G S Björn

Photochromic pigments and photoregulation in blue-green algae

Indirect evidence makes it very likely that chromatic adaptation and some other photophysiological responses of blue-green algae are mediated by one or several photoreversibly photochromic biliproteins, with properties somewhat analogous to the phytochrome of higher plants; the search for proteins with properties to fit this theory continues. Ohki and Fujita [3] failed to detect in vivo photochromism in Tolypothrix tenuis, and also could not detect photochromism in crude aqueous extracts or purified pigments from healthy cells of the same organism. They could, however, induce photochromism both in crude extracts and in purified phycocyanin and allophycocyanin by treatment with the chaotropic reagents, guanidine hydrochloride or urea. They therefore argued that the photochromism previously attributed to special pigments, phycochromes, by Scheibe [ 161 and by us, is an artefact and due to partially denatured phycocyanin and allophycocyanin. In a later publication [4], Ohki and Fujita [3] observed that they obtained photochromic extracts even without treatment with guanidine hydrochloride or urea, provided they first bleached the cells with strong white light (15 Wm-’) in a medium free of combined nitrogen. From cells pretreated in this way, they obtained a phycocyanin fraction claimed to exhibit the green minus red difference spectrum of phycochrome a, and an allophycocyanin fraction showing the green minus red difference spectrum of phycochrome c. Their difference spectrum for the phycocyanin fraction is, however, rather different from our difference spectra for phycochrome a, with a much smaller absorbance change in the green region compared to the large change in the red region. Ohki and Fujita [3] conclude that “we must be more careful in our evaluations of a possible role of these photoreversible pigments in the photobiological phenomena of bluegreen algae”. Stimulated by these experiments of Ohki and Fujita, one of the present authors (G. S. Bjorn, unpublished) has grown Nostoc muscorum in weak light (lo00 and 5001x white fluorescent light corresponding to 2.6 and 1.3 WnC2, respectively) in the presence of nitrate, and extracted the pigment with the avoidance of oxygen (mercaptoethanol was added to the extraction medium and deoxygenated water was used for preparing the column for isoelectric focusing). Even with these precautions preparations exhibiting phycochrome c activity were obtained (phycochrome a was not tested for). In contrast to results reported by Ohki and Fujita, also crude extracts from Tolypothrix tenuis grown as above, in our hands, were photochromic with a difference spectrum peaking at 620 nm. Further denaturation studies have been carried out by G. S. Bjorn [S]. Treatment with 6 M urea decreases photochromicity. Gentle heating (up to 60°C) increases photochromicity, especially when related to the absorbance, which is decreased by heating. Photochromic preparations of allophycocyanin have also been studied by Ohad et a/ . [S]. In contrast to us and to Ohki and Fujita [3], they found allophycocyanin to account for both phycochrome a and phycochrome c activity. All photochromism disappeared when the solution was passed through a column incorporating immobilized allophycocyanin antibody, while use of the phycocyanin antibody had no effect. At pH 7.2, when the allophycocyanin is aggregated and absorbs maximally at 650 nm, the photochromism was of phycochrome c type, while at pH 3.6 both the absorption and the photochromic change had maxima at 620nm. At low pH, the changes in the green region were highly variable. A different kind of photochromic pigment, designated phycochrome b, has been further investigated by G. S. Bjorn [6,7]. This pigment, which Seems to be identical to the a-subunit of phycoerythrocyanin, exhibits maximum and opposite changes at 500 and 570 nm, and can be driven between two states by light of these wavelengths. It is photochromic both in free form and as part of dissolved phycoerythrocyanin. Exactly the same photochromic changes, and with the same action spectra, were also observed with intact cells of Tolypothrix distorta. No physiological light effects corresponding to these photochromic changes have been observed (but c j next section). A preliminary experiment to test the possibility that the changes are associated with a regulation of energy

Characterization of a non-detergent PS II-cytochrome b/f preparation (BS)

A non-detergent photosystem II preparation, named BS, has been characterized by countercurrent distribution, light saturation curves, absorption spectra and fluorescence at room and at low temperature (−196°C). The BS fraction is prepared by a sonication-phase partitioning procedure (Svensson P and Albertsson P-Å, Photosynth Res 20: 249–259, 1989) which removes the stroma lamellae and the margins from the grana and leaves the appressed partition region intact in the form of vesicles. These are closed structures of inside-out conformation. They have a chlorophyll a/b ratio of 1.8–2.0, have a high oxygen evolving capacity (295 μmol O2 per mg chl h), are depleted in P700 and enriched in the cytochrome b/f complex. They have about 2 Photosystem II reaction centers per 1 cytochrome b/f complex.The plastoquinone pool available for PS II in the BS vesicles is 6–7 quinones per reaction center, about the same as for the whole thylakoid. It is concluded, therefore, that the plastoquinone of the stroma lamellae is not available to the PS II in the grana and that plastoquinone does not act as a long range electron transport shuttler between the grana and stroma lamellae.Compared with Photosystem II particles prepared by detergent (Triton X-100) treatment, the BS vesicles retain more cytochrome b/f complex and are more homogenous in their surface properties, as revealed by countercurrent distribution, and they have a more efficient energy transfer from the antenna pigments to the reaction center.

Margareta Ryberg (1946–2012): a personal tribute

We pay tribute to the life and work of Margareta Ryberg (1946–2012). She was an expert on the different forms of protochlorophyll(ide), their protein partners, and their transformations in angiosperms; on the structural aspects, and the nature of prolamellar bodies, as well as on the localization of light-dependent NADPH:protochlorophyllide oxido-reductase. She was a great teacher, who also loved gardening and handicraft. But above all, she was a beloved wife, mother, grandmother, and friend who will be deeply missed.

STUDIES ON ENERGY DISSIPATION IN PHYCOBILISOMES USING THE KENNARD-STEPANOV RELATION BETWEEN ABSORPTION AND FLUORESCENCE EMISSION SPECTRA

Absorption and fluorescence emission spectra were measured at room temperature (ca. 22°C) for solutions of phycocyanin‐1, phycocyanin‐2 and allophycocyanin from Phormidium luridum, and also for phycobilisome preparations from various blue‐green algae (Anabaena variabilis, Nostoc muscorum strain A, Nostoc sp. strain Mac, Phormidium luridum). Kennard‐Stepanov (KS) temperatures (T) were computed using the Kennard‐Stepanov relationship F() = b A() ‐5 exp(‐h/kT), where F() stands for fluorescence (energy per wavelength interval) as a function of wavelength (), A() is absorbance as a function of wavelength, b a proportionality factor, and h, c and k are Plancks constant, the velocity of light and Boltzmanns constant, respectively.