Light dependence of quantum yields of Photosystem II and CO2 fixation in C3 and C4 plants

The light dependence of quantum yields of Photosystem II (_II) and of CO₂ fixation were determined in C₃ and C₄ plants under atmospheric conditions where photorespiration was minimal. Calculations were made of the apparent quantum yield for CO₂ fixation by dividing the measured rate of photosynthesis by the absorbed light [A/I=_CO2 and of the true quantum yield by dividing the estimated true rate of photosynthesis by absorbed light [(A+R_l)/I_a=_CO2·], where R_L is the rate of respiration in the light. The dependence of the _II/_CO2 and _II/_CO2 ^* ratios on light intensity was then evaluated. In both C₃ and C₄ plants there was little change in the ratio of _II/_CO2 at light intensities equivalent to 10–100% of full sunlight, whereas there was a dramatic increase in the ratio at lower light intensities. Changes in the ratio of _II/_CO2 can occur because respiratory losses are not accounted for, due to changes in the partitioning of energy between photosystems or changes in the relationship between PS II activity and CO₂ fixation. The apparent decrease in efficiency of utilization of energy derived from PS II for CO₂ fixation under low light intensity may be due to respiratory loss of CO₂. Using dark respiration as an estimate of R_L, the calculated _II/_CO2 ^* ratio was nearly constant from full sunlight down to approx 5% of full sunlight, which suggests a strong linkage between the true rate of CO₂ fixation and PS II activity under varying light intensity. Measurements of photosynthesis rates and _II were made by illuminating upper versus lower leaf surfaces of representative C₃ and C₄ monocots and dicots. With the monocots, the rate of photosynthesis and the ratio of _II/_CO2 exhibited a very similar patterns with leaves illuminated from the adaxial versus the abaxial surface, which may be due to uniformity in anatomy and lack of differences in light acclimation between the two surfaces. With dicots, the abaxial surface had both lower rates of photosynthesis and lower _II values than the adaxial surface which may be due to differences in anatomy (spongy versus palisade mesophyll cells) and/or light acclimation between the two surfaces. However, in each species the response of _II/_CO2 to varying light intensity was similar between the two surfaces, indicating a comparable linkage between PS II activity and CO₂ fixation.Abbreviations A measured rate of CO₂ assimilation - A+R_L true rate of CO₂ assimilation; e - CO₂ estimate of electrons transported through PSII per CO₂ fixed by RuBP carboxylase - f fraction of light absorbed by Photosystem II - F'_m yield of PSII chlorophyll fluorescence due to a saturating flash of white light under steady-state photosynthesis - F_s variable yield of fluorescence under steady-state photosynthesis; PPFD-photosynthetic photon flux density - I_a absorbed PPFD - PS II Photosystem II - R_d rate of respiration in the dark - R_I rate of respiration in the light estimated from measurement of R_d or from analysis of quantum yields - apparent quantum yield of CO₂ assimilation under a given condition (A/absorbed PPFD) - true quantum yield of CO₂ assimilation under a given condition [(A+R_L)/(absorbed PPFD)] - quantum yield for photosynthetic O₂ evolution - electrons transported via PS II per quantum absorbed by PS II Supported by USDA Competitive Grant 90-37280-5706.     

Mechanisms for controlling balance between light input and utilisation in the salt tolerant alga Dunaliella C9AA

The yield of photosynthetic O₂ evolution was measured in cultures of Dunaliella C9AA over a range of light intensities, and a range of low temperatures at constant light intensity. Changes in the rate of charge separation at Photosystem I (PS I) and Photosystem II (PS II) were estimated by the parameters _{PS I} and _{PS II} . _{PS I} is calculated on the basis of the proportion of centres in the correct redox state for charge separation to occur, as measured spectrophotometrically. _{PS II} is calculated using chlorophyll fluorescence to estimate the proportion of centres in the correct redox state, and also to estimate limitations in excitation delivery to reaction centres. With both increasing light intensity and decreasing temperature it was found that O₂ evolution decreased more than predicted by either _{PS I} or _{PS II}. The results are interpreted as evidence of non-assimilatory electron flow; either linear whole chain, or cyclic around each photosystem.Abbreviations F₀ dark level of chlorophyll fluorescence yield (PS II centres open) - F_m maximum level of chlorophyll fluorescence yield (PS II centres closed) - F_v variable fluorescence (F_m-F₀) - PS I Photosystem I - PS II Photosystem II - P₇₀₀ reaction centre chlorophyll(s) of PS I - q_N coefficient of non-photochemical quenching of chlorophyll fluorescence - q_P coefficient of photochemical quenching of fluorescence yield - q_E high-energy-state quenching coefficient - _{PS I} yield of PS I - _{PS II} yield of PS II - _S yield of photosynthetic O₂ evolution - _P intrinsic yield of open PS II centres     

Potential mechanisms of low-temperature tolerance of C<Subscript>4</Subscript> photosynthesis in<Emphasis Type="Italic"> Miscanthus</Emphasis> ×<Emphasis Type="Italic"> giganteus</Emphasis>: an in vivo analysis

Miscanthus × giganteus (Greef & Deuter ex Hodkinson & Renvoize) is unique among C₄ species in its remarkable ability to maintain high photosynthetic productivity at low temperature, by contrast to the related C₄ NADP-malic enzyme-type species Zea mays L. In order to determine the in vivo physiological basis of this difference in photosynthesis, water vapor and CO₂ exchange and modulated chlorophyll fluorescence were simultaneously monitored on attached leaf segments from plants grown and measured at 25/20°C or 14/11°C (day/night temperature). Analysis of the response of photosynthesis to internal CO₂ concentration suggested that ribulose bisphosphate carboxylase/oxygenase (Rubisco) and/or pyruvate orthophosphate dikinase (PPDK) play a more important role in determining the response to low temperature than does phosphoenolpyruvate carboxylase (PEPc). For both species at both temperatures, the linear relationship between operating efficiency of whole-chain electron transport through photosystem II (_PSII) and the efficiency of CO₂ assimilation (_CO2) was unchanged and had a zero intercept, suggesting the absence of non-photosynthetic electron sinks. The major limitation at low temperature could not be solely at Rubisco or at any other point in the Calvin cycle, since this would have increased leakage of CO₂ to the mesophyll and increased _PSII/_CO2. This in vivo analysis suggested that maintenance of high photosynthetic rates in M. × giganteus at low temperature, in contrast to Z. mays, is most likely the result of different properties of Rubisco and/or PPDK, reduced susceptibility to photoinhibition, and the ability to maintain high levels of leaf absorptance during growth at low temperature.     

The occurrence of photoinhibition in an over-wintering crop of oil-seed rape (Brassica napus L.) and its correlation with changes in crop growth.

The maximum quantum yield of CO₂ uptake (), as a measure of light-limited photosynthetic efficiency, of a Brassica napus crop was measured on most days from mid-October until mid-April. During the winter, was decreased by up to 50%. From January to March, leaves exposed to direct sunlight on days with minimum air temperatures near or below 0° C showed significant reductions in . However, control leaves, artificially shaded from direct sunlight on these days, did not show any decrease. This provides statistical evidence for a light-dependent inhibition of CO₂ uptake in the field, termed here photoinhibition. Recovery of during warmer interludes was slow, requiring approx. 2–3 d. Concurrent measurements of light interception by the crop canopy and dry-matter accumulation showed that the efficiency with which intercepted light was converted into dry matter varied, declin between January and March to 33% of the value recorded in the warmer autumn months. Conversion efficiency was significantly and positively correlated with quantum yield. In a closed crop canopy during winter, light will be limiting for photosynthesis for much of the time. Under these conditions depression of at the leaf level may contribute significantly to decreased dry-matter accumulation at the crop level, since the light-limited rate of CO₂ uptake is likely to govern canopy photosynthetic rate.Abbreviations and Symbols C mean crop growth rate - E_c crop conversion efficiency - F_m, F_v maximum, variable chlorophyll fluorescence - L instantaneous leaf area index - PPFD photosynthetically active photon flux density - quantum yield of CO₂ uptake for absorbed light P.K.F. was in receipt of a research studentship from the Science and Engineering Research Council.     

The regulation of component processes of photosynthesis in transgenic tobacco with decreased phosphoribulokinase activity

Tobacco plants (Nicotiana tabacum L.) transformed with an inverted cDNA encoding ribulose 5-phosphate kinase (phosphoribulokinase,PRK; EC 2.7.1.19) were employed to study the in vivo relationship between photosynthetic electron transport and the partitioning of electron transport products to major carbon metabolism sinks under conditions of elevated ATP concentrations and limited ribulose 1,5-bisphosphate (RuBP) regeneration. Simultaneous measurements of room temperature chlorophyll fluorescence and CO₂ gas exchange were conducted on intact leaves. Under ambient CO₂ concentrations and light intensities above those at which the plants were grown, transformants with only 5% of PRK activity showed down-regulation of PS II activity and electron transport in response to a decrease in net carbon assimilation when compared to wild-type. This was manifested as a decline in the efficiency of PS II electron transport (_{PS II}), an increase in dissipation of excess absorbed light in the antennae of PS II and a decline in: total linear electron transport (J₁), electron transport dedicated to carbon assimilation (J_A) and electron transport allocated to photorespiration (J_L). The transformants showed no alteration in the Rubisco specificity factor measured in vitro and calculated in vivo but had a relatively smaller ratio of RuBP oxygenation to carboxylation rates (v_o/v_c), due to a higher CO₂ concentration at the carboxylation site (C_c). The relationship between _{PS II} and _{CO 2}was similar in transformants and wild-type under photorespiratory conditions demonstrating no change in the intrinsic relationship between PS II function and carbon assimilation, however, a novel result of this study is that this similar relationship occurred at different values of quantum flux, J₁, J_A, J_L and v_o/v_c in the transformant. For both wild-type and transformants, an assessment was made of the possible presence of a third major sink for electron transport products, beside RuBP oxygenation and carboxylation, the data provided no evidence for such a sink.Abbreviations C_c CO₂ concentration at the site of carboxylation - C_i intercellular CO₂ concentration - g_m mesophyll conductance to CO₂ - J₁ total linear electron flow - J_A linear electron flow allocated to CO₂ assimilation - J_c linear electron flow supporting carbon reduction and oxidation cycles - J_L linear electron flow allocated to photorespiration (RuBP oxygenation and fixation of released photorespiratory CO₂) - PRK phosphoribulokinase - q_P, q_N coefficients for photochemical and non-photochemical quenching of fluorescence respectively - Rubisco ribulose 1,5-bisphosphate carboxylase-oxygenase - S Rubisco specificity to CO₂/O₂ - v_c, v_o rates of RuBP carboxylation and RuBP oxygenation, respectively - _{CO 2} relative quantum yield of CO₂ assimilation - _C maximum _{CO 2} under non-photorespiratory conditions - _exc the efficiency of excitation capture by open PS II centres - _{PS II} relative quantum yield of PS II electron transport     

“Differentiation Induction” culture of human leukemic myeloid cells stimulates high production of macrophage differentiation inducing factor

A suitable procedure for the production of human monokines was defined as differentiation-induction culture. Human monocytic leukemia THP-1 cells were well-differentiated from nonfunctional promonocytes into macrophage-like cells by the induction with a combination of mezerein, retinoic acid, and aMycoplasma fermentans extract. The differentiated THP-1 cells secreted a high amount of macrophage differentiation-inducing factor (DIF) activity and concomitantly produced other known monokines, such as tumor necrosis factor- (TNF-), interleukin-1 (IL-1) and interleukin-1 (IL-1), into the medium. These results suggest that other novel human monokines may also be found in the conditioned medium of THP-1 cells induced by the differentiation-induction culture conditions defined in this study. Macrophage DIF was purified to homogeneity and NH₂-terminal amino acid sequence analysis revealed that macrophage DIF is very similar or identical to human leukemia inhibitory factor (LIF). The cDNA encoding human LIF was isolated using the polymerase chain reaction, and a clone producing 3.7 g/10⁶ cells day recombinant LIF was selected from Chinese hamster ovary (CHO) cells which were transfected with the LIF cDNA. The recombinant LIF production in CHO cells was quantified using MTT reduction assay with M1 cells.     

The quantum efficiency of photosystem II and its relation to non-photochemical quenching of chlorophyll fluorescence; the effect of measuring-and growth temperature

The relation between the quantum yield of oxygen evolution of open photosystem II reactions centers (_p), calculated according to Weis and Berry (1987), and non-photochemical quenching of chlorophyll fluorescence of plants grown at 19°C and 7°C was measured at 19°C and 7°C. The relation was linear when measured at 19°C, but when measured at 7°C a deviation from linearity was observed at high values of non-photochemical quenching. In plants grown at 7°C this deviation occurred at higher values of non-photochemical quenching than in plants grown at 19°C. The deviations at high light intensity and low temperature are ascribed to an increase in an inhibition-related, non-photochemical quenching component (q_I).The relation between the quantum yield of excitation capture of open photosystem II reaction centers (_exe), calculated according to Genty et al. (1989), and non-photochemical quenching of chlorophyll fluorescence was found to be non-linear and was neither influenced by growth temperature nor by measuring temperature.At high PFD the efficiency of overall steady state electron transport measured by oxygen-evolution, correlated well with the product of q _N and the efficiency of excitation capture (_exe) but it deviated at low PFD. The deviations at low light intensity are attributed to the different populations of chloroplasts measured by gas exchange and chlorophyll fluorescence and to the light gradient within the leaf.Abbreviations F₀ basic fluorescence - F₀ basic fluorescence, thylakoid in energized state - F_m maximal fluorescence - F_m maximum fluorescence in energized state - F_s steady state fluorescence - F_v maximal variable fluorescence - PFD photon flux density - PS II_rc Photosystem II reaction center - q_F0 quenching of basic fluorescence - q_E energy related quenching - q_N non-photochemical quenching:-q_f-total quenching - q_I inhibition-related quenching - q_p photochemical quenching - q_r quenching due to state transition - R_d dark respiration - _p PS II efficiency of excitation capture of open PS II_rc - _pe extrapolated minimal value of _p - _p0 extrapolated maximal value of _p - _si quantum efficiency of linear electron transport, calculated from gas exchange measurements based on incident light - _sf quantum efficiency of linear electron transport, calculated from fluorescence measurements, based on incident measuring light     

A Salt-Tolerant Cultivar of Wheat Maintains Photosynthetic Activity by Suppressing Sodium Uptake

The effects of NaCl treatment on the photosynthetic machinery in wheat (Triticum aestivum L.) cultivars differing in salt tolerance were investigated by comparison with iso-osmotic PEG treatment. Both cultivars similarly reduced the photosystem 2 (PS2) energy conversion efficiency (_PS2) rapidly when plants were exposed to a 100 mM NaCl solution, though no decline was detected under the iso-osmotic PEG treatment. There was no correlation between the reduction of the leaf relative water content (RWC) and the _PS2 in the two iso-osmotic stress treatments. In contrast, a decline of _PS2 along with the increase of the leaf sodium content above 4 % dry matter was detected under the NaCl treatment, while no such correlation was detected with other cations. The recovery of _PS2 after photoinhibitory irradiation was repressed by the NaCl treatment as the increase of the duration of the treatment. Norin 61 subjected to the 100 mM NaCl treatment for 10 d showed a decline of the _PS2 after 1 h moderate irradiation of 400 mol m^–2 s^–1 PPFD. Thus the concentrated Na⁺ within a leaf under salinity treatments may decrease the stability of PS2 functions and lead to photochemical inactivation.     

Virulence as a consequence of genome instability of a novel temperate bacteriophage, ΦRsG1, of Rhodobacter sphaeroides Y

A novel temperate bacteriophage, designated RsG1, was isolated from Rhodobacter sphaeroides Y (previously designated Rhodopseudomonas sphaeroides) following exposure to mitomycin C. The phage morphology, as revealed from electron microscopy, showed a hexagonal head (90 by 46.5 nm) connected with a tail (116 by 9.4 nm), to which a collar was proximally attached. A morphologically similar phage was also produced by spontaneous lysis of the cells. While RsG1 did not grow on any other bacterial strain tested, spontaneously produced phage particles propagated (and formed plaques) on R. sphaeroides Y still carrying RsG1 in the prophage state. The genome of RsG1 consisted of double stranded linear DNA with cohesive ends and a GC-content of 71.8 mol%. The DNA molecules formed circles in vitro with a mean contour length of 17.18±0.4 m, which corresponds to a size of 49 kbase pairs (kb). On the other hand, DNA extracted from the virulent phage particles was heterogeneous and consisted of two DNA species of different size, occurring in a ratio of about 1:1. These molecules also circularized having contour lengths of 17.18±0.4 m and 14.02±0.41 m corresponding to 49 and 40 kb, respectively. Restriction digest analysis of the two DNA species and DNA from RsG1 indicated that they are similar, and allowed the indentification of an 11.5 kb EcoRI fragment that carries the cohesive ends. Because DNA from RsG1 and the 49 kb DNA of the virulent phage particles were indistinguishable with the criteria applied, it is suggested that phage particles containing the 40 kb DNA represent the virulent type of phage, termed RsG1.1.     

Different Relationship Between Electron Transport and CO2 Assimilation in two Zea mays Cultivars as Influenced by Increasing Irradiance

Gas exchange and fluorescence parameters were measured simultaneously in two Zea mays L. cultivars (Liri and 121C D8) to assess the relationship between the quantum yield of electron transport (_PS2) and the quantum yield of CO₂ assimilation (_CO2) in response to photosynthetic photon flux density (PPFD). The cv. Liri was grown under controlled environmental conditions in a climate chamber (CC) while cv. 121C D8 was grown in CC as well as outdoors (OT). By exposing the two maize cultivars grown in CC to an increasing PPFD, higher photosynthetic and photochemical rates were evidenced in cv. Liri than in cv. 121C D8. In Liri plants the _PS2/_CO2 ratio increased progressively up to 27 with increasing PPFD. This suggests that the reductive power was more utilised in non-assimilatory processes than in CO₂ assimilation at high PPFD. On the contrary, by exposing 121C D8 plants to increasing PPFD, _PS2/_CO2 was fairly constant (around 11–13), indicating that the electron transport rate was tightly down regulated by CO₂ assimilation. Although no significant differences were found between _PS2/_CO2 of the 121C D8 maize grown under CC and OT by exposing them to high PPFD, the photosynthetic rate and photochemical rates were higher in OT maize plants.     

Two useful dimensionless parameters that combine physiological, operational and bioreactor design parameters for improved control of dissolved oxygen

Two new dimensionless parameters ( and ) are proposed for calculating the proportional, integral, and derivative constants of a dissolved oxygen proportional integral-derivative (PID) feed-back control algorithm from knowledge of the growth rate, bioreactor design and operation variables. The values of and were determined for a broad range of Reynolds numbers (between 1000 to 40 000) during the exponential growth phase of two highly different processes: fermentations of recombinant Escherichia coli and cultures of human hematopoietic cells. The utility of and for use in dissolved oxygen self-tunning adaptive control algorithms is discussed.     

The Instron technique as a measure of immediate-past wall extensibility

The relationship between the plastic-extensibility values (PEx) obtained in the Instron technique and the growth parameter, wall extensibility () has been evaluated for Avena sativa L. coleoptile cell walls. The possibility that PEx is proportional to the growth rate rather than to has been eliminated by showing that turgor-driven changes in the growth rate do not cause comparable changes in PEx. For Avena coleoptiles, PEx appears to be a measure of the average over the previous 60–90 min rather than a measure of the instantaneous of the growth equation. This is indicated by the fact that while PEx and the growth rate start to change simultaneously after addition of indole-3-acetic acid or KCN, the growth rate reaches a new, constant value 60–90 min before a new plateau value of PEx is obtained. Similar results are obrained with soybean (Glycine max L.) hypocotyl walls, indicating that the relationship between PEx and the parameter is a general one, although the period over which is averaged differs from tissue to tissue. In addition, it is shown that PEx can be measured more than once on the same section; a new potential for plastic extension is regenerated whenever the force vectors are changed even slightly. It is concluded that PEx is a measure of those domains in the wall where a wall-loosening event has occurred which has not been eliminated by further wall synthesis or other biochemical events.Abbreviations and symbols DP Instron plastic compliance - IAA indole-3-acetic acid - PEx Instron plastic extensibility - instantaneous wall extensibility     

Factors Limiting Photosynthetic Recovery in Sweet Pepper Leaves After Short-Term Chilling Stress Under Low Irradiance

The effects of chilling treatment (4 °C) under low irradiance, LI (100 mol m^–2 s^–1) and in the dark on subsequent recovery of photosynthesis in chilling-sensitive sweet pepper leaves were investigated by comparing the ratio of quantum yields of photosystem (PS) 2 and CO₂ assimilation, _PS2/_CO2, measured in normal air (21 % O₂, NA) and low O₂-air (2% O₂, LOA), and by analyzing chlorophyll (Chl) a fluorescence parameters. Chilling treatment in the dark had little effect on F_v/F_m and _PS2/_CO2, but it caused the decrease of net photosynthetic rate (P _N) under saturating irradiance after 6-h chilling treatment, indicating that short-term chilling alone did not induce PS2 photoinhibition. Furthermore, photorespiration and Mehler reaction also did not obviously change during subsequent recovery after chilling stress in the dark. During chilling treatment under LI, there were obvious changes in F_v/F_m and _PS2/_CO2, determined in NA or LOA. F_v/F_m could recover fully in 4 h at 25 °C, and _PS2/_CO2 increased at the end of the treatment, as determined in both NA and LOA. During subsequent recovery, _PS2/_CO2 in LOA decreased faster than in NA. Thus the Mehler reaction might play an important role during chilling treatment under LI, and photorespiration was an important process during the subsequent recovery. The recovery of PN under saturating irradiance determined in NA and LOA took about 50 h, implying that there were some factors besides CO₂ assimilation limiting the recovery of photosynthesis. From the progress of reduced P700 and the increase of the Mehler reaction during chilling under LI we propose that active oxygen species were the factors inducing PS1 photoinhibition, which prevented the recovery of photosynthesis in optimal conditions because of the slow recovery of the oxidizable P700.     

Effects of <Emphasis Type="Italic">in vitro</Emphasis>rooting environments and irradiance on growth and photosynthesis of strawberry plantlets during acclimatization

Strawberry (Fragaria ananassaDuch. cv. Fengxiang) plantlets were cultured under two in vitroenvironments for rooting, and then acclimatized under two ex vitroirradiance conditions. At the end of rooting stage plant height, fresh weight and specific leaf area of T1-plants grown under high sucrose concentration (3 sucrose), low photosynthetic photon flux density (30 mol m^–2 s^–1) and normal CO₂ concentration (350–400 l l^–1) were significantly higher than those of T2-plantlets grown under low sucrose concentration (0.5), high photosynthetic photon flux density (90 mol m^–2 s^–1) and elevated CO₂ concentration (700–800 l l^–1). But T2-plantlets had higher net photosynthetic rate (Pn), effective photochemical quantum yield of PSII (_PSII), effective photosynthetic electron transport rate (ETR), photochemical quenching (q_P) and ratio of chlorophyll fluorescence yield decrease (Rfd). After transfer, higher irradiance obviously promoted the growth of plantlets and was beneficial for the development of photosynthetic functions during acclimatization. T2-plantlets had higher fresh weight, leaf area, _PSII and ETR under higher ex vitroirradiance condition.     

Physiological relationship between the temperate phages lambda and phi80

Summary This work deals with the ability of phage 80 to provide defective mutants of with their missing functions. Functions Involved in Recombination. As shown by others, the Int mechanism of 80 cannot excise prophage . However, 80 efficiently excises recombinants from tandem dilysogens, using its Ter mechanism. Likewise, the nonspecific mechanism Red is interchangeable between 80 and . Maturation of DNA by 80. The Ter recombinants excised by 80 from tandem dilysogens are packaged into a 80 protein coat. This contrasts with the fact, already mentionned by Dove, that 80 is extremely inefficient for packaging phage superinfecting a -lysogen. The latter result is also found when the helper phage is a hybrid with the left arm of (80hy4 or 80hy41 — see Fig. 1). However, the maturation of the superinfecting is much more efficient if the 80hy used as a helper has the att-N region of (like 80hy1). Conversely a with the att-N region of 80 (hy6 — see Fig. 1) is packaged more efficiently by 80 or 80hy4 than by 80hy1. It is suggested that the maturation of chromosome superinfecting an immune cell requires a recombination with the helper phage. Vegetative Functions. Among the replicative functoons O and P, the latter only can be supplied by 80. That N mutants are efficiently helped by 80 does not tell that 80 provides the defective with an active N product; the chromosomes are simply packaged into a 80 coat. This shows that 80 is unable to switch on the late genes of . That neither 80 nor any of the 80hy tested can provide an active N product is shown in a more direct way by their complete failure to help N ^-r₁₄; this phage carries a polar mutation which makes the expression of genes O and P entirely N-dependant. The maturation of a N ^- by 80 contrasts with the fact that mutants affected in late genes (A, F or H) are not efficiently helped by 80. This suggests that the products coded by these genes are not interchangeable between 80 and , and that packaging of DNA into 80 coats is possible but inhibited when late proteins are present in the cell. Activation of the Late Genes. Among the im ₈₀ h ⁺ hybrids tested, only 80hy41 is able to switch on the late genes of a N defective mutant. This hybrid differs from the other hybrids studied here, by the fact that it has the Q-S-R region of (see Fig. 1). The results are consistant with the view that the product of Q gene is sufficient for activating the late genes of a DNA. N would thus control the expression of late genes only indirectly by controlling the expression of gene Q (Couturier & Dambly have independantly reached the same conclusion, 1970). Furthermore the failure of 80 and of the 80hy1 and 80hy4 to activate the late genes of would imply that these phages are unable to provide an Q product active on the chromosome Reciprocally, switches on the late genes of prophage 80hy41, but not of prophages 80hy1 and 80hy4. This suggests that the initiation of late genes expression takes place at a main specific site located in the Q-S-R region of the chromosome. The expression of the late genes would thus be sequential, and proceed through the left arm only when steaky ends cohere. Similar conclusions were reached independantly by Toussaint (1969) and by Herskowitz and Signer (1970).

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Ce travail a été réalisé dans le cadre du contrat d'association Euratom-U. L. B. 007-61-10 ABIB et avec l'aide du Fonds de la Recherche Fondamentale Collective.     

Identification of Photosynthetic Mutants of Arabidopsis by Automatic Screening for Altered Effective Quantum Yield of Photosystem 2

Quantification of chlorophyll (Chl) fluorescence is a versatile tool for analysing the photosynthetic performance of plants in a non-intrusive manner. A pulse-amplitude modulated fluorometer was combined with a CNC router for the automated measurement of the effective quantum yield of photosystem 2 (₂) of Arabidopsis thaliana plants. About 90 000 individual plants representing 7 500 lines derived from En-transposon and T-DNA mutagenised Arabidopsis populations were screened for mutants with altered ₂. Forty-eight recessive ₂ mutations were identified of which most exhibit also altered pigmentation and increased photosensitivity. For three ₂ mutants the corresponding mutated genes were identified that code all for chloroplast-located proteins. Comparison of the ₂ mutant screen with other screening methods based on the measurement of Chl fluorescence shows that the ₂ mutants identified are different to mutants identified by high Chl fluorescence. Some ₂ mutants, on the contrary, are common to mutants identified by screens based on non-photochemical quenching.     

Three-dimensional structures of photosynthetic reaction centers

In this article, the three-dimensional structures of photosynthetic reaction centers (RCs) are presented mainly on the basis of the X-ray crystal structures of the RCs from the purple bacteria Rhodopseudomonas (Rp.) viridis and Rhodobacter (Rb.) sphaeroides. In contrast to earlier comparisons and on the basis of the best-defined Rb. sphaeroides structure, a number of the reported differences between the structures cannot be confirmed. However, there are small conformational differences which might provide a basis for the explanation of observed spectral and functional discrepancies between the two species.A particular focus in this review is on the binding site of the secondary quinone (Q_B), where electron transfer is coupled to the uptake of protons from the cytoplasm. For the discussion of the Q_B site, a number of newlydetermined coordinate sets of Rp. viridis RCs modified at the Q_B site have been included. In addition, chains of ordered water molecules are found leading from the cytoplasm to the Q_B site in the best-defined structures of both Rp. viridis and Rb. sphaeroides RCs.Abbreviations B_A accessory bacteriochlorophyll in the active branch - B_B accessory bacteriochlorophyll in the inactive branch - D primary electron donor (special pair) - D_L special pair bacteriochorophyll bound by the L subunit - D_M special pair bacteriochorophyll bound by the M subunit - Q_A primary electron acceptor quinone - Q_B secondary electron acceptor quinone - RC reaction center - Rb. Rhodobacter - Rp. Rhodopseudomonas - _A bacteriopheophytin in the active branch - _B bacteriopheophytin in the inactive branch     

Quantifying Protein Folding Transition States with ΦT

A number of reaction coordinates have been proposed for reduced-dimensionalityrepresentations of a protein's folding free energy surface. We discuss in detail the entropic reaction coordinate _T = SS, recently introduced to quantify the conservation of mutations and the location of the folding transition state based on experimental temperature-tuning data. Numerical simulations illustrate the advantages as well as the limitations of _T. _T can be determined from experiment,computation, and analytical theory; _T can also be used to investigate structurally localized perturbations of the free energy surface. However, _T is only a relative reaction cordinate; furthermore, proteins undergo cold denaturation at sufficiently low temperatures, and care must be taken ininterpreting _T near the region where G/T = 0, particularly if the heat capacity change upon folding is small.