Yuval Cohen

A Monte Carlo study of the Gross-Neveu model

Abstract Several lattice versions of the Gross-Neveu model are constructed and studied using Monte Carlo methods. The expected shiral structures are confirmed by the numerical results. The correct asymptotic freedom behaviour is recovered with the appropriate number of species taken into account. The models differ in their number of soft modes and their strong coupling behaviour. In some of them, chiral symmetry is restored at a finite coupling. The large-N, finite-temperature transition is also found in good agreement with the theoretical value.

Effects of Iranian crude oil on the red sea octocoral heteroxenia fuscescens

Abstract Acute toxicity and sublethal effects of Iranian crude oil on colonies of the Red Sea octocoral Heteroxenia fuscescens were studied under static and continuous flow assay conditions. Static toxicity bioassays conducted in small (3 litre) jars at 41% 0 salinity and 22°C showed that the concentration of crude oil added at the start fatal to 50% of the test colonies in 96 h was 12 ml/litre. Colonies surviving exposure to high sublethal levels of crude oil were adversely affected both during treatment and afterwards. Tank tests conducted in large (1500 litre), deep (2 m) containers and flowing sea water demonstrated that Heteroxenia were more resistant to crude oil than when assayed in jars; no deaths were observed in tanks during exposure for 168 h to initial concentrations of 10 ml/litre (15 litres added at surface). Tank assays demonstrated a depth protective effect; the number of colonies exhibiting signs of stress decreased with increasing distance from the oil film at the surface. Gas chromotographic analysis of the hydrocarbon composition of Heteroxenia exposed to high sublethal levels of crude showed that petroleum derived hydrocarbons were incorporated into tissues. The highest level of pollutant hydrocarbons found in these colonies was about 1% of their endogenic hydrocarbon content. It is concluded that while crude oil may not be acutely toxic to Heteroxenia , exposure to high sublethal oil levels may result in long term deleterious effects.

Stable assembly of PsaE into cyanobacterial photosynthetic membranes is dependent on the presence of other accessory subunits of photosystem I.

We studied assembly of the PsaE subunit of photosystem I into photosynthetic membranes of cyanobacterial mutant strains that lack specific photosystem I subunits. Radiolabeled PsaE was incubated with photosynthetic membranes, and their binding and assembly were assayed by resistance to removal by chaotropic agents and proteolytic digestion. PsaE incorporated into the wild-type membranes was resistant to these treatments. In the absence of PsaD, it was resistant to proteolytic digestion, but was removed by NaBr. When the membranes were isolated from a mutant strain in which the psaF and psaJ genes have been inactivated, PsaE assembled in vitro could not be removed. PsaE could associate with the membranes of the strain DF in which the psaD, psaJ and psaF genes have been mutated. However, the radiolabeled PsaE associated with these membranes was removed both by the proteolytic as well as by the chaotropic agents. Characterization of PsaE present in vivo revealed similar results. These observations suggest that PsaD and PsaF/J may interact with PsaE and stabilize it in the photosystem I complex.

Assembly and processing of subunit II (PsaD) precursor in the isolated photosystem-I complex

The precursor of photosystem I (PSI) subunit II (pre‐subunit II) synthesized in vitro, was found to bind to the holo‐PSI complex, both within the thylakoids and outside, after detergent extraction of PSI from the membranes. Chloroplast stromal fraction added to the purified PSI complexes, containing the labeled pre‐subunit II, induced the processing of the precursor to the mature form. This implies that processing can occur within the isolated complex, after the integration of the precursor. The results presented suggest that certain aspects of biogenesis of membranal protein complexes can be studied in detergent‐extracted purified complexes.

Assembly of the chlorophyll-protein complexes*

The biogenesis of photosynthetic complexes in plants and algae is a multi-step process that involves intricate coordination of steps in two intracellular compartments, the chloroplast and the cytoplasm. The process initiates with the transcription and translation of the various polypeptide subunits. The nuclear-encoded Chl-binding proteins are translated on cytoplasmic ribosomes as precursors that have a transit (leader) sequence at their amino-terminus. The precursors are post-translationally imported into the chloroplasts, proteolytically processed into their mature forms, inserted into the thylationally imported into the chloroplasts, proteolytically processed into their mature forms, inserted into the thylakoid membrane, and bound to their co-factors (and pigments) and with other subunits to form an active complex. The order and mechanisms by which these events occur, are currently being discovered. Electrostatic interactions, the ‘positive inside rule’, interhelix interactions, interactions with lipids and chaperone proteins affect the insertion and stabilization of the Chl-proteins in the thylakoids. This review describes the events occurring during the integration and organization of the Chl-proteins.

The carboxyl-terminal region of the spinach PsaD subunit contains information for its specific assembly into plant thylakoids

The assembly of the multi-subunit membrane-protein Photosystem I (PS I) complex involves incorporation of peripheral proteins into the complex. Here we studied assembly of the PsaD subunit of the cyanobacterial and plant PS I into the thylakoid membranes. We generated partial and chimeric psaD genes from which labeled proteins were synthesized in vitro. Assembly of these proteins into the cyanobacterial or plant thylakoids was assayed. The deletion of leader sequence and N-terminal extension of spinach prePsaD did not inhibit its assembly into spinach or cyanobacterial thylakoids. Addition of these sequences to the cyanobacterial PsaD did not enable it to assemble into plant thylakoids. Moreover, these additions significantly decreased the ability of the chimeric proteins to assemble into cyanobacterial thylakoids. In contrast, when the carboxyl-terminal half of cyanobacterial PsaD was replaced by the corresponding region of the spinach PsaD, the chimeric protein could assemble into both spinach and cyanobacterial thylakoids. Therefore, information in the carboxyl-terminal region of spinach PsaD is crucial for its assembly into plant thylakoids.

Characterization of the Sequential Light-Regulated Assembly of Photosystem I Core Complex

The biogenesis and assembly of the photosynthetic complexes of higher plants proceed by a multi-step process in the two intercellular compartments; the chloroplast and the cytoplasm. Hence a high degree of coordination is required between these different compartments in each step of the process. The process of biogenesis initiates with the transcription and translation of the various polypeptide subunits taking place both in the chloroplast (for the chloroplast-encoded subunits) and in the nucleus and cytoplasm (for the nuclear-encoded subunits) (Archer and Keegstra, 1991; Chitnis and Thornber, 1988). The polypeptide subunits synthesized in the cytoplasm are made as precursors having a leader (transit) sequence in their amino-terminus (Von-Heijne et al., 1989). These precursors are post-translationally imported into the chloroplasts in an energy dependent process (requiring ATP), probably via a receptor situated in the envelope membrane (Archer and Keegstra, 1991; Chitnis and Thornber, 1988). In the chloroplast the precursors enter the thylakoid membrane. They are processed into their mature form and associate with their cofactors (pigments and metal clusters) and with the chloroplast-encoded subunits to form the fully active complex. The order and mechanism in which these events occur, are not yet fully understood. Many experimental systems have been developed in order to try to dissect and follow the different stages of these complex processes and thereby determine the temporal sequence of the assembly of the complexes found in the membrane.

On Some of the In-Organello Processes Involved in the Biogenesis of Chlorophyll-Protein Complexes

The biogenesis and assembly of chlorophyll-protein complexes consist of many steps. These are initiated with the transcription and translation of the different polypeptide components constituting the complexes. For the nuclear-encoded subunits the synthesis takes place in the cytoplasm, and they are synthesized as precursors, which are later imported into the chloroplast. Within the organelle, the precursors are inserted into the thylakoid membranes, as well as being processed to their mature forms. The different nuclear- and chloroplast-encoded subunits assemble together, and bind the pigments and other cofactors to form the active pigmented-complex. In the present article, we discuss only the in organello processes of the biogenesis. We describe the pathways taken by two nuclear-encoded thylakoid proteins, the precursor of the main light-harvesting chlorophyll-protein of photosystem II (pLHCP) and the precursor of photosystem I subunit II (pre subunit II). These polypeptide subunits, that are located in two different photosynthetic complexes, differ from each other. While pLHCP is an integral membrane protein, which binds pigments, photosystem I-subunit II is a peripheral membrane protein, located on the stromal side of the thylakoids, and is not predicted to span it. The differences and the common features of the in organello biogenesis pathways of these two proteins are discussed.