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A role of the Qв binding protein in the mechanism of cyanobacterial adaptation to light intensity?
(1986)
Growth of the unicellular blue-green alga Anacystis nidulans in media containing sublethal concentrations of DCMU-type inhibitors of photosynthetic electron transport in strong white light gave rise to shade type appearance in this organism, as characterized by an increased ratio of phycocyanin to chlorophyll and reduced ratios, both, of carotenoids to chlorophyll and of total chlorophyll to P700. Shade type in Anacystis was caused neither by phenolic inhibitors tested nor by those known to bind to the cytochrome b6/f-complex. Surprisingly enough, the molar ratio of phycocyanin to chlorophyll in artificially shade adapted Anacystis1 grown in strong white light in the presence of 10-6 м atrazine, was found to increase with temperature for a given light intensity and with light intensity for a given temperature.
Mutants of Anaeystis with a reduced binding capacity for DCMU-type herbicides due to an amino acid exchange in the 32 kDa Qв-binding polypeptide, also called D-1 protein, were ob- served to show shade type appearance in strong light, to respond very little to changes in light intensity and to show a reduced capability to further change their appearance to shade type by binding of competitors of Ob to the 32 kDa polypeptide.
In Anaeystis a concentration of atrazine (10-7 м), ten times lower than the one causing the highest rate of shade adaptation (10-6 м), was shown to induce an optimum in cell density, which in turn resulted in an optimum in light-dependent O2 evolution. Both factors together might be responsible for the so-called greening effect observed in higher plants treated with sublethal concentrations of DCMU-type inhibitors of photosynthetic electron transport.
The cyanobacteria Anabaena cylindrica and Synechococcus leopoliensis (= Anacystis nidulans) were grown at different levels of UV-B radiation (439. 717, 1230 and 1405 J m -2d-1 weighted according Caldwell, 1971) for 2 days. Dry weight was hardly affected but phycocyanin content of both species decreased linearly to the level of UV-B radiation. Contents of protein, carotenoids and chlorophyll a were reduced only after exposure to high doses (1230 J m-2d-1) of UV-B radiation. Photosynthetic 14CO2 fixation of Anabaena cells was reduced linearly with increasing UV-B dose whereas no effect could be observed in Synechococcus. A depression of photosynthetic 15N-nitrate uptake was found after UV-B stress in both species. UV-B irradiance caused an increase of 15N-incorporation into glutamine, but no effect was noted for incorporation into alanine or aspartic acid. An increase of 15N-excess in glutamic acid linear with the UV-B dose was observed in Synechococcus, only. Patterns of 14C-labelled photosynthetic products were either less affected by UV-B radiation (Anabaena) or an enhancement of 14C-label in total amino acids was detected (Synechococcus). The amount of total free amino acids increased parallel to the level of UV-B radiation. Only, the high dose of UV-B (1405 J m-2d-1, weighted) results in a decrease of the glutamine pool. Our results indicate an inhibition of glutamate synthase by UV-B irradiation in Anabaena, only. Results were discussed with reference to the damage of the photosynthetic apparatus.