Reactivation of photosynthesis in the photoinhibited green alga Chlamydomonas reinhardtii: Role of dark respiration and of light

Effect of quality, quantity and minimum duration of light on the process of recovery was investigated in the photoinhibited cells of the green alga Chlamydomonas reinhardtii. Complete and rapid reactivation of photosynthesis took place in diffuse white light of 25 μmol m(-2) s(-1). The recovery was partial (< 10%) in the dark. Far red (725 nm), red (660 nm) and blue light (480 nm) in the range of 10 to 75 μmol m(-2) s(-1) did not enhance the process of reactivation. Photoinhibited cells incubated in dark for 15 min when exposed for 5 min to diffuse light (25 μmol m(-2) s(-1)) showed complete reactivation. Even exposure of 15 min dark incubated photoinhibited cells to photoinhibitory light (2500 μmol m(-2) s(-1)) for 5 s fully regained the photosynthesis. The study indicated a very precise and triggering effect of light in the process of reactivation. The dark respiratory inhibitor KCN and uncouplers FCCP and CCCP increased the susceptibility of C. reinhardtii to photoinhibition and also prevented photoinhibited cells to reactivate fully even after longer period of incubation under suitable reactivating conditions. Of the various possibilities envisaged to assign the role of dark respiration in recovery process, supply of ATP by mitochondrial respiration appeared sound and pertinent.

Association of a cucumber mosaic virus strain with mosaic disease of banana, Musa paradisiaca--an evidence using immuno/nucleic acid probe

Virus causing severe chlorosis/mosaic disease of banana was identified as a strain of cucumber mosaic virus (CMV). Association of CMV with the disease was established by Western immunoblot using polyclonal antibodies to CMV-T and slot blot hybridization with nucleic acid probe of CMV-P genome.

Purification and characterization of homoserine dehydrogenase from spinach leaves

Homoserine dehydrogenase (HSDH) has been purified to homogeneity from spinach leaves using ammonium sulphate fractionation followed by ion exchange chromatography, gel filtration and FPLC techniques. The purified enzyme has a relative molecular mass of 220,000 and subunit molecular mass of 55,000 and probably occurs as a tetramer. The enzyme was found to be sensitive to threonine and also exhibited aspartate kinase (AK) activity, which was also sensitive to threonine suggesting that it is a bifunctional protein. The enzyme protein also gave a positive cross reaction with antibodies raised against purified AK isoenzymes. Both HSDH and AK activities were stimulated by calcium and calmodulin.

Molecular cloning of Indian tomato leaf curl virus genome following a simple method of concentrating the supercoiled replicative form of viral DNA

DNA-A and DNA-B components of the genome of a whitefly transmitted virus causing yellowing and leaf curl in tomato (ITLCV) were cloned following a simple procedure for isolation of the double stranded replicative form of viral DNA from infected tomato plants. The method is based on extraction of total DNA from infected plants followed by concentration of the double stranded replicative form of viral DNA by an alkaline denaturation procedure identical to that used for isolation of plasmid DNA from Escherichia coli. The attempted cloning of DNA showed that 95% of the transformants contained plasmids with an insert of either DNA-A (2.75 kb) or DNA-B (2.55 kb). Cloned DNA-A and DNA-B when used as probes could detect DNA-A/DNA-B in total nucleic acid obtained from fresh diseased tissue. Both DNA-A and DNA-B are needed for infection and they have a common region of 166 bases with about 94% nucleotide sequence homology, a characteristic of all bipartite geminiviruses. Comparison of the amino acid sequence of the putative coat protein product of ITLCV with some other mono- and bipartite geminiviruses revealed a maximum of 86% homology with Indian cassava mosaic virus.

Translocation of adenine nucleotides in the mitochondria of male sterile and male fertile sorghum

The translocation of ATP from the inside to the outside of the mitochondrial membrane has been studied in a male sterile (2219 A) and a male fertile (2219 B) line of sorghum. The translocation of ATP was found to be substantially lower in case of mitochondria from the male sterile line. The affinity of adenine nucleotides to the translocator proteins of the mitochondrial membrane was found to be almost one half in case of 2219 A as compared to 2219 B when the Km for ATP-ADP was determined by two different assays. It is proposed that the inadequate supply of ATP in the cytosol resulting from its inefficient translocation may contribute to the male sterility in this line.

Lysine- and threonine-sensitive aspartokinase isoenzymes from spinach leaves share common antigenic determinants

The lysine- and threonine-sensitive isoenzymes of aspartate kinase were purified to homogeneity from spinach leaves and polyclonal antibodies were raised in rabbits. The antibodies were characterized by various immunological tests like Ouchterlonys-double-diffusion, titrations of the inhibition of enzyme activity and ELISA. The antibodies against the lysine-sensitive isoenzyme could recognise as little as 50 ng of the pure antigen protein and that against the threonine-sensitive form could recognise 200 ng of the protein in the ELISA tests. The immunological tests have also shown that the lysine and threonine sensitive isoenzymes of aspartate kinase share some common antigenic determinants and differ in others.

Inactivation of leucine aminotransferase with diethylpyrocarbonate and rose bengal: evidence for an active site histidine residue

Modification of leucine aminotransferase by diethylpyrocarbonate or rose bengal-sensitized photo-oxidation caused rapid inactivation of the enzyme. The inactivation of leucine aminotransferase depended on the concentration of the reagent, the time of incubation and exhibited pseudo-first order kinetics. Rose bengal-sensitized photo-oxidation was maximum at pH 6.5 and 9. Substrates leucine and alpha-ketoglutarate protected the enzyme against inactivation by these reagents, thus suggesting participation of histidine residue at the substrate binding site.

Detection of aspartate kinase in rat liver and its activation by Ca++ and calmodulin

The enzyme aspartate kinase (EC 2.7.2.4) has been detected in rat liver (animal tissue) for the first time. This enzyme, like the aspartate kinase from bacteria and plants is inhibited by lysine and threonine. Further, the activity of the enzyme is stimulated over two fold by Ca++ and calmodulin and inhibited by EGTA, a Ca++ chelator and trifluoperazine, an anti-calmodulin compound.

The oxygen-dependent deactivation and reactivation of spinach ribulose-1,5-bisphosphate carboxylase

Ribulose-1,5-bisphosphate carboxylase-oxygenase (3-phospho-D-glycerate carboxy-lyase (dimerizing), EC 4.1.1.39) is deactivated by the removal of oxygen, and reversibly reactivated by its readdition to the enzyme solution. A short pulse of oxygen to the anaerobic enzyme solution is sufficient to trigger the reactivation process; the Ka value for this reaction was estimated as 0.12 mM oxygen. The enzyme could not be reactivated under anaerobic conditions by an organic oxidant (benzoylperoxide) or by sulfhydryl group reducing reagents (dithiothreitol or beta-mercaptoethanol), suggesting that the process of reactivation was oxygen specific. Furthermore, the inhibition of the reactivation by superoxide anion scavengers such as Tiron (1,2-dihydroxybenzene-3,5-disulfonic acid), copper penicillamine, hydroxylamine, nitroblue tetrazolium, and ascorbate, indicated that the monovalent reduced oxygen was involved as the reacting species in this process. The deactivation of the enzyme associated with the removal of oxygen was also sensitive to the presence of scavengers of O2(-), suggesting that superoxide anion was also involved in the deactivation process. Both the carboxylase and the oxygenase activities were similarly affected under all the experimental conditions studied. On the basis of these results it is argued that the enzyme molecules are able to reduce oxygen and that superoxide anion causes the deactivation or reactivation of the enzyme.

The activation of ribulose-1,5-bisphosphate carboxylase-oxygenase from spinach by oxygen

The effect of oxygen on ribulose-1,5-bisphosphate carboxylase-oxygenase from spinach was investigated. Both activities were deactivated by removal of oxygen and reversibly reactivated by oxygenation of the enzyme solution. The change in enzyme activities was accompanied by conformational changes as studied by the use of intrinsic and extrinsic fluorescent probes. The analysis of cysteine sulfhydryl groups accessible to 5,5'-dithiobis-(2-nitrobenzoic acid) revealed that the number of these groups changed with the oxygen concentration. The kinetic of the exposure of eight cysteine residues was similar to the loss of enzyme activities. The modification of these groups with 5,5'-dithiobis-(2-nitrobenzoic acid) caused almost complete loss of both the activities. The enzyme isolated from a photolithotrophic organism, Chromatium vinosum, was not affected by oxygen removal. During air--argon transitions, neither the enzyme conformation nor the number of accessible sulfhydryl groups changed.

Inhibition of the nitrate reductase complex by dibromothymoquinone

The plastoquinone antagonist 2,5-dibromothymoquinone was found to inhibit NO-3 reduction from NADH by the nitrate reductase complex from wheat. It accepts electrons from NADH through the NADH dehydrogenase activity of the nitrate reductase. However, it does not inhibit the reduction of 2,6-dichlorophenol-indophenol by the enzyme. This suggests that the two compounds may be accepting electrons at different places from the enzyme. Further it was observed that reduced DCIP could be oxidized by DBMIB in the absence of NADH indicating that the electron flow in the nitrate reductase complex may take place in a unidirectional way.

Characterization of the low temperature thermoluminescence band Zv in leaf: an explanation for its variable nature

Out of the six thermoluminescence bands reported for a mature leaf, one band (Zv) appearing at the lowest temperatures is dependent on the temperature of illumination. The characteristics of this band in fresh leaf are compared with those is a leaf heated to 60 degrees C for 5 min. It is concluded here that this band, following illumination at temperatures lower than 173 K, is part of Arnold and Azzi's Z band (Arnold, W. and Azzi, J.R. (1971) Photochem. Photobiol. 14, 233--240). However, it is part of peak I when observed subsequent to illumination beyond 173 K. An explanation for the appearance of this band at different temperatures is proposed.