Effect of lanthanides on oxidation of Mn2+ cations via a high-affinity Mn-binding site in photosystem II membranes.

Lanthanide cations (La3+ and Tb3+) bind to the Ca-binding site of the oxygen-evolving complex in Ca-depleted PSII membranes and irreversibly inhibit the oxygen evolution. Оn the other hand, EPR measurement of Mn2+ concentration in buffer revealed that lanthanide cations inhibit the light-dependent oxidation of Mn2+ cations via the high-affinity Mn-binding site in Mn-depleted PSII membranes, which suggests that they bind to and inhibit the high-affinity Mn-binding site of the oxygen-evolving complex. The inhibition is irreversible, bound Ln3+ cation could not be washed out from the sample. Calcium ion inhibits oxidation of Mn2+ (5 µM) at very high concentration (tens mM) and the inhibition is reversible. In this work we measured the reduction rate of exogenic electron acceptor 2,6-dichlorophenolindophenol during the oxidation of Mn2+ cations in the Ca-depleted PSII and in the Ca-depleted PSII treated with lanthanides after extraction of Mn cluster from these preparations. We found that irreversible binding of the lanthanide cation to the Ca-binding site in the Ca-depleted PSII membranes leads to a partial inhibition of the high-affinity Mn-binding site.

Competitive interaction of Mn(II) and Fe(II) cations with the high-affinity Mn-binding site of the photosystem II: evolutionary aspect.

The evolutionary origin of the oxygen-evolving complex (OEC) in the photosystem II (PSII) is still unclear, as is the nature of electron source for the photosystem before the OEC had appeared. Johnson et al. (in PNAS 110:11238, 2013) speculated that Mn(II) cations were the source of electrons for transitional photosystems. However, Archean oceans also contained Fe(II) cations at concentrations comparable or higher than that of Mn(II). Fe(II) cations can bind to the high-affinity (НА) Mn-binding site in the OEC (Semin et al. in Biochemistry 41:5854, 2002). Now we have investigated the competitive interaction of Mn(II) and Fe(II) cations with the HA site in the Mn-depleted PSII membranes (PSII[-Mn]). Fe cations, oxidized under illumination, bind strongly to the HA site and, thus, prevent the interaction of Mn(II) with this site. If the Mn(II) and Fe(II) cations, at relatively equal concentration, are simultaneously present in the buffer, together with PSII(-Mn) membranes, there is competition between these two cations for the binding site, which manifests itself in partial inhibition of the Mn(II) oxidation and the blocking of the HA site by Fe(II) cations. If the concentration of Fe(II) cations is several times higher than the concentration of Mn(II), the HA site is completely blocked and the oxidation of Mn(II) cations is inhibited; under saturating light, the effectiveness of this inhibitory effect increases. This may be due to the generation of H2O2 on the acceptor side of the photosystem, which significantly accelerates the rate of the turnover reaction of Mn(II) on the HA site.

Ca2+ effects on Fe(II) interactions with Mn-binding sites in Mn-depleted oxygen-evolving complexes of photosystem II and on Fe replacement of Mn in Mn-containing, Ca-depleted complexes.

Fe(II) cations bind with high efficiency and specificity at the high-affinity (HA), Mn-binding site (termed the "blocking effect" since Fe blocks further electron donation to the site) of the oxygen-evolving complex (OEC) in Mn-depleted, photosystem II (PSII) membrane fragments (Semin et al. in Biochemistry 41:5854, 2002). Furthermore, Fe(II) cations can substitute for 1 or 2Mn cations (pH dependent) in Ca-depleted PSII membranes (Semin et al. in Journal of Bioenergetics and Biomembranes 48:227, 2016; Semin et al. in Journal of Photochemistry and Photobiology B 178:192, 2018). In the current study, we examined the effect of Ca2+ cations on the interaction of Fe(II) ions with Mn-depleted [PSII(-Mn)] and Ca-depleted [PSII(-Ca)] photosystem II membranes. We found that Ca2+ cations (about 50 mM) inhibit the light-dependent oxidation of Fe(II) (5 µM) by about 25% in PSII(-Mn) membranes, whereas inhibition of the blocking process is greater at about 40%. Blocking of the HA site by Fe cations also decreases the rate of charge recombination between QA- and YZ•+ from t1/2 = 30 ms to 46 ms. However, Ca2+ does not affect the rate during the blocking process. An Fe(II) cation (20 µM) replaces 1Mn cation in the Mn4CaO5 catalytic cluster of PSII(-Ca) membranes at pH 5.7 but 2 Mn cations at pH 6.5. In the presence of Ca2+ (10 mM) during the substitution process, Fe(II) is not able to extract Mn at pH 5.7 and extracts only 1Mn at pH 6.5 (instead of two without Ca2+). Measurements of fluorescence induction kinetics support these observations. Inhibition of Mn substitution with Fe(II) cations in the OEC only occurs with Ca2+ and Sr2+ cations, which are also able to restore oxygen evolution in PSII(-Ca) samples. Nonactive cations like La3+, Ni2+, Cd2+, and Mg2+ have no influence on the replacement of Mn with Fe. These results show that the location and/or ligand composition of one Mn cation in the Mn4CaO5 cluster is strongly affected by calcium depletion or rebinding and that bound calcium affects the redox potential of the extractable Mn4 cation in the OEC, making it resistant to reduction.

pH dependence of photosystem II photoinhibition: relationship with structural transition of oxygen-evolving complex at the pH of thylakoid lumen.

Ca-depleted photosystem II membranes (PSII[-Ca]) do not contain PsbP and PsbQ proteins protecting the Mn4CaO5 cluster of the PSII oxygen-evolving complex (OEC). Therefore, the Mn ions in the PSII(-Ca) membranes can be reduced by exogenous bulky reductants or the charged reductant Fe(II). We have recently found that the resistance of Mn ions in the OEC to the Fe(II) action is pH dependent and that this reductant is less effective at pH 5.7 than at pH 6.5 (Semin et al. J Photochem Photobiol B 178:192, 2018). Taking these data into account, we investigated the photoinhibition in different PSII membranes at pH 5.7 and 6.5 and found that the resistance to photoinhibition of PSII and PSII(-Ca) membranes with a Mn cluster is higher at pH 5.7 than at pH 6.5, whereas the resistance of the Mn-depleted PSII membranes is pH independent. In thylakoids, light generates the transmembrane ΔpH, leading to the acidulation of lumen that results in pH 5.7. The uncouplers (NH4Cl or nigericin) that significantly prevent acidulation increase the rate of PSII photoinhibition in thylakoids. We suggest that the structural transition in the OEC at pH 5.7 plays a role of a built-in mechanism increasing the resistance of OEC to photoinhibition under illumination, since it is accompanied by a pH decrease in lumen to 5.7. The coincidence of these pH values, i.e. lumen pH under illumination and pH of the maximal resistance of the Mn cluster to the reduction by reductants, can point at the pH-dependent mechanism of PSII self-protection from photoinactivation.

Effect of sucrose-bound polynuclear iron oxyhydroxide nanoparticles on the efficiency of electron transport in the photosystem II membranes.

Effect of water-soluble and stable sucrose-bound iron oxyhydroxide nanoparticles [Fe[III] sucrose complex (FSC)] on the efficiency of electron transport in the photosystem II membranes was studied. FSC significantly increases (by a factor 1.5) the rate of light-induced oxygen evolution in the presence of alternative electron acceptor 2,6-dichloro-p-benzoquinone (DCBQ). Without DCBQ, FSC only slightly (5%) provides the oxygen evolution. Electron transport supported by pair DCBQ + FSC is inhibited by diuron. Maximum of stimulating effect was recorded at Fe(III) concentration 5 µM. In the case of another benzoquinone electron acceptor (2-phenyl-p-benzoquinone and 2,3-dimethyl-p-benzoquinone) and 2,6-dichlorophenolindophenol, stimulating effect of FSC was not observed. Incubation of PSII membranes at different concentrations with FSC is accompanied by binding of Fe(III) by membrane components but only about 50% of iron can be extracted by membranes from Fe(III) solution at pH 6.5. This result implies the heterogeneity of FSC solution in a buffer. The heterogeneity depends on pH and decreases with its rising. At pH around 9.0 Fe(III), sucrose solution is homogeneous. The study of pH effect has shown that stimulation of electron transport is induced only by iron cations which can be bound by membranes. Not extractable iron pool cannot activate electron transfer from oxygen-evolving complex to DCBQ.

Nonlinear saturation of the large scale flow in a laboratory model of the quasibiennial oscillation.

The quasibiennial oscillation (QBO) is the nearly periodic reversal of the large scale flow generated by internal waves in the equatorial stratosphere. Using a laboratory model experiment, we study the instability that generates the QBO and investigate its nonlinear regime. We report the first quantitative measurements of the nonlinearly saturated velocity of the flow. We show that the QBO is generated by a bifurcation that is either supercritical or subcritical depending on the dominant dissipative process. This is confirmed by a nonlinear analysis in the vicinity of the instability threshold.

Creation of a 3Mn/1Fe cluster in the oxygen-evolving complex of photosystem II and investigation of its functional activity.

Extraction of Mn cations from the oxygen-evolving complex (OEC) of Ca-depleted PSII membranes (PSII[-Ca,4Mn]) by reductants like hydroquinone (H2Q) occurs with lower efficiency at acidic pH (2Mn/reaction center [RC] are extracted at pH5.7) than at neutral pH (3Mn/RC are extracted at pH6.5) [Semin et al. Photosynth. Res. 125 (2015) 95]. Fe(II) also extracts Mn cations from PSII(-Ca,4Mn), but only 2Mn/RC at pH6.5, forming a heteronuclear 2Mn/2Fe cluster [Semin and Seibert, J. Bioenerg. Biomembr. 48 (2016) 227]. Here we investigated the efficiency of Mn extraction by Fe(II) at acidic pH and found that Fe(II) cations can extract only 1Mn/RC from PSII(-Ca,4Mn) membranes at pH 5.7, forming a 3Mn/1Fe cluster. Also we found that the presence of Fe cations in a heteronuclear cluster (2Mn/2Fe) increases the resistance of the remaining Mn cations to H2Q action, since H2Q can extract Mn cations from homonuclear Mn clusters of PSII(-Ca,4Mn) and PSII(-Ca,2Mn) membranes but not from the heteronuclear cluster in PSII(-Ca,2Mn,2Fe) membranes. H2Q also cannot extract Mn from PSII membranes obtained by incubation of PSII(-Ca,4Mn) membranes with Fe(II) cations at pH5.7, which suggests the formation of a heteronuclear 3Mn/1Fe cluster in the OEC. Functional activity of PSII with a 3Mn/1Fe cluster was investigated. PSII preparations with a 3Mn/1Fe cluster in the OEC are able to photoreduce the exogenous electron acceptor 2,6-dichlorophenolindophenol, possibly due to incomplete oxidation of water molecules as is the case with PSII(-Ca,2Mn,2Fe) samples. However, in the contrast to PSII(-Ca,2Mn,2Fe) samples PSII(-Ca,3Mn,1Fe) membranes can evolve O2 at a low rate in the presence of exogenous Ca2+ (at about 27% of the rate of O2 evolution in native PSII membranes). The explanation for this phenomenon (either water splitting and production of molecular O2 by the 3Mn/1Fe cluster or apparent O2 evolution due to minor contamination of PSII(3Mn,1Fe) samples with PSII(-Ca,4Mn) membranes) is discussed.

Mechanism of inhibition and decoupling of oxygen evolution from electron transfer in photosystem II by fluoride, ammonia and acetate.

Ca(2+) extraction from oxygen-evolving complex (OEC) of photosystem II (PSII) is accompanied by decoupling of oxygen evolution/electron transfer processes [Semin et al. Photosynth. Res. 98 (2008) 235] and appearance of a broad EPR signal at g=2 (split "S3" signal) what can imply the relationship between these effects. Split signal have been observed not only in Ca-depleted PSII but also in PSII membranes treated by fluoride anions, sodium acetate, and NH4Cl. Here we investigated the question: can such compounds induce the decoupling effect during treatment of PSII like Ca(2+) extraction does? We found that F(-), sodium acetate, and NH4Cl inhibit O2 evolution in PSII membranes more effectively than the reduction of artificial electron acceptor 2,6-dichlorophenolindophenol, i.e. the action of these compounds is accompanied by decoupling of these processes in OEC. Similarity of effects observed after Ca(2+) extraction and F(-), CH3COO(-) or NH4Cl treatments suggests that these compounds can inactivate function of Ca(2+). Such inactivation could originate from disturbance of the network of functionally active hydrogen bonds around OEC formed with participation of Ca(2+). This inhibition effect is observed in the region of low concentration of inhibitors. Increasing of inhibitor concentration is accompanied by appearance of other sites of inhibition.

[Effect of nucleocapsid on multimerization of gypsy structural protein Gag].

The structural protein (Gag) of the gypsy Drosophila retrovirus lacks matrix, but contains capsid and nucleocapsid domains. The Gag forms virus-like particles in a bacterial cell; besides, its capsid alone is able to form aggregates. However, aggregates assembled from the capsid were variable in size and displayed much less organization than particles formed by the whole Gag. The nucleocapsid exerts influence on the organization and structure of particles, and this function is directed by sequence of amino acid residues at its N-terminus (a nucleocapsid proximal part). The particle assembling occurs in the presence of any RNAs or single stranded DNA oligonucleotides.

Rapid degradation of the tetrameric Mn cluster in illuminated, PsbO-depleted photosystem II preparations.

A "decoupling effect" (light-induced electron transport without O2 evolution) was observed in Ca-depleted photosystem II (PSII(-Ca)) membranes, which lack PsbP and PsbQ (Semin et al. (2008) Photosynth. Res., 98, 235-249). Here PsbO-depleted PSII (PSII(-PsbO)) membranes (which also lack PsbP and PsbQ) were used to examine effects of PsbO on the decoupling. PSII(-PsbO) membranes do not reduce the acceptor 2,6-dichlorophenolindophenol (DCIP), in contrast to PSII(-Ca) membranes. To understand why DCIP reduction is lost, we studied light effects on the Mn content of PSII(-PsbO) samples and found that when they are first illuminated, Mn cations are rapidly released from the Mn cluster. Addition of an electron acceptor to PSII(-PsbO) samples accelerates the process. No effect of light was found on the Mn cluster in PSII(-Ca) membranes. Our results demonstrate that: (a) the oxidant, which directly oxidizes an as yet undefined substrate in PSII(-Ca) membranes, is the Mn cluster (not the Y(Z) radical or P680(+)); (b) light causes rapid release of Mn cations from the Mn cluster in PSII(-PsbO) membranes, and the mechanism is discussed; and (c) rapid degradation of the Mn cluster under illumination is significant for understanding the lack of functional activity in some PSII(-PsbO) samples reported by others.

[The structural protein Gag of the gypsy retrovirus forms virus-like particles in the bacterial cell].

The amino acid sequence of the drosophila retrovirus MDG4 (gypsy) structural protein Gag does not contain a canonical motif known for the majority of vertebrate retroviruses. Moreover, the protein translation can theoretically begin with two separated initiation codons located within its unique open reading frame. We designed constructs for expression of two theoretically possible variants of Gag polypeptide and investigated an ability of the each product to form virus-like particles in the bacterial cell, i.e. in the absence of eukaryotic cell factors. The results obtained showed that the both variants of the gypsy protein Gag form globular particles in the bacterial cell.

Viscometer using drag force measurements.

A robust and precise viscometer using the forces exerted by a laminar flow inside a small duct is presented: the force is measured on a long cylindrical sensor dipped into the flow. Two devices of respective volumes 1.4 and 0.031 ml have been realized, demonstrating that the technique is usable with small fluid volumes. Several Newtonian and non-Newtonian fluids have been tested at shear rates ranging from 0.3 to 10 s(-1) for the first device and from 85 to 2550 s(-1) for the second one. For Newtonian fluids, of viscosities ranging from 10(-3) to 0.1 Pa s, the linear response of the device has been verified and a 90% agreement with the values provided by commercial rheometers is obtained. For non-Newtonian polymer solutions, the variation of the force with the flow velocity allows one to determine the dependence of the viscosity on the shear rate. Two shear thinning polymer solutions with a power law behavior at intermediate shear rates have been investigated and their rheological parameters have been determined.

Investigation of the low-affinity oxidation site for exogenous electron donors in the Mn-depleted photosystem II complexes.

In the manganese-depleted photosystem II (PSII[-Mn]) preparations, oxidation of exogenous electron donors is carried out through the high-affinity (HA) and the low-affinity (LA) sites. This paper investigates the LA oxidation site in the PSII(-Mn) preparations where the HA, Mn-binding site was blocked with ferric cations [[11] B.K. Semin, M.L. Ghirardi, M. Seibert, Blocking of electron donation by Mn(II) to Y(Z)(*) following incubation of Mn-depleted photosystem II membranes with Fe(II) in the light, Biochemistry 41 (2002) 5854-5864.]. In blocked (PSII[-Mn,+Fe]) preparations electron donation by Mn(II) cations to Y(Z)(*) was not detected at Mn(II) concentration 10 microM (corresponds to K(m) for Mn(II) oxidation at the HA site), but detected at Mn concentration 100 microM (corresponds to K(m) for the LA site) by fluorescence measurements. Comparison of pH-dependencies of electron donation by Mn(II) through the HA and the LA sites revealed the similar pK(a) equal to 6.8. Comparison of K(m) for diphenylcarbazide (DPC) oxidation at the LA site and K(d) for A(T) thermoluminescence band suppression by DPC in PSII(-Mn,+Fe) samples suggests that there is relationship between the LA site and A(T) band formation. The role of D1-His190 as an oxidant of exogenous electron donors at the LA site is discussed. In contrast to electrogenic electron transfer from Mn(II) at the HA site to Y(Z)(*), photovoltage due to Mn(II) oxidation in iron-blocked PSII(-Mn) core particles was not detected.

Effect of calcium chelators on the formation and oxidation of the slowly relaxing reduced plastoquinone pool in calcium-depleted PSII membranes. Investigation of the F0 yield.

The F(0) fluorescence yield in intact photosystem II (PSII), Ca-depleted PSII (PSII(-Ca/NaCl)), and Mn-depleted PSII membranes was measured before and after dim light treatment (1-2 min), using flash-probe fluorescence and fluorescence induction kinetic measurements. The value of F(0) after the light treatment (F'(0)) was larger than F(0) in dark-adapted PSII membranes and depended on the appearance of the slowly relaxing, reduced plastoquinone pool (t(1/2) = 4 min) formed during preillumination, which was not totally reoxidized before the F'(0) measurement. In PSII(-Ca/NaCl) such a pool also appeared, but the F'(0) yield was even higher than in intact PSII membranes. In Mn-depleted PSII membranes, the pool did not form. Interestingly, the yield of F'(0) in Ca-depleted PSII membranes prepared using chelators (EGTA and citrate) or containing 5 mM EGTA was significantly lower than in PSII(-Ca/NaCl) samples prepared without chelators. These data indicate that chelators inhibit the reduction of Q(A) and Q(B) and formation of the slowly relaxing plastoquinone pool, or alternatively they increase the rate of its oxidation. Such an effect can be explained by coordination of the chelator molecule to the Mn cluster in PSII(-Ca/NaCl) membranes, rather than different amounts of residual Ca2+ in the membranes (with or without the chelator), since the remaining oxygen-evolving activity (approximately 15%) in PSII(-Ca/NaCl) samples did not depend on the presence of the chelator. Thus, chelators of calcium cations not only have an effect on the EPR properties of the S2 state in PSII(-Ca/NaCl) samples, but can also influence the PSII properties determining the rate of plastoquinone pool reduction and/or oxidation. The effect of some toxic metal cations (Cd, Cu, Hg) on the formation of the slowly relaxing pool in PSII membranes was also studied.

[Diversity of LTR retrotransposons and their role in genome reorganization].

Current views of retrotransposons possessing long terminal repeats (LTRs) are described. The existing classification and element types isolated by genome organization are considered. Experimental data are summarized to demonstrate that the replicative cycle of a retrotransposon is not restricted to a single cell and that LTR retrotransposons are transferred between somatic cells with a rate comparable with the element transposition rate within the genome of one cell. The major mechanisms mediating the role of LTR retrotransposons in reorganization of the genome are considered with regard to the strategies of their horizontal and vertical transfer.

pH dependence of the efficiency of binding of iron cations to the donor side of photosystem II.

Light-induced interaction of Fe(II) cations with the donor side of Mn-depleted photosystem II (PS II(-Mn)) results in the binding of iron cations and blocking of the high-affinity (HAZ) Mn-binding site. The pH dependence of the blocking was measured using the diphenylcarbazide/2,6-dichlorophenolindophenol test. The curve of the pH dependence is bell-shaped with pK1 = 5.8 and pK2 = 8.0. The pH dependence of the O2-evolution mediated by PS II membranes is also bell-shaped (pK2 = 7.6). The pH dependence of the process of electron donation from exogenous donors in PS II(-Mn) was studied to determine the location of the alkaline pH sensitive site of the electron transport chain. The data of the study showed that the decrease in the iron cation binding efficiency at pH > 7.0 during blocking was determined by the donor side of the PS II(-Mn). Mössbauer spectroscopy revealed that incubation of PS II(-Mn) membranes in a buffer solution containing 57Fe(II) + 57Fe(III) was accompanied by binding only Fe(III) cations. The pH dependence of the nonspecific Fe(III) cation binding is also described by the same bell-shaped curve with pK2 = 8.1. The treatment of the PS II(-Mn) membranes with the histidine modifier diethylpyrocarbonate resulted in an increase in the iron binding strength at alkaline pH. It is suggested that blocking efficiency at alkaline pH is determined by competition between OH- and histidine ligand for Fe(III). Because the high-affinity Mn-binding site contains no histidine residue, this fact can be regarded as evidence that histidine is located at another (other than high-affinity) Fe(III) binding site. In other words, this means that the blockage of the high-affinity Mn-binding site is determined by at least two iron cations. We assume that inactivation of oxygen-evolving complex and inhibition of photoactivation in the alkaline pH region are also determined by competition between OH- and a histidine residue involved in coordination of manganese cation outside the high-affinity site.

[Errantiviruses of Drosophila]. / Erantivirusy Drosophila.

The view on Drosophila long terminal repeat (LTR) retrotransposons, which have three reading frames, as endogenous retroviruses or errantiviruses (ERVs, according to the latest ICTV nomenclature) is discussed. Data on the biology of ERVs and the mechanisms of their involvement in genetic instability of Drosophila are considered.

[Retrotransposon mdg3 is transferred between somatic cells of unrelated species in coculture]. / Retrotranspozon MDG3 sposoben peremeshchat'sia mezhdu somaticheskimi kletkami nerodstvennykh vidov pri ikh sovmestnom kul'tivirovanii.

Since retrovirus-like particles of gypsy (mdg4) are capable of interspecific transfer, other Drosophila melanogaster gypsy-related retrotransposons were tested for this property. As a donor and a recipient, D. melanogaster and D. virilis cultured cells were used. Recipient cell DNA was analyzed with probes directed to mdg1, mdg3, 17.6, 297, 412, or B104/roo. Transfer was demonstrated for mdg3, which lacks env. The possible mechanism of transfer is discussed.