Born 1930, Buzau (Romania); Ph.D. 1963, Hebrew Univ.; Sen. Lect. 1966; Assoc. Prof. 1973; Prof. 1977; Director, The Minerva Avron Even-Ari Center for Photosynthesis Research (1995-1998). Emeritus, 1998

E-mail: ohad@vms.huji.ac.il; Tel: 972-2-658-5423/4; Fax: 972-2-658-6448


Hestrin Prize, the Israel Biochemical Society, 1964; The Max-Planck Scientific Award in cooperation with K. Kloppstech, Hannover, 1993; Director of Minerva Avron Even-Ari Center for Photosynthesis Research 1995-1998; Doctor Honoris Causa, Stockholm University, 1998

Research Interests:

  1. Stress responses in photosynthesis.
  2. Dynamics of the photosynthetic apparatus in terms of its biogenesis, regulation of its activity and mechanism of stress reaction and protection.

Research Projects:

  1. Assembly of Photosystem II (PSII), light stress induced degradation and turnover of PSII proteins; mechanism of light stress effects on PSII; regulation of energy distribution between the two photosystems via phosphorylation of the chlorophyll binding proteins of the light harvesting antennae, LHCII.
  2. Mechanism of photoinhibition and recovery of reaction center II activity
  3. Control of the LHCII kinase system by redox activation/deactivation and mechanism of cytochrome b6/f in this process; development of the LHCII-kinase system.
  4. Light induced conformational changes in chl-protein complexes exposing phosphoryaltion sites to protein-kinases

Abstracts of Current Research:

Inhibition of Photosystem II activity by saturating single turnover flashes in calicum-depleted and active Photosystem II

Inhibition of Photosystem II (PSII) activity induced by continuous light or by saturating single turnover flashes was investigated in Ca2+-, Mn-depleted and active PS II enriched membrane fragments. While Ca2+- and Mn-depleted PSII were more damaged under continuous illumination, active PSII was more susceptible to flash-induced photoinhibition. The extent of photoinactivation as a function of the duration of the dark interval between the saturating single turnover flashes was investigated. The highest amount of damage was seen at the longest dark interval (32 s) which was long enough to allow charge recombination between the S2 or S3 and QB- in the majority of centres in the active PSII sample. Illumination with groups of consecutive flashes (spacing between the flashes 0.1 s followed by 32 s dark interval) resulted in a binary oscillation of the loss of PSII-activity in active samples as has been shown previously (N. Keren, H. Gong, I. Ohad, 1995, J. of Biol. Chem. 270, 806-814). Ca2+- and Mn-depleted PSII show in addition to the inhibition of water-splitting activity an up-shift of the midpoint potential of QA. It was proposed that this changes the pathway of charge recombination within PSII disfavouring the population of excited states (G.N. Johnson, A.W. Rutherford, A. Krieger (1995) Biochim. Biophys. Acta 1229, 201-207). The lower susceptibility of Ca2+- and Mn-depleted PSII to flash induced photoinhibition are explained within this model.

GTP enhances the degradation of the Photosystem II D1 protein irrespective of its conformational heterogeneity at the QB site

The light exposure history and/or binding of different herbicides at the QB site may induce heterogeneity of photosystem II (PSII) acceptor side conformation that affects D1 protein degradation under photoinhibitory conditions. GTP was recently found to stimulate the D1 protein degradation of photoinactivated PSII (Spetea, C., Hundal, T., Lohman, F., and Andersson, B. (1999) Proc. Natl. Acad. Sci. U.S.A. 96, 6547-6552). Here we report that GTP enhances the cleavage of the D1 protein D-E loop following exposure of thylakoid membranes to either high light, low light or repetitive single turnover flashes, but not to trypsin. GTP does not stimulate D1 protein degradation in the presence of herbicides known to affect the accessibility of the cleavage site to proteolysis. However, GTP stimulates the degradation that can be induced in some PSII conformers even in darkness following binding of the PNO8 herbicide (Nakajima, Y., Yoshida, S., Inoue, Y., and Ono, T. (1996) J. Biol. Chem. 271, 17383-17389). Both the PNO8- and the light-induced primary cleavage of the D1 protein occur in the grana membrane domains. The subsequent migration of PSII cores containing the D1 protein fragments to the stroma domains for secondary proteolysis is light-activated. We conclude that the GTP effect is not confined to a specific photoinactivation pathway nor to the conformational state of the PSII acceptor side. Consequently, GTP does not interact with the D1 protein cleavage site but rather enhances the activity of the endogenous proteolytic system.

Regulation of thylakoid protein phosphorylation at the substrate level: Reversible light-induced conformational changes expose the phosphorylation site of the light harvesting complex II (LHCII)

Light-dependent activation of thylakoid protein phosphorylation regulates the energy distribution between photosystems I and II of oxygen evolving photosynthetic eukaryotes as well as the turnover of photosystem II proteins. So far the only known effect of light on the phosphorylation process was the redox-dependent regulation of the membrane bound protein kinase(s) activity via plastoquinol bound to the cytochrome bf complex and the redox state of thylakoid dithiols. Using a partially purified thylakoid protein-kinase and isolated native chlorophyll a/b light-harvesting complex of photosystem II (LHCII), as well as recombinant LHCII, we find that illumination of the chlorophyll-protein substrate exposes the phosphorylation site to the kinase. Light does not activate the phosphorylation of the LHCII apoprotein nor the recombinant pigment-reconstituted complex lacking the N-terminal domain that contains the phosphothreonine site. The suggested light-induced conformational change exposing the N-terminal domain of LHCII to the kinase is evidenced also by an increase in its accessibility to tryptic cleavage following light exposure. Light activates preferentially the trimeric form of LHCII and the process is paralleled by chlorophyll fluorescence quenching. Both phenomena are slowly reversible in darkness. Light-induced exposure of the LHCII N-terminal domain to the endogenous protein-kinase(s) and tryptic cleavage occurs also in thylakoid membranes. These results demonstrate that light may regulate thylakoid protein phosphorylation not only via the signal transduction chain connecting redox reactions to the protein kinase activation, but also by affecting the conformation of the chlorophyll-protein substrate.

Recovery of photosystem II activity in photoinhibited Synechocystis cells: light-dependent translation activity is required besides light independent synthesis of the D1 protein Sabine Constant, Yael Eisenberg-DomovitchItzhak Ohad and Diana Kirilovsky

Irreversible photoinactivation of photosystem II (PSII) results in the degradation of the reaction center II D1 protein. In Synechocystis PCC 6714 cells recovery of PSII activity requires illumination. The rates of photoinactivation and recovery of PSII activity in the light are similar in cells grown in minimal (MM) or glucose containing medium (GM). Re-assembly of PSII with newly synthesized proteins requires degradation of the D1 protein of the photoinactivated PSII. This process may occur in darkness in both type of cells. The degraded D1 protein is however only partially replaced by†newly synthesized protein in MM cells in darkness while a high level of D1 protein synthesis occurs in darkness in the GM cells. The newly synthesized D1 protein in darkness appears to be assembled with other PSII proteins. However, PS II activity is not recovered in such cells. Illumination of the cells in absence but not in presence of protein synthesis inhibitors allows recovery of PS II activity.

Publications (since 1995)

Keren, N., Gong, H. and Ohad I. (1995) Oscillations of reaction center II-D1 protein degradation in vivo induced by repetitive light flashes. Correlation between the level of RCII-QB- andprotein degradation in low light. J. Biol. Chem. 270, 806-814.

Mor, T.S., Ohad, I., Hirschberg, J. and Pakrasi, H. (1995) An unusual organization of the genes encoding cyt.b559 in C. reinhardtii: psbE and psbF genes are separately transcribed from different regions of the plastid chromosome. Mol. Gen. Genetics 246, 600-604.

Gong, H. and Ohad, I. (1995) Rapid turnover of the RCII-D1 protein in the dark induced by photoinactivation of photosystem II in Scenedesmus wild-type and the PSII-donor defective LF-1 mutant cells. Biochim. Biophys. Acta 1228, 181-188.

Zer, H. and Ohad, I. (1995) Photoinactivation of Photosystem II induces changes in the photochemical reaction center II abolishing the regulatory role of the QB site in the D1 protein degradation. Eur. J. Biochem. 231, 448-453.

Eisenberg-Domovich, Y., Oelmuller, R., Herrmann, R.G. and Ohad, I. (1995) Role of the RCII-D1 protein in the reversible association of the oxygen evolving complex proteins with the lumenal side of photosystem II. J. Biol. Chem. 270, 30181-30186.

Vener, A., van Kan, P.J.M., Gal, A., Andersson B. and Ohad, I. (1995) Activation/deactivation cycle of the redox-controlled thylakoid protein phosphorylation. J. Biol. Chem. 270, 25225-25232.

Sokolenko, A., Fulgosi, H., Gal, A., Altschmied, L., Ohad I. and Hermann, R.G. (1995) The 64 kDa polypeptide of spinach may not be the LHCII kinase but a lumen-located polyphenol oxidase. FEBS Lett. 371, 176-180.

Vermass, W.F.J., Shen, G. and Ohad I. (1996) Chimeric CP47 mutants of the cyanobacterium sp. PCC 6803 carrying spinach sequences. Photosynth. Res. 48 (1-2), 147-162.

Andersson, B., Adamska, I., Kloppstech, K., Lindahl, M. and Itzhak Ohad (1996) Proteolytic activities associated with the photosynthetic membrane in: Plant Membrane Biology (Moeler I. M. and Brodelius, P. eds.) 38, 107-126

Gal, A., Herrmann, R.G. and Ohad I. (1997) Affinity labeling by ATP and GTP of the nuclear encoded OEC23 protein of the photosystem II oxygen evolving complex. Photochem. Photobiol. 36 (B), 307-311.

Vener, A.V., van Kan, P.J.M., Rich, P.R., Ohad I. and Andersson, B. (1997) Plastoquinol at the Qo-site of reduced cytochrome bf mediates signal transduction between light and protein phosphorylation: Thylakoid protein kinase deactivation by a single turnover flash. Proc. Natl. Acad. Sci.USA, 94, 1585-1590.

Keren. N., Berg, A., van Kan, P.J.M., Levanon, H. and Ohad, I.. (1997) Mechanism of Photosystem II photoinactivation and D1 protein degradation at low light: the role of back electron flow. Proc. Natl. Acad. Sci. USA 94, 1579-1584

Mor, T.S., Hundal, T., Ohad, I. and Andersson, B. (1997) The fate of cytochrome b559 during anaerobic photoinhibition and its recovery processes. Photosynth. Res. 53, 205-213.

Hassidim, M., Keren, N., Ohad, I., Reinhold, L. and Kaplan A. (1997) Acclimation of Synechoccous strain WH7803 to ambient CO2 concentrations and to elevated light intensity. J. Phycol. 33, 811-817

Gal, A., Zer, H. and Ohad, I. (1997) Redox controlled thylakoid protein kinase(s): News and views. Physiol. Plant. 100, 869-885.

Tyystjaervi, T., Tyystjaervi, E., Ohad I., Aro-E-M. (1998) Exposure of Synechocystis 6803 cells to series of single turnover flashes increases the psbA transcript level by activating transcription and down-regulating psbA mRNA degradation. FEBS Lett. 436, 483-487

Vener,A., Ohad I. and Andersson B. (1998) Thylakoid protein phosphorylation. Curr. Op. in Plant Biol. 1, 217-223

Keren, N. and Ohad, I. (1998) State transition and photoinhibition. in: Advances in Photosynthesis series "Molecular biology of Chlamydomonas:chloroplast and mitochondria" (J. David Rochaix , Michel Goldscmidt-Clermont and Sabeah Merchant eds.) Kluwer Academic Publishers Vol.7. pp.569-596.

Zer, H., Vink, M., Keren, N., Dilly-Hartwig, H. G., Paulsen, H., Herrmann, R. G., Andersson, B. and Ohad I. (1999) Regulation of thylakoid protein phosphorylation at the substrate level: Reversible light-induced conformational changes exposes the phosphorylation site of the light harvesting complex II (LHCII). Proc. Natl. Acad. Sci. USA 96, 8277-8282.

Tal, S., Keren, N., Hirschberg, J. and Ohad, I. (1999) Photosystem II activity and turnover of the D1 protein are impaired in the psbA Y112L mutant of Synechocisitis PCC 6803 sp. Photochem. Photobiol. B: Biol. 48 120-126.

Franke, C., Loyal, R., Ohad, I., and Haehnel, W. (1999) In vitro assembly of a b2 cytochrome b559-like complex from the chemically synthesized b-subunit encoded by the Synechocystis sp.6803 psbF gene. FEBS Lett. 442, 75-78 .

Meetam, M., Keren, N., Ohad, I. and Pakrasi, H. B. (1999) The PsbY protein is not essential for oxygenic photosynthesis in the cyanobacteruim Synechocystis sp. PCC 6803. Plant Physiol. 121, 1267-1272.

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