Sequential analysis of a Simon task--evidence for an attention-shift account

We investigated the attention-shift hypothesis of the Simon effect by analysing the effect of repeating relevant colour or irrelevant location of the stimulus in four serial reaction time tasks. In Experiment 1 with short response-stimulus intervals (RSI), we assume that there is no time to engage attention at the fixation cross before the onset of a new stimulus. In agreement with the hypothesis, Experiment 1 reveals no Simon effect when the stimulus location is repeated. In Experiment 2 with long RSI, we observe a Simon effect for location repetitions and alternations. In Experiment 3 with long RSI, we hinder the disengagement of attention by displaying the stimulus after response execution. As expected, the Simon effect is reduced for location repetitions. In Experiment 4 with stimuli additionally presented at the fixation cross, responses are faster if the attention shift towards the centrally presented stimulus corresponds with the location of the required response. Additionally, we argue that binding of the stimulus features into an object or event file better explains the so-called blocking of the automatic response-priming route after a noncorresponding trial.

Involvement of zeaxanthin and of the Cbr protein in the repair of photosystem II from photoinhibition in the green alga Dunaliella salina

A light-sensitive and chlorophyll (Chl)-deficient mutant of the green alga Dunaliella salina (dcd1) showed an amplified response to irradiance stress compared to the wild-type. The mutant was yellow-green under low light (100 micromol photons m(-2) s(-1)) and yellow under high irradiance (2000 micromol photons m(-2) s(-1)). The mutant had lower levels of Chl, lower levels of light harvesting complex II, and a smaller Chl antenna size. The mutant contained proportionately greater amounts of photodamaged photosystem (PS) II reaction centers in its thylakoid membranes, suggesting a greater susceptibility to photoinhibition. This phenotype was more pronounced under high than low irradiance. The Cbr protein, known to accumulate when D. salina is exposed to irradiance stress, was pronouncedly expressed in the mutant even under low irradiance. This positively correlated with a higher zeaxanthin content in the mutant. Cbr protein accumulation, xanthophyll cycle de-epoxidation state, and fraction of photodamaged PSII reaction centers in the thylakoid membrane showed a linear dependence on the chloroplast 'photoinhibition index', suggesting a cause-and-effect relationship between photoinhibition, Cbr protein accumulation and xanthophyll cycle de-epoxidation state. These results raised the possibility of zeaxanthin and Cbr involvement in the PSII repair process through photoprotection of the partially disassembled, and presumably vulnerable, PSII core complexes from potentially irreversible photooxidative bleaching.

Hydrogen production. Green algae as a source of energy

Hydrogen gas is thought to be the ideal fuel for a world in which air pollution has been alleviated, global warming has been arrested, and the environment has been protected in an economically sustainable manner. Hydrogen and electricity could team to provide attractive options in transportation and power generation. Interconversion between these two forms of energy suggests on-site utilization of hydrogen to generate electricity, with the electrical power grid serving in energy transportation, distribution utilization, and hydrogen regeneration as needed. A challenging problem in establishing H(2) as a source of energy for the future is the renewable and environmentally friendly generation of large quantities of H(2) gas. Thus, processes that are presently conceptual in nature, or at a developmental stage in the laboratory, need to be encouraged, tested for feasibility, and otherwise applied toward commercialization.

Hydrogen production. Green algae as a source of energy

Hydrogen gas is thought to be the ideal fuel for a world in which air pollution has been alleviated, global warming has been arrested, and the environment has been protected in an economically sustainable manner. Hydrogen and electricity could team to provide attractive options in transportation and power generation. Interconversion between these two forms of energy suggests on-site utilization of hydrogen to generate electricity, with the electrical power grid serving in energy transportation, distribution utilization, and hydrogen regeneration as needed. A challenging problem in establishing H(2) as a source of energy for the future is the renewable and environmentally friendly generation of large quantities of H(2) gas. Thus, processes that are presently conceptual in nature, or at a developmental stage in the laboratory, need to be encouraged, tested for feasibility, and otherwise applied toward commercialization.

Absence of lutein, violaxanthin and neoxanthin affects the functional chlorophyll antenna size of photosystem-II but not that of photosystem-I in the green alga Chlamydomonas reinhardtii

Chlamydomonas reinhardtii double mutant npq2 lor1 lacks the beta, epsilon-carotenoids lutein and loroxanthin as well as all beta,beta-epoxycarotenoids derived from zeaxanthin (e.g. violaxanthin and neoxanthin). Thus, the only carotenoids present in the thylakoid membranes of the npq2 lor1 cells are beta-carotene and zeaxanthin. The effect of these mutations on the photochemical apparatus assembly and function was investigated. In cells of the mutant strain, the content of photosystem-II (PSII) and photosystem-I (PSI) was similar to that of the wild type, but npq2 lor1 had a significantly smaller PSII light-harvesting Chl antenna size. In contrast, the Chl antenna size of PSI was not truncated in the mutant. SDS-PAGE and Western blot analysis qualitatively revealed the presence of all LHCII and LHCI apoproteins in the thylakoid membrane of the mutant. The results showed that some of the LHCII and most of the LHCI were assembled and functionally connected with PSII and PSI, respectively. Photon conversion efficiency measurements, based on the initial slope of the light-saturation curve of photosynthesis and on the yield of Chl a fluorescence in vivo, showed similar efficiencies. However, a significantly greater light intensity was required for the saturation of photosynthesis in the mutant than in the wild type. It is concluded that zeaxanthin can successfully replace lutein and violaxanthin in most of the functional light-harvesting antenna of the npq2 lor1 mutant.

Microalgae: a green source of renewable H(2)

This article summarizes recent advances in the field of algal hydrogen production. Two fundamental approaches are being developed. One involves the temporal separation of the usually incompatible reactions of O(2) and H(2) production in green algae, and the second involves the use of classical genetics to increase the O(2) tolerance of the reversible hydrogenase enzyme. The economic and environmental impact of a renewable source of H(2) are also discussed.

Photosynthetic apparatus organization and function in the wild type and a chlorophyll b-less mutant of Chlamydomonas reinhardtii. Dependence on carbon source

The assembly, organization and function of the photosynthetic apparatus was investigated in the wild type and a chlorophyll (Chl) b-less mutant of the unicellular green alga Chlamydomonas reinhardtii, generated via DNA insertional mutagenesis. Comparative analyses were undertaken with cells grown photoheterotrophically (acetate), photomixotrophically (acetate and HCO3-) or photoautotrophically (HCO3-). It is shown that lack of Chl b diminished the photosystem-II (PSII) functional Chl antenna size from 320 Chl (a and b) to about 95 Chl a molecules. However, the functional Chl antenna size of PSI remained fairly constant at about 290 Chl molecules, independent of the presence of Chl b. Western blot and kinetic analyses suggested the presence of inner subunits of the Chl a-b light-harvesting complex of PSII (LHCII) and the entire complement of the Chl a-b light-harvesting complex of PSI (LHCI) in the mutant. It is concluded that Chl a can replace Chl b in the inner subunits of the LHCII and in the entire complement of the LHCI. Growth of cells on acetate as the sole carbon source imposes limitations in the photon-use efficiency and capacity of photosynthesis. These are manifested as a lower quantum yield and lower light-saturated rate of photosynthesis, and as lower variable to maximal (Fv/Fmax) chlorophyll fluorescence yield ratios. This adverse effect probably originates because acetate shifts the oxidation-reduction state of the plastoquinone pool, and also because it causes a decrease in the amount and/or activity of Rubisco in the chloroplast. Such limitations are fully alleviated upon inclusion of an inorganic carbon source (e.g. bicarbonate) in the cell growth medium. Further, the work provides evidence to show that transformation of green algae can be used as a tool by which to generate mutants exhibiting a permanently truncated Chl antenna size and a higher (per Chl) photosynthetic productivity of the cells.

Insulin-like growth factor inhibits vascular contraction to 5-hydroxytryptamine: involvement of tyrosine phosphatase

This study tests the hypothesis that insulin-like growth factor 1 (IGF-1)-induced vasodilation is due to the stimulation of tyrosine phosphatase. Rat aortic segments (endothelium intact) were placed in muscle baths for force measurement. Segments were contracted to serotonin [5-hydroxytyptamine (5-HT), 10(-7)-10(-5) M] before and after incubation with IGF-1 (10-100 nM; 90 min). IGF-1 caused a 20% inhibition of 5-HT-induced contractions. This inhibition was reversed by the tyrosine phosphatase inhibitors sodium orthovanadate and molybdate. Orthovanadate did not alter inhibitory properties of the calcium channel antagonist verapamil, suggesting that the phosphatase inhibitors were relatively specific. IGF-1-induced inhibition was not altered by blockade of nitric oxide synthase. Western blot analysis confirmed that the 5-HT-induced stimulation of tyrosine phosphorylation of the 42-kDa extracellular signal-regulated mitogen-activated protein kinase protein was reduced by IGF-1 (52% inhibition), an inhibition that was attenuated by orthovanadate. These data are consistent with the hypothesis that the vasodilator activity of IGF-1 is mediated by the activation of a tyrosine phosphatase.

Sustained photobiological hydrogen gas production upon reversible inactivation of oxygen evolution in the green alga Chlamydomonas reinhardtii

The work describes a novel approach for sustained photobiological production of H(2) gas via the reversible hydrogenase pathway in the green alga Chlamydomonas reinhardtii. This single-organism, two-stage H(2) production method circumvents the severe O(2) sensitivity of the reversible hydrogenase by temporally separating photosynthetic O(2) evolution and carbon accumulation (stage 1) from the consumption of cellular metabolites and concomitant H(2) production (stage 2). A transition from stage 1 to stage 2 was effected upon S deprivation of the culture, which reversibly inactivated photosystem II (PSII) and O(2) evolution. Under these conditions, oxidative respiration by the cells in the light depleted O(2) and caused anaerobiosis in the culture, which was necessary and sufficient for the induction of the reversible hydrogenase. Subsequently, sustained cellular H(2) gas production was observed in the light but not in the dark. The mechanism of H(2) production entailed protein consumption and electron transport from endogenous substrate to the cytochrome b(6)-f and PSI complexes in the chloroplast thylakoids. Light absorption by PSI was required for H(2) evolution, suggesting that photoreduction of ferredoxin is followed by electron donation to the reversible hydrogenase. The latter catalyzes the reduction of protons to molecular H(2) in the chloroplast stroma.

Transforming growth factor beta, fibrogenesis and hyperglycemia in patients with chronic pancreatitis

It has been suggested that transforming growth factor beta (TGFb) mediates liver fibrosis, which can be monitored by the serum determination of the N-terminal peptide of type III procollagen (PIIIP) and laminin. Fibrosis is also an important phenomenon in patients with chronic pancreatitis (CP). In 23 patients with CP, 38 with liver cirrhosis (LC) and 20 healthy controls we compared the serum patterns of PIIIP, laminin and TGFb and assessed whether in CP these markers are correlated with exocrine and endocrine function. In patients with LC, PIIIP and laminin levels were significantly higher, whereas TGFb levels were significantly lower than those of controls. In CP patients, no significant variations were found for PIIIP and laminin, although levels were high in 7/23 and in 5/23 patients, respectively. TGFb levels in CP patients were higher than those in LC patients, levels being raised in 6/23 patients. In LC patients an inverse correlation was found between PIIIP and TGFb, whereas in CP patients, a direct correlation was found between TGFb and PIIIP. Moreover, in CP patients, there was also a positive correlation between TGFb and fasting serum glucose levels, while laminin was correlated with PABA test results.

IN CONCLUSION

serum biochemical markers of liver fibrosis can be considered of limited value in assessing pancreatic fibrosis; in liver cirrhosis there may be a negative feed-back regulation between TGFb production and the fibrogenetic process; and in chronic pancreatitis TGFb appears to favor fibrosis on the one hand and the development of hyperglycemia on the other.

Primary sclerosing cholangitis (PSC): clinical, laboratory and survival analysis in children and adults

BACKGROUND

Primary sclerosing cholangitis (PSC) is an uncommon disorder, rarely diagnosed in children, moreover, data on its natural history and survival are still lacking.

AIM

The study was undertaken to compare clinical, laboratory and survival rates in two series of PSC: one in a pediatric group (group A) and the other in an adult population (group B).

METHODS

Group A included 9 patients (5 males, 4 females, mean age 10 yrs, range 7-15); group B included 28 patients (19 males, 9 females, mean age 32 years, range 19-60). The mean follow-up was 5.2 years in group A and 6.9 years in group B (range 1-14 years). ERCP and colonoscopy were performed in each case. Survival was analyzed using the Kaplan-Meier method.

RESULTS

At presentation children showed significantly higher levels of IgG and AST compared to adults (p<0.05); moreover, interface hepatitis occurred in 50% of children and in 14.2% in adults (p=ns). During follow-up the following major events occurred: oesophageal bleeding (n=2) in group A; progressive liver failure (n=6), cholangiocarcinoma (n=3), colonic cancer (n=1) in group B. Liver transplantation (OLTx) was performed in 4 adults (one died after a retransplantation). No deaths were observed in children. The Kaplan-Meier curve in adults shows a 65% rate of survival at 10 years.

CONCLUSIONS

The present findings on PSC suggest a more severe activity of the disease in children than in adults at presentation; nonetheless, the prognosis seems to be better in children than in adults. The Mayo score prognostic index does not predict the development of liver/colonic cancer. A poor outcome (defined as death or being listed for OLTx) only occurred in adults.

Oxygen-derived free radical production by peripheral blood neutrophils in chronic cholestatic liver diseases

BACKGROUND/AIMS

Patients with chronic cholestasis, particularly those with associated cirrhosis, are susceptible to infectious complications. From animal models it has been postulated that cholestasis affects systemic polymorphonuclear leukocyte (PMNL) function by impeding chemotaxis, phagocytosis and superoxide release, which are necessary for an adequate immune response. The aim of this study was to evaluate neutrophil activity in the production of oxygen-derived free radicals in chronic cholestatic liver diseases.

METHODOLOGY

The following groups were included in the study: 27 primary biliary cirrhosis (PBC) patients, 12 primary sclerosing cholangitis (PSC) patients, and 3 control groups (29 healthy subjects, 19 patients with HCV-related cirrhosis and 23 ulcerative colitis (UC) patients). Peripheral neutrophils were isolated from heparinized blood samples and PMNL activity was measured by free radical production, using a chemiluminometer, after stimulation with fMLP, PMA and Zymosan. The effect of liver disease severity and degree of cholestasis on PMNL function was also evaluated.

RESULTS

Both PBC and PSC patients exhibited a normal PMNL activity compared to healthy subjects after the three stimuli used. In PBC patients only (but not in PSC patients), the histological stage of the disease seems to positively influence ROS production. Stage IV PBC patients showed a significantly higher PMNL activity compared to HCV-related cirrhotic patients. PSC patients failed to show any difference according to the association with UC.

CONCLUSIONS

The increased susceptibility to bacterial infections in patients with chronic cholestatic liver disease is not related to an impaired PMNL activity. However, our findings may support the influence of biohumoral factors (cytokines?) on PMNL activation.

Photosystem-II damage and repair cycle in chloroplasts: what modulates the rate of photodamage ?

Organisms that rely on oxygenic photosynthesis are subject to the effects of photo-oxidative damage, which impairs the function of photosystem-II (PSII). This phenomenon has the potential to lower rates of photosynthesis and diminish plant growth. Experimental evidence shows that the steady-state oxidation-reduction level of the primary quinone acceptor (QA) of PSII is the parameter that controls photodamage under a variety of physiological and environmental conditions. When QA is reduced, excitation energy at PSII is dissipated via a charge-recombination reaction. Such non-assimilatory dissipation of excitation generates singlet oxygen that might act to covalently modify the photochemical reaction center chlorophyll. Under steady-state photosynthesis conditions, the reduction state of QA increases linearly with irradiance, thereby causing a correspondingly linear increase in the probability of photodamage. It is concluded that there is a low probability that photodamage will occur when QA is oxidized and excitation energy is utilized in electron transport, and a significantly higher probability when QA is reduced in the course of steady-state photosynthesis.

Tumour necrosis factor alpha and cellular proliferation in primary biliary cirrhosis

AIMS

1) To evaluate serum levels and tissue expression of Tumour necrosis factor alpha in primary biliary cirrhosis: 2) to correlate serum tumour necrosis factor alpha levels and cellular proliferation with the severity and prognosis of liver disease.

METHODS

Twenty-nine primary biliary cirrhosis patients (6 stage I, 8 II, 8 III, and 7 IV) entered the study. Serum tumour necrosis factor alpha was measured by EIA (Innogenetics, Antwerp, Belgium). Tissue tumour necrosis factor alpha and Ki-67 were tested by indirect immunoperoxidase staining on liver sections.

RESULTS

Serum tumour necrosis factor alpha increased with the severity of histological stage (from 10.8 +/- 11 pg/ml in stage II to 17.1 +/- 10 in stage III and 22.8 +/- 8.7 in stage IV, p < 0.036). A positive correlation was also found between tumour necrosis factor alpha serum levels and the Mayo score (p < 0.05). A weak and sporadic expression of tumour necrosis factor alpha was observed in the inflammatory infiltrate around the bile ducts. Tissue Ki-67 (expressed as the labelling index in the hepatocellular nuclei) was evaluated in all stages of the disease (1.09 +/- 0.6% in stage I, 1.14 +/- 0.6% in stage II, 2.11 +/- 1.9% in stage III, and 2.67 +/- 2.8% in stage IV; the labelling index was significantly lower in early stages (I/II) than in late stages (III/IV), p < 0.05. A strong correlation between Ki-67 and the Mayo score was observed (p < 0.0005).

CONCLUSIONS

1) tumour necrosis factor alpha production seems related to the severity and the prognosis of primary biliary cirrhosis; 2) liver mononuclear cells in the inflammatory infiltrate do not seem to be the major site of tumour necrosis factor alpha release; 3) cellular proliferation is correlated with the severity of liver disease.

Separation of oligonucleotides and DNA fragments by capillary electrophoresis in dynamically and permanently coated capillaries, using a copolymer of acrylamide and beta-D-glucopyranoside as a new low viscosity matrix with high sieving capacity

New copolymers of acrylamide and beta-D-glucopyranoside were synthesized and characterized. The different reactivity of the two monomers towards radical polymerization meant we could control the growth of the polymer chains whose length was inversely related to the number of glucose residues incorporated in the copolymers. The properties of these polymers were investigated in the separation of oligonucleotides and double-stranded DNA by capillary electrophoresis (CE) in coated and uncoated capillaries. The new copolymers were a suitable matrix for CE due to their high-resolving capacity and low viscosity. We also looked into the advantages of a new method of dynamic suppression of electroosmotic flow based on the addition of small amounts (0.03-0.05%) of dimethylacrylamide to the sieving and to the running buffer. A complete test was run on the reproducibility and efficiency of separations carried out in a permanently and dynamically coated capillary, and the advantages and disadvantages of the two methods were compared.

Photoinhibitory damage is modulated by the rate of photosynthesis and by the photosystem II light-harvesting chlorophyll antenna size

We investigated the effect of photosynthetic electron transport and of the photosystem II (PSII) chlorophyll (Chl) antenna size on the rate of PSII photoinhibitory damage. To modulate the rate of photosynthesis and the light-harvesting capacity in the unicellular chlorophyte Dunaliella salina Teod., we varied the amount of inorganic carbon in the culture medium. Cells were grown under high irradiance either with a limiting supply of inorganic carbon, provided by an initial concentration of 25 mM NaHCO3, or with supplemental CO2 bubbled in the form of 3% CO2 in air. The NaHCO3-grown cells displayed slow rates of photosynthesis and had a small PSII light-harvesting Chl antenna size (60 Chl molecules). The half-time of PSII photodamage was 40 min. When switched to supplemental CO2 conditions, the rate of photodamage was retarded to a t1/2 = 70 min. Conversely, CO2-supplemented cells displayed faster rates of photosynthesis and a larger PSII light-harvesting Chl antenna size (500 Chl molecules). They also showed a rate of photodamage with t1/2 = 40 min. When depleted of CO2, the rate of photodamage was accelerated (t1/2 = 20 min). These results indicate that the in-vivo susceptibility to photodamage is modulated by the rate of forward electron transport through PSII. Moreover, a large Chl antenna size enhances the rate of light absorption and photodamage and, therefore, counters the mitigating effect of forward electron transport. We propose that under steady-state photosynthesis, the rate of light absorption (determined by incident light intensity and PS Chl antenna size) and the rate of forward electron transport (determined by CO2 availability) modulate the oxidation/reduction state of the primary PSII acceptor QA, which in turn defines the low/high probability for photodamage in the PSII reaction center.

The regulation of photosynthetic electron transport during nutrient deprivation in Chlamydomonas reinhardtii

The light-saturated rate of photosynthetic O2 evolution in Chlamydomonas reinhardtii declined by approximately 75% on a per-cell basis after 4 d of P starvation or 1 d of S starvation. Quantitation of the partial reactions of photosynthetic electron transport demonstrated that the light-saturated rate of photosystem (PS) I activity was unaffected by P or S limitation, whereas light-saturated PSII activity was reduced by more than 50%. This decline in PSII activity correlated with a decline in both the maximal quantum efficiency of PSII and the accumulation of the secondary quinone electron acceptor of PSII nonreducing centers (PSII centers capable of performing a charge separation but unable to reduce the plastoquinone pool). In addition to a decline in the light-saturated rate of O2 evolution, there was reduced efficiency of excitation energy transfer to the reaction centers of PSII (because of dissipation of absorbed light energy as heat and because of a transition to state 2). These findings establish a common suite of alterations in photosynthetic electron transport that results in decreased linear electron flow when C. reinhardtii is limited for either P or S. It was interesting that the decline in the maximum quantum efficiency of PSII and the accumulation of the secondary quinone electron acceptor of PSII nonreducing centers were regulated specifically during S-limited growth by the SacI gene product, which was previously shown to be critical for the acclimation of C. reinhardtii to S limitation (J.P. Davies, F.H. Yildiz, and A.R. Grossman [1996] EMBO J 15: 2150-2159).

S-adenosylmethionine versus ursodeoxycholic acid in the treatment of intrahepatic cholestasis of pregnancy: preliminary results of a controlled trial

OBJECTIVES

To evaluate the efficacy of S-adenosylmethionine (SAMe) and ursodeoxycholic acid (UDCA) in intrahepatic cholestasis of pregnancy (ICP).

METHODS

Twenty patients in the last trimester of pregnancy were randomly assigned to receive either SAMe (1000 mg/day i.m.) or UDCA (450 mg/day) until delivery; the treatment lasted at least 15 days in all cases.

RESULTS

After UDCA the women exhibited significantly lower levels of total bile acids (P < 0.02), but no significant differences were noted in AST, ALT, or alkaline phosphatase. All ten patients showed a complete resolution of pruritus. After SAMe no significant changes were noted in pruritus, total bile acids or liver function tests. No adverse reactions on mother or child were recorded during either UDCA or SAMe treatment and the outcome of pregnancy was favorable in both groups.

CONCLUSIONS

These findings show that UDCA is more effective than SAMe in controlling pruritus and total bile acids, which are considered a prognostic parameter in ICP with respect to the fetus. Nevertheless, before UDCA is introduced as an effective and safe treatment for ICP, which also has a beneficial effect on fetal prognosis, we believe these results should be confirmed and extended in other clinical trials.

Photoinhibition and repair in Dunaliella salina acclimated to different growth irradiances

The light-dependent rate of photosystem-II (PSII) damage and repair was measured in photoautotrophic cultures of Dunaliella salina Teod. grown at different irradiances in the range 50-3000 μmol photons · m^-2· s^-1. Rates of cell growth increased in the range of 50-800 μmol photons·m^-2·s^-1, remained constant at a maximum in the range of 800-1,500 μmol photons·m^-2 ·s^-1, and declined due to photoinhibition in the range of 1500-3000 μmol photons·m^-2·s^-1. Western blot analyses, upon addition of lincomycin to the cultures, revealed first-order kinetics for the loss of the PSII reaction-center protein (D1) from the 32-kDa position, occurring as a result of photodamage. The rate constant of this 32-kDa protein loss was a linear function of cell growth irradiance. In the presence of lincomycin, loss of the other PSII reaction-center protein (D2) from the 34-kDa position was also observed, occurring with kinetics similar to those of the 32-kDa form of D1. Increasing rates of photodamage as a function of irradiance were accompanied by an increase in the steady-state level of a higher-molecular-weight protein complex (≈ 160-kDa) that cross-reacted with D1 antibodies. The steady-state level of the 160-kDa complex in thylakoids was also a linear function of cell growth irradiance. These observations suggest that photodamage to D1 converts stoichiometric amounts of D1 and D2 (i.e., the D1/D2 heterodimer) into a ≈160-kDa complex. This complex may help to stabilize the reaction-center proteins until degradation and replacement of D1 can occur. The results indicated an intrinsic half-time of about 60 min for the repair of individual PSII units, supporting the idea that degradation of D1 after photodamage is the rate-limiting step in the PSII repair process.

Chromatic regulation inChlamydomonas reinhardtii alters photosystem stoichiometry and improves the quantum efficiency of photosynthesis

The work addressed the adjustment of the photosystem ratio in the green algaChlamydomonas reinhardtii. It is shown that green algae, much like cyanophytes and higher plants, adjust and optimize the ratio of the two photosystems in chloroplasts in response to the quality of irradiance during growth. Such adjustments are compensation reactions and helpC. reinhardtii to retain a quantum efficiency of oxygen evolution near the theoretical maximum. Results show variable amounts of PS I and a fairly constant amount of PS II in chloroplasts and suggest that photosystem stoichiometry adjustments, occurring in response to the quality of irradiance during plant growth, are mainly an adjustment in the concentration of PS I. The work delineates chromatic effects on chlorophyll accumulation in the chloroplast ofC. reinhardtii from those pertaining to the regulation of the PS I/PS II ratio. The detection of the operation of a molecular feedback mechanism for the PS I/PS II ratio adjustment in green algae strengthens the notion of the highly conserved nature of this mechanism among probably all oxygen evolving photosynthetic organisms. Findings in this work are expected to serve as the basis of future biochemical and mutagenesis experiments for the elucidation of the photosystem ratio adjustment in oxygenic photosynthesis.

Localization and characterization of a novel 20 kDa polypeptide in the chloroplast of the green alga Dunaliella salina

Recent work with the green alga Dunaliella salina showed the presence of a approximately 20 kDa chloroplast protein that was recognized by polyclonal antibodies raised against the isolated LHC-II [Webb M.R. and Melis A. (1995) Plant Physiol. 107: 885]. In this report, a characterization of the approximately 20 kDa polypeptide is presented. It is shown that it is localized in the chloroplast envelope membrane of D. salina. The abundance of this protein is constant on a per cell basis and independent of the light regime during cell growth. The approximately 20 kDa polypeptide is easily degraded to a approximately 19 kDa product during sample preparation. A limited amino acid sequence of 21 residues from the free N-terminus of the approximately 19 kDa product was obtained. On the basis of this partial sequence, it was concluded that the approximately 20 kDa polypeptide is not a degradation product of a known LHC-II but rather a novel protein. The approximately 20 kDa polypeptide did not cross-react with antibodies raised against the Cbr (carotene biosynthesis-related) gene product and showed a different electrophoretic mobility from the latter. Light-shift experiments suggest that the approximately 20 kDa polypeptide is not an ELIP (early light-inducible protein). Possible functions of the approximately 20 kDa protein are discussed.

Characterization of a 160 kD Photosystem II reaction center complex isolated from photoinhibited Dunaliella salina thylakoids

Photoinhibition in the green alga Dunaliella salina is accompanied by the formation of inactive Photosystem II reaction centers. In SDS-PAGE analysis, the latter appear as 160 kD complexes. These complexes are structurally stable, enough to withstand re-electrophoresis of excised gel slices from the 160 kD region. Western blot analyses with specific polyclonal antibodies raised against the D1 or D2 reaction center proteins provided evidence for the presence of both of these polypeptides in the re-electrophoresed 160 kD complex. Incubation of excised gel slices from the 160 kD region, under aerobic conditions at 4°C for a prolonged period of time, caused a break-up of the 160 kD complex into a ∽52 kD D1-containing and ∽80 and ∽26 kD D2-containing pieces. Western blot analysis with polyclonal antibodies raised against the apoproteins of CPI (reaction center proteins of PS I) did not show cross-reaction either with the 160 kD complex or with the ∽52, ∽80 and ∽26 kD pieces. The results show the presence of both D1 and D2 in the 160 kD complex and strengthen the notion of a higher molecular weight D1- and D2-containing complex that forms upon disassembly of photodamaged PS II units.

The role of chloroplast-encoded protein biosynthesis on the rate of D1 protein degradation in Dunaliella salina

Mechanistic aspects of the Photosystem II (PS II) damage and repair cycle in Dunaliella salina were investigated. The work addressed the role of chloroplast-encoded protein biosynthesis on the rate of the D1 protein (chloroplast psbA gene product) degradation, following photoinhibition of PS II under in vivo conditions. Cells were grown under different light-intensities and the rate of D1 photodamage and degradation was measured via pulse-chase measurements with ((35)S)sulfate. It is shown that no detectable difference exists in the rate of D1 degradation in D. salina, measured in the presence or absence of lincomycin, a chloroplast protein biosynthesis inhibitor. The results suggest that de novo D1 biosynthesis does not play a role in the regulation of D1 degradation. In low-light (100 μmol photons m(-2) s(-1)) grown cells, the rate of photodamage to D1 did not exceed the rate of its degradation and replacement. In high-light (2200 μmol photons m(-1) s(-1)) grown cells, the rate of D1 photodamage was faster than the rate of its degradation, resulting in a significant accumulation of photoinactivated PS II centers in the chloroplast thylakoids (chronic photoinhibition). The latter was coincident with the appearance of a 160 kD complex that contained photodamaged D1. Electron micrographs of D. salina thylakoids revealed extensive grana stacks in the thylakoid membrane of low-light grown cells. Only rudimentary appressions consisting of simple membrane pairings were found in the high-light grown cells. The results are discussed in terms of the regulation of D1 degradation in chloroplasts under in vivo conditions.