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Biomedical subjects

P J Nixon

Publications and source records attributed to P J Nixon.

At least 19 recordsLinked to original sources
PubMed2005

A survey of demand for specialist restorative dental services.

OBJECTIVES: To determine the level of demand from general dental practitioners (GDPs) for specialist restorative dental services in the Yorkshire region. To investigate barriers and promoters to referral and GDPs' perceptions of restorative mono-specialists. METHODS: A postal questionnaire was sent to 301 randomly selected GDPs (stratified for location) registered in the six Family Health Service Units in Yorkshire. Questionnaires were piloted prior to release; reminders were sent to non-respondents. RESULTS: A response rate of 72% (n=217) was achieved, of these 195 questionnaires were useable (65% useable response rate). Results showed a large demand for restorative specialist services. Main promoters for National Health Service (NHS) referral were dentolegal issues (77% GDPs ranked this as a top three promoter) and for private referral increasing patient expectations (78%). The top barriers against referral were length of waiting lists for NHS patients (79% GDPs ranked this as a top three barrier) and high cost of treatment for private patients (88%). Excessive distance to specialist centre was the greatest barrier common to both NHS and private referrals. Fifty-eight per cent of GDPs would prefer to refer private patients to mono-specialists, compared with 5% who would prefer restorative specialists. CONCLUSIONS: There is a strong demand for specialist restorative services which may increase in the future. Results indicate a high regard for mono-specialists. Overall demand is for a prompt, locally based, low cost referral service.

Attitude of Health Personnel↗
PubMed2003

Alcohol and drug use among vocational dental practitioners.

OBJECTIVE: This study was designed to investigate the prevalence of alcohol and drug use in vocational dental practitioners (VDPs). DESIGN: Anonymous self-report questionnaire. SUBJECTS AND METHODS: All UK vocational dental practitioners (n = 719) who started practice in the summer of 1999 were questioned on their use of alcohol, tobacco, cannabis and other illicit drugs whilst a VDP and before graduation. RESULTS: Eighty-nine per cent of males and 88% of females reported alcohol use. Of these, 35% of males and 36% of females drank above sensible levels (up to 21 units per week for males, 14 units for females), with 48% of males and 52% of females "binge drinking". Thirty-three per cent of respondents reported cannabis use whilst a VDP. Regular drug use (use at least once a month) other than cannabis whilst a VDP was found to be highest among males, with regular ecstasy use reported by 4% and regular cocaine use reported by 2%. Regular tobacco use (ten or more cigarettes per day) was reported by 9% of VDPs. Regular tobacco users were found to be six times more likely to use cannabis on a regular basis, three times more likely to use ecstasy on a regular basis and four times more likely to use cocaine on a regular basis than those VDPs who reported non regular tobacco use. CONCLUSION: VDPs are drinking above sensible limits, binge drinking, using tobacco and indulging in illicit drug use to a degree that will cause health problems. However, use of all substances had reduced since qualification.

Alcohol Drinking↗
PubMed2001

Site-directed mutations at D1-His198 and D2-His197 of photosystem II in Synechocystis PCC 6803: sites of primary charge separation and cation and triplet stabilization.

Site-directed mutations were introduced to replace D1-His198 and D2-His197 of the D1 and D2 polypeptides, respectively, of the photosystem II (PSII) reaction center of Synechocystis PCC 6803. These residues coordinate chlorophylls P(A) and P(B) which are homologous to the special pair Bchlorophylls of the bacterial reaction centers that are coordinated respectively by histidines L-173 and M-200 (202). P(A) and P(B) together serve as the primary electron donor, P, in purple bacterial reaction centers. In PS II, the site-directed mutations at D1 His198 affect the P(+)--P-absorbance difference spectrum. The bleaching maximum in the Soret region (in WT at 433 nm) is blue-shifted by as much as 3 nm. In the D1 His198Gln mutant, a similar displacement to the blue is observed for the bleaching maximum in the Q(y) region (672.5 nm in WT at 80 K), whereas features attributed to a band shift centered at 681 nm are not altered. In the Y(Z*)--Y(Z)-difference spectrum, the band shift of a reaction center chlorophyll centered in WT at 433--434 nm is shifted by 2--3 nm to the blue in the D1-His198Gln mutant. The D1-His198Gln mutation has little effect on the optical difference spectrum, (3)P--(1)P, of the reaction center triplet formed by P(+)Pheo(-) charge recombination (bleaching at 681--684 nm), measured at 5--80 K, but becomes visible as a pronounced shoulder at 669 nm at temperatures > or =150 K. Measurements of the kinetics of oxidized donor--Q(A)(-) charge recombination and of the reduction of P(+) by redox active tyrosine, Y(Z), indicate that the reduction potential of the redox couple P(+)/P can be appreciably modulated both positively and negatively by ligand replacement at D1-198 but somewhat less so at D2-197. On the basis of these observations and others in the literature, we propose that the monomeric accessory chlorophyll, B(A), is a long-wavelength trap located at 684 nm at 5 K. B(A)* initiates primary charge separation at low temperature, a function that is increasingly shared with P(A)* in an activated process as the temperature rises. Charge separation from B(A)* would be potentially very fast and form P(A)(+)B(A)(-) and/or B(A)(+)Pheo(-) as observed in bacterial reaction centers upon direct excitation of B(A) (van Brederode, M. E., et al. (1999) Proc. Natl. Acad Sci. 96, 2054--2059). The cation, generated upon primary charge separation in PSII, is stabilized at all temperatures primarily on P(A), the absorbance spectrum of which is displaced to the blue by the mutations. In WT, the cation is proposed to be shared to a minor extent (approximately 20%) with P(B), the contribution of which can be modulated up or down by mutation. The band shift at 681 nm, observed in the P(+)-P difference spectrum, is attributed to an electrochromic effect of P(A)(+) on neighboring B(A). Because of its low-energy singlet and therefore triplet state, the reaction center triplet state is stabilized on B(A) at < or =80 K but can be shared with P(A) at >80 K in a thermally activated process.

Bacteriochlorophylls↗
PubMed2001

Photosynthetic water oxidation in cytochrome b(559) mutants containing a disrupted heme-binding pocket.

The role of cytochrome b(559) in photosynthetic oxygen evolution has been investigated in three chloroplast mutants of Chlamydomonas reinhardtii, in which one of the two histidine axial ligands to the heme, provided by the alpha subunit, has been replaced by the residues methionine, tyrosine, and glutamine. Photosystem two complexes functional for oxygen evolution could be assembled in the methionine and tyrosine mutants up to approximately 15% of wild type levels, whereas no complexes with oxygen evolution activity could be detected in the glutamine mutant. PSII supercomplexes isolated from the tyrosine and methionine mutants were as active as wild type in terms of light-saturated rates of oxygen evolution but in contrast to wild type contained no bound heme despite the presence of the alpha subunit. Oxygen evolution in the tyrosine and methionine mutants was, however, more sensitive to photoinactivation than the WT. Overall, these data establish unambiguously that a redox role for the heme of cytochrome b(559) is not required for photosynthetic oxygen evolution. Instead, our data provide new evidence of a role for cytochrome b(559) in the protection of the photosystem two complex in vivo.

Animals↗
PubMed2001

The contribution of cone responses to rat electroretinograms.

The contribution of rods and cones to the scotopic electroretinogram (ERG) of small animals is unclear, with a recent report suggesting that the mouse has no cone a-wave. The present study considered the contribution of cones to the ERG of the rat. Dark-adapted Long Evans rats (n = 4) had ERG signals collected following a single flash, which stimulated rods and cones (mixed response), or a twin-fash paradigm (short interstimulus interval, 1 s), which isolated cone responses. Rod signals were derived by digital subtraction of the cone signal from the mixed rod/cone ERG. The rat a-wave was found to be dominated by rod responses but cone responses contributed substantially (45%) to post-receptoral waveforms (b-wave) at higher light levels.

Animals↗
PubMed2000

Assignment of the Qy absorbance bands of photosystem II chromophores by low-temperature optical spectroscopy of wild-type and mutant reaction centers.

Photosystem II (PSII) contains a collection of pheophytins (Pheo) and chlorophylls (Chl) that have unique absorbance spectra depending on their electronic structure and the surrounding protein environment. Despite numerous efforts to identify the spectra of each cofactor, differing assignments of the chromophore absorbance bands and electrochromic effects have led to conflicting models of pigment organization and chromophore interactions in PSII. We have utilized low-temperature measurements on well-defined redox states, together with the use of site-directed mutants, to make spectral assignments of several reaction center (RC) chromophores. Cryogenic (77 K) optical spectroscopy has been used to trap the bound redox-active quinone, Q(A), in the reduced form and measure the effect of the redox state of Q(A) on PSII chromophores without interference from other redox-active cofactors. The Q(A)(-) minus Q(A) difference spectrum contains a number of features that represent the perturbation of Pheo and Chl absorbance bands upon Q(A) reduction. Using site-directed mutants in which the axial ligand of the D1-side monomeric core Chl, P(A), is changed (D1-H198Q) or the hydrogen-bonding environment of the D1-side Pheo is modified (D1-Q130E), we have assigned the Q(y)() absorbance bands of four chromophores shifted by Q(A) reduction including both RC Pheos, the D1-side monomeric accessory Chl (B(A)), and one other Chl in PSII. The absorbance maximum of B(A) was identified at 683.5 nm from least-squares fits of the D1-H198Q minus wild type (WT) Q(A)(-) minus Q(A) double-difference spectrum; this assignment provides new evidence of a secondary effect of site-directed mutation on a RC chromophore. The other chromophores were assigned from simultaneous fits of the WT and D1-Q130E spectra in which the parameters of only the D1-side Pheo were allowed to vary. The D1-side and D2-side Pheos were found to have lambda(max) values at 685.6 and 669.3 nm, respectively, and another Chl influenced by Q(A)(-) was identified at 678.8 nm. These assignments are in good agreement with previous spectral analyses of intact PSII preparations and reveal that the number of chromophores affected by Q(A) reduction has been underestimated previously. In addition, the assignments are generally consistent with chromophore positions that are similar in the PSII RC and the bacterial photosynthetic RC.

Bacteriochlorophylls↗
PubMed2000

Chlororespiration.

The term 'chlororespiration' is used to describe the activity of a putative respiratory electron transler chain within the thylakoid membrane of chloroplasts and was originally proposed by Bennouon in 1982 to explain effects on the redox state of the plastoquinone pool in green algae in the absence of photosynthetic plastoquinone electrontransfer. In his original model, Bennoun suggested that the pool could be reduced through the action of a NAD(P) H dehydrogenase and could be oxidized by oxygen at an oxidase. At the same time an electrochemical gradient would be generated across the membrane. This review describes the current status of the chlororespiration model in light of the recent discoveries of novel respiratory components chloroplast thylakoid membrane.

Animals↗
PubMed2000

Degradation of the Photosystem II D1 and D2 proteins in different strains of the cyanobacterium Synechocytis PCC 6803 varying with respect to the type and level of psbA transcript.

The turnover of the D1 and D2 proteins of Photosystem II (PSII) has been investigated by pulse-chase radiolabeling in several strains of the cyanobacterium Synechocystis PCC 6803 containing different types and levels of the psbA transcript. Strains lacking psbA1 and psbA3 gene and containing high levels of the psbA2 transcript showed the selective synthesis of D1 whose degradation could be slowed down by the protein synthesis inhibitor lincomycin. In contrast, in strains containing just the psbA3 gene, the intensity of the D1 protein labeling was lower and labeling of the D2 and CP43 proteins was stimulated in comparison to the psbA2-containing strains. In addition, the rate and selectivity of the D1 degradation and its dependence on the presence of lincomycin was proportional to the level of the psbA3 transcript in the particular strain. Consequently, there was parallel, lincomycin-independent and slowed-down breakdown of the D1 and D2 proteins in strains with the lowest level of psbA3 transcript. These results are discussed in terms of a model in which the rate of D1 and D2 degradation in cyanobacteria is affected not only by the rate of PSII photodamage, but also by the availability of newly synthesized D1 protein. Moreover, the comparison of the non-oxygen-evolving D1 mutants D170A** and Y161F*** differing by the presence of tyrosine Z has indicated a minor role of the oxidized form of this secondary PSII electron donor in the donor side mechanism of D1 and D2 protein breakdown.

Algal Proteins↗
PubMed1998

Modulation of quantum yield of primary radical pair formation in photosystem II by site-directed mutagenesis affecting radical cations and anions.

Pigment-protein interactions play a significant role in determining the properties of photosynthetic complexes. Site-directed mutants of Synechocystis PCC 6803 have been prepared which modify the redox potential of the primary radical pair anion and cation. In one set of mutants, the environment of P680, the primary electron donor of Photosystem II, has been modified by altering the residue at D1-His198. It has been proposed that this residue is an axial ligand to the magnesium cation. In the other set, the D1-Gln130 residue, which is thought to interact with the C9-keto group of the pheophytin electron acceptor, has been changed. The effect of these mutations is to alter the free energy of the primary radical pair state, which causes a change in the equilibrium between excited singlet states and radical pair states. We show that the free energy of the primary radical pair can be increased or decreased by modifications at either the D1-His198 or the D1-Gln130 sites. This is demonstrated by using three independent measures of quantum yield and equilibrium constant, which exhibit a quantitative correlation. These data also indicate the presence of a fast nonradiative decay pathway that competes with primary charge separation. These results emphasize the sensitivity of the primary processes of PS II to small changes in the free energy of the primary radical pair.

Alanine↗
PubMed1998

The chloroplast-encoded alpha subunit of cytochrome b-559 is required for assembly of the photosystem two complex in both the light and the dark in Chlamydomonas reinhardtii.

The role of cytochrome b-559 in the photosystem two (PSII) complex has been investigated through the construction of a psbE null mutant by transformation of the chloroplast genome of the green alga Chlamydomonas reinhardtii. No PSII activity could be detected in this mutant either in oxygen evolution assays or by analysis of variable chlorophyll fluorescence. Immunoblotting experiments showed that the absence of PSII activity in the mutant was due to the loss of the PSII complex in both light-grown and dark-grown cultures. In contrast, the photosystem one reaction center polypeptide, PsaA, was present at wild-type levels in the mutant. RNA gel blot assays confirmed that the transcript levels for the psbA, psbD, and psbF genes were unaffected by disruption of the psbE gene, suggesting a post-transcriptional effect on their expression. Pulse-labeling experiments showed that either synthesis of PSII subunits was impaired in the psbE null mutant or there was extremely rapid degradation of newly synthesized subunits. Interestingly, the PsbE and PsbF subunits accumulated to wild-type levels in a psbA deletion mutant of C. reinhardtii, FuD7, which fails to synthesize D1 and assemble PSII. Our results provide evidence for a role for cytochrome b-559 in the early steps of assembly of the PSII complex, possibly as a redox-controlled nucleation factor that determines the level of PSII within the thylakoid membrane.

Amino Acid Sequence↗
PubMed1998

The chloroplast Ndh complex mediates the dark reduction of the plastoquinone pool in response to heat stress in tobacco leaves.

We have examined the effects of heat stress on electron transfer in the thylakoid membrane of an engineered plastid ndh deletion mutant, delta1, incapable of performing the Ndh-mediated reduction of the plastoquinone pool in the chloroplast. Upon heat stress in the dark, the rate of PSII-independent reduction of PSI after subsequent illumination by far-red light is dramatically enhanced in both delta1 and a wild-type control plant (WT). In contrast, in the dark, only the WT shows an increase in the reduction state of the plastoquinone pool. We conclude that the heat stress-induced reduction of the intersystem electron transport chain can be mediated by Ndh-independent pathways in the light but that in the dark the dominant pathway for reduction of the plastoquinone pool is catalysed by the Ndh complex. Our results therefore demonstrate a functional role for the Ndh complex in the dark.

Chlorophyll↗
PubMed1998

Identification of a functional respiratory complex in chloroplasts through analysis of tobacco mutants containing disrupted plastid ndh genes.

The plastid genomes of several plants contain homologues, termed ndh genes, of genes encoding subunits of the NADH:ubiquinone oxidoreductase or complex I of mitochondria and eubacteria. The functional significance of the Ndh proteins in higher plants is uncertain. We show here that tobacco chloroplasts contain a protein complex of 550 kDa consisting of at least three of the ndh gene products: NdhI, NdhJ and NdhK. We have constructed mutant tobacco plants with disrupted ndhC, ndhK and ndhJ plastid genes, indicating that the Ndh complex is dispensible for plant growth under optimal growth conditions. Chlorophyll fluorescence analysis shows that in vivo the Ndh complex catalyses the post-illumination reduction of the plastoquinone pool and in the light optimizes the induction of photosynthesis under conditions of water stress. We conclude that the Ndh complex catalyses the reduction of the plastoquinone pool using stromal reductant and so acts as a respiratory complex. Overall, our data are compatible with the participation of the Ndh complex in cyclic electron flow around the photosystem I complex in the light and possibly in a chloroplast respiratory chain in the dark.

Chlorophyll↗
PubMed1998

The plastid ndh genes code for an NADH-specific dehydrogenase: isolation of a complex I analogue from pea thylakoid membranes.

The plastid genomes of several plants contain ndh genes-homologues of genes encoding subunits of the proton-pumping NADH:ubiquinone oxidoreductase, or complex I, involved in respiration in mitochondria and eubacteria. From sequence similarities with these genes, the ndh gene products have been suggested to form a large protein complex (Ndh complex); however, the structure and function of this complex remains to be established. Herein we report the isolation of the Ndh complex from the chloroplasts of the higher plant Pisum sativum. The purification procedure involved selective solubilization of the thylakoid membrane with dodecyl maltoside, followed by two anion-exchange chromatography steps and one size-exclusion chromatography step. The isolated Ndh complex has an apparent total molecular mass of approximately 550 kDa and according to SDS/PAGE consists of at least 16 subunits including NdhA, NdhI, NdhJ, NdhK, and NdhH, which were identified by N-terminal sequencing and immunoblotting. The Ndh complex showed an NADH- and deamino-NADH-specific dehydrogenase activity, characteristic of complex I, when either ferricyanide or the quinones menadione and duroquinone were used as electron acceptors. This study describes the isolation of the chloroplast analogue of the respiratory complex I and provides direct evidence for the function of the plastid Ndh complex as an NADH:plastoquinone oxidoreductase. Our results are compatible with a dual role for the Ndh complex in the chlororespiratory and cyclic photophosphorylation pathways.

Chloroplasts↗
PubMed1998

Mutation of residue threonine-2 of the D2 polypeptide and its effect on photosystem II function in Chlamydomonas reinhardtii.

The D2 polypeptide of the photosystem II (PSII) complex in the green alga Chlamydomonas reinhardtii is thought to be reversibly phosphorylated. By analogy to higher plants, the phosphorylation site is likely to be at residue threonine-2 (Thr-2). We have investigated the role of D2 phosphorylation by constructing two mutants in which residue Thr-2 has been replaced by either alanine or serine. Both mutants grew photoautotrophically at wild-type rates, and noninvasive biophysical measurements, including the decay of chlorophyll fluorescence, the peak temperature of thermoluminescence bands, and rates of oxygen evolution, indicate little perturbation to electron transfer through the PSII complex. The susceptibility of mutant PSII to photoinactivation as measured by the light-induced loss of PSII activity in whole cells in the presence of the protein-synthesis inhibitors chloramphenicol or lincomycin was similar to that of wild type. These results indicate that phosphorylation at Thr-2 is not required for PSII function or for protection from photoinactivation. In control experiments the phosphorylation of D2 in wild-type C. reinhardtii was examined by 32P labeling in vivo and in vitro. No evidence for the phosphorylation of D2 in the wild type could be obtained. [14C]Acetate-labeling experiments in the presence of an inhibitor of cytoplasmic protein synthesis also failed to identify phosphorylated (D2.1) and nonphosphorylated (D2.2) forms of D2 upon sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Our results suggest that the existence of D2 phosphorylation in C. reinhardtii is still in question.

Amino Acid Sequence↗
PubMed1997

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.

Binding Sites↗
PubMed1996

Comparison of primary charge separation in the photosystem II reaction center complex isolated from wild-type and D1-130 mutants of the cyanobacterium Synechocystis PCC 6803.

We compare primary charge separation in a photosystem II reaction center preparation isolated from a wild-type (WT) control strain of the cyanobacterium Synechocystis sp. PCC 6803 and from two site-directed mutants of Synechocystis in which residue 130 of the D1 polypeptide has been changed from a glutamine to either a glutamate (mutant D1-Gln130Glu), as in higher plant sequences, or a leucine residue (mutant D1-Gln130Leu). The D1-130 residue is thought to be close to the pheophytin electron acceptor. We show that, when P680 is photoselectively excited, the primary radical pair state P680+Ph- is formed with a time constant of 20-30 ps in the WT and both mutants; this time constant is very similar to that observed in Pisum sativum (a higher plant). We also show that a change in the residue at position D1-130 causes a shift in the peak of the pheophytin Qx-band. Nanosecond and picosecond transient absorption measurements indicate that the quantum yield of radical pair formation (phi RP), associated with the 20-30-ps component, is affected by the identify of the D1-130 residue. We find that, for the isolated photosystem II reaction center particle, phi RP higher plant > phi RP D1-Gln130Glu mutant > phi RP WT > phi RP D1-Gln130Leu mutant. Furthermore, the spectroscopic and quantum yield differences we observe between the WT Synechocystis and higher plant photosystem II, seem to be reversed by mutating the D1-130 ligand so that it is the same as in higher plants. This result is consistent with the previously observed natural regulation of quantum yield in Synechococcus PS II by particular changes in the D1 polypeptide amino acid sequence (Clark, A.K., Hurry, V. M., Gustafsson, P. and Oquist, G. (1993) Proc. Natl. Acad. Sci. U.S.A. 90, 11985-11989).

Cyanobacteria↗
PubMed1995

Deletion of the PEST-like region of photosystem two modifies the QB-binding pocket but does not prevent rapid turnover of D1.

The rapid turn-over of the D1 polypeptide of the photosystem two complex has been suggested to be due to the presence of a "PEST"-like sequence located between putative transmembrane helices IV and V of D1 (Greenberg, B. M., Gaba, V., Mattoo, A. K. and Edelman, M. (1987) EMBO J. 6, 2865-2869). We have tested this hypothesis by constructing a deletion mutant (delta 226-233) of the cyanobacterium Synechocystis sp. PCC 6803 in which residues 226-233 of the D1 polypeptide, containing the PEST-like sequence, have been removed. The resulting mutant, delta PEST, is able to grow photoautotrophically and give light-saturated rates of oxygen at wild type levels. However electron transfer on the acceptor side of the complex is perturbed. Analysis of cells by thermoluminescence and by monitoring the decay in quantum yield of variable fluorescence following saturating flash excitation indicates that Q-B, but not Q-A, is destabilized in this mutant. Electron transfer on the donor side of photosystem two remains largely unchanged in the mutant. Turnover of the D1 polypeptide as examined by pulse-chase experiments using [35S]methionine was enhanced in the delta PEST mutant compared to strain TC31 which is the wild type control. We conclude that the PEST sequence is not absolutely required for turnover of the D1 polypeptide in vivo although deletion of residues 226-233 does have an effect on the redox equilibrium between QA and QB.

Amino Acid Sequence↗