Photosystem II Cyclic Heterogeneity and Photoactivation in the Diazotrophic, Unicellular Cyanobacterium Cyanothece Species ATCC 51142

The unicellular, diazotrophic cyanobacterium Cyanothece sp. ATCC 51142 demonstrated important modifications to photosystem II (PSII) centers when grown under light/dark N₂-fixing conditions. The properties of PSII were studied throughout the diurnal cycle using O₂-flash-yield and pulse-amplitude-modulated fluorescence techniques. Nonphotochemical quenching (q_N) of PSII increased during N₂ fixation and persisted after treatments known to induce transitions to state 1. The q_N was high in cells grown in the dark, and then disappeared progressively during the first 4 h of light growth. The photoactivation probability, ε, demonstrated interesting oscillations, with peaks near 3 h of darkness and 4 and 10 h of light. Experiments and calculations of the S-state distribution indicated that PSII displays a high level of heterogeneity, especially as the cells prepare for N₂ fixation. We conclude that the oxidizing side of PSII is strongly affected during the period before and after the peak of nitrogenase activity; changes include a lowered capacity for O₂ evolution, altered dark stability of PSII centers, and substantial changes in q_N.     

Rising CO2 Interacts with Growth Light and Growth Rate to Alter Photosystem II Photoinactivation of the Coastal Diatom Thalassiosira pseudonana

We studied the interactive effects of pCO₂ and growth light on the coastal marine diatom Thalassiosira pseudonana CCMP 1335 growing under ambient and expected end-of-the-century pCO₂ (750 ppmv), and a range of growth light from 30 to 380 µmol photons·m⁻²·s⁻¹. Elevated pCO₂ significantly stimulated the growth of T. pseudonana under sub-saturating growth light, but not under saturating to super-saturating growth light. Under ambient pCO₂ susceptibility to photoinactivation of photosystem II (σ_i) increased with increasing growth rate, but cells growing under elevated pCO₂ showed no dependence between growth rate and σ_i, so under high growth light cells under elevated pCO₂ were less susceptible to photoinactivation of photosystem II, and thus incurred a lower running cost to maintain photosystem II function. Growth light altered the contents of RbcL (RUBISCO) and PsaC (PSI) protein subunits, and the ratios among the subunits, but there were only limited effects on these and other protein pools between cells grown under ambient and elevated pCO₂.     

Dependence of Root Growth Rate on Holoploid DNA Content

Russian Journal of Developmental Biology - Analysis of root growth rate’s dependence on the holoploid DNA content was carried out on different plant species. Such dependence was found weakly...     

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.     

Xiphinema krugi,Species Complex or Complex of Cryptic Species?

Fourteen morphologically putative populations of X. krugi were clearly separated into four different profiles by RFLP analysis (Alu I and Hinf I), sequencing of the ITS-1 region, and subsequent Maximum Likelihood phylogenetic analyses. These four profiles were further supported by a principal component analysis of morphometric characters that yielded four taxonomic clusters matching those produced by the molecular data. Sequence homology was greater amongst populations that represented the same RFLP profile than between profiles and similar both between representative populations of the RFLP profiles and putative closely related Xiphinema species. This study suggests that X. krugi is a potential species complex comprised of at least four distinct genotypes.     

Heterogeneity and Photoinhibition of Photosystem II Studied with Thermoluminescence

Thermoluminescence (TL) signals were recorded from grana stacks, margins, and stroma lamellae from fractionated, dark-adapted thylakoid membranes of spinach (Spinacia oleracea L.) in the absence and in the presence of 2,6-dichlorphenylindophenol (DCMU). In the absence of DCMU, the TL signal from grana fractions consisted of a homogenous B-band, which originates from recombination of the semi-quinone Q_B⁻ with the S₂ state of the water-splitting complex and reflects active photosystem II (PSII). In the presence of DCMU, the B-band was replaced by the Q-band, which originates from an S₂Q_A⁻ recombination. Margin fractions mainly showed two TL-bands, the B- and C-bands, at approximately 50°C in the absence of DCMU, and Q- and C-bands in the presence of DCMU. The C-band is ascribed to a Tyr_D⁺-Q_A⁻ recombination. In the absence of DCMU, the fractions of stromal lamellae mainly gave rise to a TL emission at 42°C. The intensity of this band was independent of the number of excitation flashes and was shifted to higher temperatures (52°C) after the addition of DCMU. Based on these observations, this band was considered to be a C-band. After photoinhibitory light treatment of uncoupled thylakoid membranes, the TL intensities of the B- and Q-bands decreased, whereas the intensity at 45°C (C-band) slightly increased. It is proposed that the 42 to 52°C band that was observed in marginal and stromal lamellae and in photoinhibited thylakoid membranes reflects inactive PSII centers that are assumed to be equivalent to inactive PSII Q_B-nonreducing centers.     

Cell Size and Growth Rate Are Modulated by TORC2-Dependent Signals

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Stoichiometry of Photosystem I, Photosystem II, and Phycobilisomes in the Red Alga Porphyridium cruentum as a Function of Growth Irradiance

Cells of the red alga Porphyridium cruentum (ATCC 50161) exposed to increasing growth irradiance exhibited up to a three-fold reduction in photosystems I and II (PSI and PSII) and phycobilisomes but little change in the relative numbers of these components. Batch cultures of P. cruentum were grown under four photon flux densities of continuous white light; 6 (low light, LL), 35 (medium light, ML), 180 (high light, HL), and 280 (very high light, VHL) microeinsteins per square meter per second and sampled in the exponential phase of growth. Ratios of PSII to PSI ranged between 0.43 and 0.54. About three PSII centers per phycobilisome were found, regardless of growth irradiance. The phycoerythrin content of phycobilisomes decreased by about 25% for HL and VHL compared to LL and ML cultures. The unit sizes of PSI (chlorophyll/P₇₀₀) and PSII (chlorophyll/Q_A) decreased by about 20% with increase in photon flux density from 6 to 280 microeinsteins per square meter per second. A threefold reduction in cell content of chlorophyll at the higher photon flux densities was accompanied by a twofold reduction in β-carotene, and a drastic reduction in thylakoid membrane area. Cell content of zeaxanthin, the major carotenoid in P. cruentum, did not vary with growth irradiance, suggesting a role other than light-harvesting. HL cultures had a growth rate twice that of ML, eight times that of LL, and slightly greater than that of VHL cultures. Cell volume increased threefold from LL to VHL, but volume of the single chloroplast did not change. From this study it is evident that a relatively fixed stoichiometry of PSI, PSII, and phycobilisomes is maintained in the photosynthetic apparatus of this red alga over a wide range of growth irradiance.     

Comparison of the Antennal Sensilla Ultrastructure of Two Cryptic Species in Bemisia tabaci

Bemisia tabaci is an important agricultural pest with worldwide distribution and host preference. Therefore, understanding the biology of this pest is important to devise specific pest control strategies. The antennae of herbivorous insects play an important role in the identification of hosts using plant volatiles. To understand the features of antennae in B. tabaci MEAM 1(formerly known as biotype ‘B’) and MED (formerly known as biotype ‘Q’), the morphology and distribution of the antennal sensilla were examined using scanning electron micrographs. The results showed that the average antennae length in MEAM 1 was longer than MED. No differences were observed in the number and distribution of antennal sensilla in MEAM 1 and MED antennae; each antenna had nine different types of sensilla. Both cryptic species possessed Microtrichia, Grooved surface trichodea sensilla, Chaetae sensilla, Coeloconic sensillaⅠandⅡ, Basiconic sensilla Ⅰ, Ⅱ and Ⅲ and Finger-like sensilla. This is the first report of Grooved surface trichodea sensilla and Basiconic sensilla Ⅱ on B. tabaci flies. The numbers of Chaetae sensilla were different in the females and males of MEAM 1 and MED, which females having 5 and males containing 7. The surface structure of Basiconic sensilla Ⅰ was different with MEAM 1 showing a multiple-pitted linen surface and MED showing a multiple-pitted pocking surface. Basiconic sensillaⅡ were double in one socket with the longer one having a multiple-pitted surface and the shorter one with a smooth surface. Basiconic Ⅲ and Finger-like sensillae were longer in MEAM 1 antennae than in MED antennae. Our results are expected to further the studies that link morphological characteristics to insect behavior and help devise strategies to control insect pests.     

Influence of the Rate of Cell Growth and Cell Density on Interferon Action in Chick Embryo Cells

Chick embryo cells became more sensitive to the action of interferon the longer they remained in culture. This phenomenon was found even before confluency had been reached. The relative insensitivity of newly seeded cells was not due to a loss of receptors. Cells synthesizing deoxyribonucleic acid (DNA) at a high rate were less sensitive to interferon action than cells synthesizing DNA at a low rate, but the inhibition of DNA synthesis had no effect on interferon action. An increase in the number of cells used for seeding resulted in an earlier appearance of increased sensitivity to interferon action. These results are discussed in relation to the induction process in animal cells.     

Multiple Regulatory Systems Coordinate DNA Replication with Cell Growth in Bacillus subtilis

In many bacteria the rate of DNA replication is linked with cellular physiology to ensure that genome duplication is coordinated with growth. Nutrient-mediated growth rate control of DNA replication initiation has been appreciated for decades, however the mechanism(s) that connects these cell cycle activities has eluded understanding. In order to help address this fundamental question we have investigated regulation of DNA replication in the model organism Bacillus subtilis. Contrary to the prevailing view we find that changes in DnaA protein level are not sufficient to account for nutrient-mediated growth rate control of DNA replication initiation, although this regulation does require both DnaA and the endogenous replication origin. We go on to report connections between DNA replication and several essential cellular activities required for rapid bacterial growth, including respiration, central carbon metabolism, fatty acid synthesis, phospholipid synthesis, and protein synthesis. Unexpectedly, the results indicate that multiple regulatory systems are involved in coordinating DNA replication with cell physiology, with some of the regulatory systems targeting oriC while others act in a oriC-independent manner. We propose that distinct regulatory systems are utilized to control DNA replication in response to diverse physiological and chemical changes.     

Replicon Size, Mean Rate of DNA Replication and the Duration of the Cell Cycle and its Component Phases in Eight Monocotyledonous Species of Contrasting DNA C Values

Various aspects of the cell cycle were measured in the apicalmeristem of primary and seminal roots of eight monocotyledonousangiosperms: Oryza sativa (0.6 pg), Zea mays (2.4 pg), Pennisetumamericanum (2.5 pg), Aegilops umbellulata (5.1 pg), Hordeumvulgare (5.5 pg), Triticum monococcum (6.2 pg), Secale cereale(8.6 pg) and Tulipa kaufmanniana (22.6 pg), representing a 38-foldvariation in DNA C values. Using 4-d-old roots of the firstseven species and 21-d-old Tulipa roots, replicon size and ratesof replication were determined by DNA fibre autoradiography,and the duration of the cell cycle and its component phasesby the percentage labelled mitoses method. When tested withDNA C value, no significant relationships existed for repliconsize, rate of DNA replication or duration of G 1. Significantpositive linear relationships were found between DNA C valueand cell cycle duration, duration of mitosis and G2 durationwhen all data were tested, but not when the Tulipa data wereexcluded. The only characters significantly related to DNA C value whenthe Tulipa data were included or excluded were the durationof S-phase, and the ratio of the interval required for a repliconto replicate its allotted DNA (Rs) to the duration of S-phase(Ds). The Rs: Ds ratio is a measure of synchrony of repliconactivation, and the higher the DNA C value the lower this ratiobecame. We concluded that there was a nucleotypic effect ofDNA C value on this ratio and that the interval between activationof replicons became protracted and hence S-phase lengthenedas C value increased. Cell cycle, NA C value, DNA replication, replicon, S-phase     

Cell Growth and Size Homeostasis in Silico

Cell growth in size is a complex process coordinated by intrinsic and environmental signals. In a research work performed by a different group, size distributions of an exponentially growing population of mammalian cells were used to infer cell-growth rate in size. The results suggested that cell growth was neither linear nor exponential, but subject to size-dependent regulation. To explain the observed growth pattern, we built a mathematical model in which growth rate was regulated by the relative amount of mRNA and ribosomes in a cell. Under the growth model and a stochastic division rule, we simulated the evolution of a population of cells. Both the sampled growth rate and size distribution from this in silico population agreed well with experimental data. To explore the model space, alternative growth models and division rules were studied. This work may serve as a starting point to understand the mechanisms behind cell growth and size regulation using predictive models.     

Influence of temperature on Photosystem II electron transfer reactions

The influence of temperature on the rate of reduction of P-680⁺, the primary donor of Photosystem II, has been studied in the range 5–294 K, in chloroplasts and subchloroplasts particles. P-680 was oxidized by a short laser flash. Its oxidation state was followed by the absorption level at 820 nm, and its reduction attributed to two mechanisms: electron donation from electron donor D₁ and electron return from the primary plastoquinone (back-reaction).Between 294 and approx. 200 K, the rate of the back-reaction, on a logarithmic scale, is a linear function of the reciprocal of the absolute temperature, corresponding to an activation energy between 3.3 and 3.7 kcal · mol^?1, in all of the materials examined (chloroplasts treated at low pH or with Tris; particles prepared with digitonin). Between approx. 200 K and 5 K the rate of the back-reaction is temperature independent, with $\text{t}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{= 1.6}\text{ms}$. In untreated chloroplasts we measured a $\text{t}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ of 1.7 ms for the back-reaction at 77 and 5 K.The rate of electron donation from the donor D₁ has been measured in darkadapted Tris-treated chloroplasts, in the range 294–260 K. This rate is strongly affected by temperature. An activation energy of 11 kcal · mol^?1 was determined for this reaction.In subchloroplast particles prepared with Triton X-100 the signals due to P-680 were contaminated by absorption changes due to the triplet state of chlorophyll a. This triplet state has been examined with pure chlorophyll a in Triton X-100. An Arrhenius plot of its rate of decay shows a temperature-dependent region (292–220 K) with an activation energy of 9 kcal · mol^?1, and a temperature-independent region (below 200 K) with $\text{t}{}_{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}\text{= 1.1}\text{ms}$.     

Inactive Photosystem II Complexes in Leaves : Turnover Rate and Quantitation

The flash-induced electrochromic shift, measured by the amplitude of the rapid absorbance increase at 518 nanometers (ΔA518), was used to determine the amount of charge separation within photosystems II and I in spinach (Spinacia oleracea L.) leaves. The recovery time of the reaction centers was determined by comparing the amplitudes of ΔA518 induced by two flashes separated by a variable time interval. The recovery of the ΔA518 on the second flash revealed that 20% of the reaction centers exhibited a recovery half-time of 1.7 ± 0.3 seconds, which is 1000 times slower than normally active reaction centers. Measurements using isolated thylakoid membranes showed that photosystem I constituted 38% of the total number of reaction centers, and that the photosystem I reaction centers were nearly fully active, indicating that the slowly turning over reaction centers were due solely to photosystem II. The results demonstrate that in spinach leaves approximately 32% of the photosystem II complexes are effectively inactive, in that their contribution to energy conversion is negligible. Additional evidence for inactive photosystem II complexes in spinach leaves was provided by fluorescence induction measurements, used to monitor the oxidation kinetics of the primary quinone acceptor of photosystem II, Q_A, after a short flash. The measurements showed that in a fraction of the photosystem II complexes the oxidation of Q_A⁻ was slow, displaying a half-time of 1.5 ± 0.3 seconds. The kinetics of Q_A⁻ oxidation were virtually identical to the kinetics of the recovery of photosystem II determined from the electrochromic shift. The key difference between active and inactive photosystem II centers is that in the inactive centers the oxidation rate of Q_A⁻ is slow compared to active centers. Measurements of the electrochromic shift in detached leaves from several different species of plants revealed a significant fraction of slowly turning over reaction centers, raising the possibility that reaction centers that are inefficient in energy conversion may be a common feature in plants.     

The Photosystem II Light-Harvesting Protein Lhcb3 Affects the Macrostructure of Photosystem II and the Rate of State Transitions in Arabidopsis

The main trimeric light-harvesting complex of higher plants (LHCII) consists of three different Lhcb proteins (Lhcb1-3). We show that Arabidopsis thaliana T-DNA knockout plants lacking Lhcb3 (koLhcb3) compensate for the lack of Lhcb3 by producing increased amounts of Lhcb1 and Lhcb2. As in wild-type plants, LHCII-photosystem II (PSII) supercomplexes were present in Lhcb3 knockout plants (koLhcb3), and preservation of the LHCII trimers (M trimers) indicates that the Lhcb3 in M trimers has been replaced by Lhcb1 and/or Lhcb2. However, the rotational position of the M LHCII trimer was altered, suggesting that the Lhcb3 subunit affects the macrostructural arrangement of the LHCII antenna. The absence of Lhcb3 did not result in any significant alteration in PSII efficiency or qE type of nonphotochemical quenching, but the rate of transition from State 1 to State 2 was increased in koLhcb3, although the final extent of state transition was unchanged. The level of phosphorylation of LHCII was increased in the koLhcb3 plants compared with wild-type plants in both State 1 and State 2. The relative increase in phosphorylation upon transition from State 1 to State 2 was also significantly higher in koLhcb3. It is suggested that the main function of Lhcb3 is to modulate the rate of state transitions.     

Single Cell Analysis Linking Ribosomal (r)DNA and rRNA Copy Numbers to Cell Size and Growth Rate Provides Insights into Molecular Protistan Ecology

Ribosomal (r)RNA and rDNA have been golden molecular markers in microbial ecology. However, it remains poorly understood how ribotype copy number (CN)‐based characteristics are linked with diversity, abundance, and activity of protist populations and communities observed at organismal levels. Here, we applied a single‐cell approach to quantify ribotype CNs in two ciliate species reared at different temperatures. We found that in actively growing cells, the per‐cell rDNA and rRNA CNs scaled with cell volume (CV) to 0.44 and 0.58 powers, respectively. The modeled rDNA and rRNA concentrations thus appear to be much higher in smaller than in larger cells. The observed rRNA:rDNA ratio scaled with CV^0.14. The maximum growth rate could be well predicted by a combination of per‐cell ribotype CN and temperature. Our empirical data and modeling on single‐cell ribotype scaling are in agreement with both the metabolic theory of ecology and the growth rate hypothesis, providing a quantitative framework for linking cellular rDNA and rRNA CNs with body size, growth (activity), and biomass stoichiometry. This study also demonstrates that the expression rate of rRNA genes is constrained by cell size, and favors biomass rather than abundance‐based interpretation of quantitative ribotype data in population and community ecology of protists.     

Changes in Cell Size and DNA Content in Sulfolobus Cultures during Dilution and Temperature Shift Experiments

Stationary-phase cultures of different hyperthermophilic species of the archaeal genus Sulfolobus were diluted into fresh growth medium and analyzed by flow cytometry and phase-fluorescence microscopy. After dilution, cellular growth started rapidly but no nucleoid partition, cell division, or chromosome replication took place until the cells had been increasing in size for several hours. Initiation of chromosome replication required that the cells first go through partition and cell division, revealing a strong interdependence between these key cell cycle events. The time points at which nucleoid partition, division, and replication occurred after the dilution were used to estimate the relative lengths of the cell cycle periods. When exponentially growing cultures were diluted into fresh growth medium, there was an unexpected transient inhibition of growth and cell division, showing that the cultures did not maintain balanced growth. Furthermore, when cultures growing at 79 degrees C were shifted to room temperature or to ice-water baths, the cells were found to "freeze" in mid-growth. After a shift back to 79 degrees C, growth, replication, and division rapidly resumed and the mode and kinetics of the resumption differed depending upon the nature and length of the shifts. Dilution of stationary-phase cultures provides a simple protocol for the generation of partially synchronized populations that may be used to study cell cycle-specific events.