations, we conclude that the D1-R269G mutation has not only altered the structure and function of PS II (QB niche being abnormal), but may also have a decreased net excitation energy transfer from the PS II core to the reaction center and/or an increased number of inactivated reaction center II. We also discuss a possible scenario for these effects using a recently constructed three dimensional model of the PS II reaction center.

Reduced turnover of the D1 polypeptide and photoactivation of electron transfer in novel herbicide resistant mutants of Synechocystis sp. PCC 6803

Two missense mutants, A263P and S264P, and a deletion mutant des-Ala263, Ser264, have been constructed in the D1 protein of the cyanobacterium Synechocystis sp PCC 6803. All were expected to induce a significant conformational change in the QB-binding region of photosystem II (PSII). Although the des-Ala263, Ser264-D1 mutant accumulated some D1 protein in the thylakoid membrane it was unable to grow photoautotrophically or evolve oxygen. Thermoluminescence and chlorophyll fluorescence studies confirmed that this deletion mutant did not show any functional PSII activity. In contrast, [S264P]D1 was able to grow photoautotrophically and give light-saturated rates of oxygen evolution at 60% of the rate of the wild-type control strain, TC31. The A263P missense mutant was also able to evolve oxygen at 50% of TC31 rates although it did not readily grow photoautotrophically. Thermoluminescence, flash oxygen yield and chlorophyll fluorescence measurements indicated that in both missense mutants electron transfer from QA to QB was significantly impaired in dark adapted cells. However, QA to QB electron transfer could be photoactivated in the mutants by background illumination. Both the A263P and S264P mutants also showed an increase in resistance to the s-triazine family of herbicides although this feature did not hold for the phenolic herbicide, ioxynil. Of particular interest was that the two missense mutants, especially S264P, possessed a slower rate of turnover of the D1 protein compared with TC31 and in vivo contained detectable levels of a 41-kDa adduct consisting of D1 and the alpha subunit of cytochrome b559. When protein synthesis was blocked by the addition of lincomycin, D1 degradation was again slower in S264P than TC31. The results are discussed in terms of structural changes in the QB-binding region which affect herbicide and plastoquinone binding and perturb the normal regulatory factors that control the degradation of the D1 protein and its synchronisation with the synthesis of a replacement D1 protein.

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

In Cyanobacteria and Chlamydomonas reinhardtii, substitution of valine for alanine at position 251 of the photosystem II D1 protein in the loop between transmembrane helices IV and V confers resistance to herbicides that reduce photosystem II function and increases sensitivity to photoinhibition. Using site-directed mutagenesis and chloroplast transformation in Chlamydomonas we have examined further the role of residue 251 in relation to D1 structure, function, and photosynthetic performance. Of the 12 different amino acid substitutions for Ala251 introduced at this position, five (Arg, Asp, Gln, Glu, and His) resulted in a nonphotosynthetic phenotype. Transformants with the Arg251 substitution synthesize a normal sized 32-kDa D1 protein with greatly reduced stability. The Gln, Glu, His, and Asp transformants make a 33-34-kDa form of the D1 protein of varying stability as well as an immunologically related polypeptide of 24-25 kDa corresponding to the N-terminal portion of D1 that is unstable and appears to be an aborted D1 translation product. All mutant forms of the D1 protein are intrinsic to the thylakoids. In contrast to previous studies in Cyanobacteria showing that residues in the IV-V loop can be mutated or deleted without loss of photosynthetic competence, our results suggest that Ala251 has a key role in the structure and function of the IV-V loop region.

Construction and characterization of a photosystem II D1 mutant (arginine-269-glycine) of Chlamydomonas reinhardtii

Numerous lines of evidence indicate that bicarbonate anion regulates electron and proton transfer processes in the photosystem II (PSII) complex of chloroplasts and cyanobacteria. On the reducing side of PSII, the addition of bicarbonate to bicarbonate-depleted (or formate-treated) membranes accelerates, especially, QA(-)-->QB(-) electron transfer kinetics. The site(s) at which bicarbonate binds is unknown. It is evident, however, from several spectroscopic studies that the bicarbonate binding site on the reducing side of PSII includes the non-heme iron located between the QA and QB sites. Since small anions may displace bicarbonate (Good, N.E. (1963) Plant Physiol. 38, 298-304) [1], it is apparent that the bicarbonate binding site is electrostatic in nature, presumably also involving positively charged amino acid residues. Previously, it had been predicted that residue arginine 269 of the PSII D1 protein may participate in bicarbonate binding. To test this hypothesis, we have generated a non-conservative mutation in the psbA gene of Chlamydomonas reinhardtii which converts residue R269 to a glycine (R269G). The R269G mutant was unable to grow photosynthetically or evolve oxygen. This phenotype is associated with a lack of the tetra-manganese water splitting complex and a reduced capacity to form a stabilized charge separated state (defined as TyrD+/QA- under the experimental conditions measured). In addition, the mutant cells have a less stable PSII complex than wild-type cells, particularly when grown in the light. It is apparent from analyses of the effect of formate on the magnitude of the QA-Fe+2 EPR signal, however, that the bicarbonate or formate binding site is not substantially affected by the R269G mutation. Although our results do not substantiate that residue R269 is the site at which bicarbonate is bound, they demonstrate the importance of R269 in the structure and function of PSII. It is apparent from analysis of the photosynthetic phenotype, that the structural perturbations on the stromal side of the D1 protein are transduced to the lumenal side of the membrane altering charge accumulating processes on the electron donor side of PSII.

Tandem sequence duplications functionally complement deletions in the D1 protein of photosystem II

Obligate photoheterotrophic mutants of the cyanobacterium Synechocystis sp. PCC 6803 that carry deletions of conserved residues in the plastoquinone-binding niche of the D1 protein were used to select for spontaneous mutations that restore photoautotrophic growth. Spontaneous pseudorevertants emerged from two deletion mutants, delta YNIV246-9 and delta NN266-7, when the cultures were maintained long after the carbon source (glucose) had been depleted from the medium and cells had reached stationary phase. Most pseudorevertants were found to contain tandem duplications of 6-45-base pair DNA sequences located close to the domain carrying the deletion; none of them restored the wild-type sequence. Three pseudorevertants isolated from the delta YNIV246-9 mutant contained a duplication (7-15 codons) of the DNA sequence immediately downstream of the deletion; the protein region encoded by this DNA may include part of the putative de helix, an important constituent of the plastoquinone-binding niche. Three pseudorevertants isolated from the delta NN266-7 mutant contained duplications corresponding to 2-8 amino acid residues adjacent to the site of the deletion. In all six pseudorevertants carrying duplications, the length of the D1 protein in the modified regions was restored to at least the length present in wild type, suggesting that a minimal length of these protein domains may be required for functional integrity. In another photoautotrophic strain isolated from delta NN266-7, no secondary mutations could be identified in the gene coding for the D1 protein; such mutations apparently reside on another protein subunit of the photosystem II complex. Photosystem II function in the pseudorevertants was altered as compared with wild type in terms of growth and oxygen evolution rates, photosystem II concentration, the semiquinone equilibrium at the acceptor side, and thermostability. A mechanism leading to tandem sequence duplication may involve DNA damage followed by DNA synthesis, strand displacement, and ligation.

Oscillations of reaction center II-D1 protein degradation in vivo induced by repetitive light flashes. Correlation between the level of RCII-QB- and protein degradation in low light

The D1 protein subunit of the photochemical reaction center II (RCII) turns over rapidly in oxygenic photosynthetic organisms exposed to the light. At high photon flux densities (PFD), photoinactivation of RCII precedes the degradation of the D1 protein. We found that the apparent quantum yield for the D1 protein degradation in Chlamydomonas cells is severalfold higher at low PFDs (10-100 mumol m-2 s-1) as compared to that observed at PFDs which induce photoinactivation of RCII (1.5-3 x 10(3) mumol m-2 s-1). Relative high levels of reduced RCII secondary plastoquinone acceptor, QB-, are induced in cells exposed to low PFDs as determined by thermoluminescence measurements. The probability of generating elevated levels of QB- which may recombine with the S2,3 oxidized states of the oxygen evolving complex decreases with increase in the light intensities at which consecutive double reduction of QB and exchange with the plastoquinone pool prevail. We have used light flashes to test if a correlation exists between the degradation of D1 protein and the relative level of QB-. D1 protein degradation could be induced in dark-incubated cells exposed to a series of 1.4 x 10(3) single light flashes given at intervals compatible with generation of elevated levels of QB- and its decay by charge recombination. Oscillations of the QB- level in cells exposed to 960-1440 series of 1 to several flashes correlated with oscillations of the D1 protein degradation in Chlamydomonas cells and in the Scenedesmus wild type but not in the LF-1 mutant lacking photosystem II donor side activity. In this mutant the "S state cycle" and QB- oscillations are abolished. We propose that the process of recombination of long lived RCII-QB- with the S2,3 states may involve damaging events related to the D1 protein degradation induced by light flashes or continuous low light in vivo.

Inactivation of the water-oxidizing enzyme in manganese stabilizing protein-free mutant cells of the cyanobacteria Synechococcus PCC7942 and Synechocystic PCC6803 during dark incubation and conditions leading to photoactivation

The previously constructed MSP (manganese stabilizing protein-psbO gene product)-free mutant of Synechococcus PCC7942 (Bockholt R, Masepohl B and Pistorius E K (1991) FEBS Lett 294: 59-63) and a newly constructed MSP-free mutant of Synechocystis PCC6803 were investigated with respect to the inactivation of the water-oxidizing enzyme during dark incubation. O2 evolution in the MSP-free mutant cells, when measured with a sequence of short saturating light flashes, was practically zero after an extended dark adaptation, while O2 evolution in the corresponding wild type cells remained nearly constant. It could be shown that this inactivation could be reversed by photoactivation. With isolated thylakoid membranes from the MSP-free mutant of PCC7942, it could be demonstrated that photoactivation required illumination in the presence of Mn(2+) and Ca(2+), while Cl(-) addition was not required under our experimental conditions. Moreover, an extended analysis of the kinetic properties of the water-oxidizing enzyme (kinetics of the S3→(S4)→S0 transition, S-state distribution, deactivation kinetics) in wild type and mutant cells of Synechococcus PCC7942 and Synechocystis PCC6803 was performed, and the events possibly leading to the reversible inactivation of the water-oxidizing enzyme in the mutant cells are discussed. We could also show that the water-oxidizing enzyme in the MSP-free mutant cells is more sensitive to inhibition by added NH4Cl-suggesting that NH3 might be a physiological inhibitor of the water oxidizing enzyme in the absence of MSP.