On the significance of electron transport systems for growth of Rhodospirillum rubrum

The inhibitory effects of 2-hydroxybiphenyl on various electron transport reactions of isolated membranes and growth in the presence of malate of either phototrophic or chemotrophic cells of Rhodospirillum rubrum were studied. 50% inhibition of both oxygen uptake of whole cells and growth under chemotrophic conditions (i.e. aerobiosis in the dark) was achieved in the presence of 0.09 mM 2-hydroxybiphenyl. With isolated membranes the same effect on NADH oxidase was obtained with 0.08 mM of inhibitor. Succinate dependent respiratory reactions were inhibited by 50% at a concentration of 0.36 mM. Growth under phototrophic conditions (i.e. anaerobiosis in the light) was inhibited by 50% in the presence of 0.17 mM (wild type strain) or 0.21 mM (blue-green mutant, strain VI) of 2-hydroxybiphenyl. Photophosphorylation and light dependent NAD⁺ reduction by succinate were inhibited by 50% at concentrations of 0.21 mM and 0.03 mM of inhibitor, respectively. After phototrophic growth of the organisms for about five doublings of cell mass in the presence of 0.18 mM of 2-hydroxybiphenyl coloured carotenoids could no longer be detected. Membrane fractions of such cultures exhibited normal activities of succinate cytochrome c reductase but activities of NADH cytochrome c reductase were decreased by 80%. In comparison with a blue green mutant, strain VI, of R. rubrum light induced absorbance changes at 865 nm as well as activities of photophosphorylation were unaffected. However, no activity of light dependent NAD⁺ reduction with succinate could be detected. The data indicate that cellular respiration as well as chemotrophic growth depend largely on NADH dependent respiration. Phototrophic growth, on the other hand, is limited by photophosphorylation while energy dependent reversed electron flow to NAD⁺, if at all, is of rathe minor importance.Abbreviation BChl bacteriochlorophyll     

The early formation of the photosynthetic apparatus in Rhodospirillum rubrum

The time dependent assembly of the photosynthetic apparatus was studied in Rhodospirillum rubrum after transfer of cells growing aerobically in the dark to low aeration. While bacteriochlorophyll (Bchl) cellular levels increase continuously levels of soluble cytochrome c ₂do not change significantly. Absorption spectra of membranes isolated at different times after transfer reveal that incorporation of carotenoids lags behind incorporation of Bchl. However, a carotenoid fraction exhibiting spectral properties of spirilloxanthin isomers was isolated apart from membranes. This carotenoid fraction even was present in homogenates from Bchl-free, aerobically grown cells. Incorporation of U-¹⁴C-proteinhydrolyzate into membrane proteins showed that proteins are mainly formed which are specific for photosynthetic membranes. Although the proportion of reaction center (RC) Bchl per light harvesting (LH) Bchl does not change the proportions of membrane proteins present in RC and LH preparations change initially. But later on the proportions of the different proteins also reach constant values. Concerning proteins characteristic for cytoplasmic membranes a differential incorporation of label can be observed. The data indicate that the photosynthetic apparatus in Rhodospirillum rubrum is assembled through a sequential mechanism.Abbreviations Bchl bacteriochlorophyll - LH light harvesting - RC reaction center - R. Rhodospirillum - R. Rhodopseudomonas     

Simultaneous presence of two terminal oxidases in the respiratory system of dark aerobically grown Rhodospirillum rubrum

Rhodospirillum rubrum CAF10, a spontaneous cytochrome oxidase defective mutant, was isolated from strain S1 and used to analyze the aerobic respiratory system of this bacterium. In spite of its lack of cytochrome oxidase activity, strain CAF10 grew aerobically in the dark although at a decreased rate and with a reduced final yield. Furthermore, aerobically grown mutant cells took up O₂ at high rates and membranes isolated from those cells exhibited levels of NADH and succinate oxidase activities which were similar to those of wild type membranes. It was observed also that whereas in both strains O₂ uptake (intact cells) and NADH and succinate oxidase activities (isolated membranes) were not affected by 0.2 mM KCN, the cytochrome oxidase activity of the wild type strain was inhibited about 90% by 0.2 mM KCN. These data indicate the simultaneous presence of two terminal oxidases in the respiratory system of R. rubrum, a cytochrome oxidase and an alternate oxidase, and suggest that the rate of respiratory electron transfer is not limited at the level of the terminal oxidases. It was also found that the aerobic oxidation of cellular cytochrome c ₂ required the presence of a functional cytochrome oxidase activity. Therefore it seems that this electron carrier, which only had been shown to participate in photosynthetic electron transfer, is also a constituent of the respiratory cytochrome oxidase pathway.Abbreviations DCIP 2,6-dichlorophenolindophenol - DMPD N,N-dimethyl-p-phenylenediamine - TMPD N,N,N,N-tetramethyl-p-phenylenediamine - Tricine N-[2-hydroxy-1,1-bis(hydroxymethyl)-ethyl]-glycine     

The role of the membrane bound cytochromes of b- and c-type in the electron transport chain of Rhodobacter capsulatus

(1) The electron transport system of heterotrophically dark-grown Rhodobacter capsulatus was investigated using the wild-type strain MT1131 and the phototrophic non-competent (Ps^-) mutant MT-GS18 carrying deletions of the genes for cytochrome c ₁ and b of the bc ₁ complex and for cytochrome c ₂. (2) Spectroscopic and thermodynamic data demonstrate that deletion of both bc ₁ complex and cyt. c ₂ still leaves several haems of c- and b-type with Em_7.0 of +265 mV and +354 mV at 551–542 nm, and +415 mV and +275 mV at 561–575 nm, respectively. (3) Analysis of the oxidoreduction kinetic patterns of cytochromes indicated that cyt. b ₄₁₅ and cyt. b ₂₇₅ are reduced by either ascorbate-diaminodurene or NADH, respectively. (4) Growth on different carbon and nitrogen sources revealed that the membrane-bound electron transport chain of both MT1131 and MT-GS18 strains undergoes functional modifications in response to the composition of the growth medium used. (5) Excitation of membrane fragments from cells grown in malate minimal medium by a train of single turnover flashes of light led to a rapid oxidation of 32% of the membrane-bound c-type haem complement. Conversely, membranes prepared from peptone/yeast extract grown cells did not show cyt. c photooxidation. These results are discussed within the framework of an electron transport chain in which alternative pathways bypassing both the cyt. c ₂ and bc ₁ complex might involve high-potential membrane bound haems of b- and c-type.Abbreviations AA antimycin A - CCCP carbonylcyanide m-chlorophenyl hydrazone - CN^- cyanide - DAD diaminodurene - Q₂H₂ ubiquinol-2 - Q-pool ubiquinone-10 pool - RC photochemical reaction center     

Oxidation of cytochrome c 2 by photosynthetic reaction centers of Rhodospirillum rubrum and Rhodopseudomonas sphaeroides in vivo. Effect of viscosity on the rate of reaction

In Rhodospirillum rubrum and Rhodopseudomonas sphaeroides it is shown that the oxidation of cytochrome c ₂ involves a diffusion limited process. From analysis of the results it follows that the electron transfer probability must be very low. This is corroborated by in vitro studies using the isolated components.     

Acclimation of barley to changes in light intensity: photosynthetic electron transport activity and components

Barley seedlings (Hordeum vulgare L. Boone) were grown at 20°C with 16 h/8 h light/dark cycle of either high (H) intensity (500 mole m^-2 s^-1) or low (L) intensity (55 mole m^-2 s^-1) white light. Plants were transferred from high to low (H L) and low to high (L H) light intensity at various times from 4 to 8 d after leaf emergence from the soil. Primary leaves were harvested at the beginning of the photoperiod. Thylakoid membranes were isolated from 3 cm apical segments and assayed for photosynthetic electron transport, Photosystem II (PS II) atrazine-binding sites (Q_B), cytochrome f(Cytf) and the P-700 reaction center of Photosystem I (PS I). Whole chain, PS I and PS II electron transport activities were higher in H than in L controls. Q_B and Cytf were elevated in H plants compared with L plants. The acclimation of H L plants to low light occurred slowly over a period of 7 days and resulted in decreased whole chain and PS II electron transport with variable effects on PS I activity. The decrease in electron transport of H L plants was associated with a decrease in both Q_B and Cytf. In L H plants, acclimation to high light occurred slowly over a period of 7 days with increased whole chain, PS I and PS II activities. The increase in L H electron transport was associated with increased levels of Q_B and Cytf. In contrast to the light intensity effects on Q_B levels, the P-700 content was similar in both control and transferred plants. Therefore, PS II/PS I ratios were dependent on light environment.Abbreviations Asc ascorbate - BQ 2,5-dimethyl-p-benzoquinone - DBMIB 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone - DCIP 2,6-dichlorophenolindophenol - H control plants grown under high light intensity - H L plants transferred from high to low light intensity - L low control plants grown under low light intensity - L H plants transferred from low to high light intensity - MV methyl viologen - P-700 photoreaction center of Photosystem I - Q_B atrazine binding site - TMPD N,N,N,N-tetramethyl-p-phenylenediamine Cooperative investigations of the United States Department of Agriculture, Agricultural Research Service, and the North Carolina Agricultural Research Service, Raleigh, NC. Paper No. 11990 of the Journal Series of the North Carolina Agricultural Research Service, Raleigh, NC 27695-7643, USA.     

Involvement of plastoquinone and lipids in electron transport reactions mediated by the cytochrome b6-f complex isolated from spinach

The isolation of a cytochrome b₆-f complex from spinach, which is depleted of plastoquinone (and lipid), is reported. The depleted complex no longer functions as a plastoquinol-plastocyanin oxidoreductase but can be reconstituted with plastoquinone and exogenous lipids. The lipid classes digalactosyldiacylglycerol, phosphatidylglycerol and phosphatidylcholine were active in reconstitution while monogalactosyldiacylglycerol and sulfoquinovosyldiacylglycerol were not. Neither plastoquinone nor lipid alone fully reconstitutes electron transport in the depleted complex. Saturation of plastoquinol-plastocyanin oxidoreductase activity in the depleted complex occurs at 1 plastoquinone per cytochrome f.     

Temperature dependence of energy transfer from the long wavelength antenna BChl-896 to the reaction center in Rhodospirillum rubrum,Rhodobacter sphaeroides (w.t. and M21 mutant) from 77 to 177K,studied by picosecond absorption spectroscopy

Decay of the bacteriochlorophyll excited state was measured in membranes of the purple bacteria Rhodospirillum (R.) rubrum, Rhodobacter (Rb.) sphaeroides wild type and Rb. sphaeroides mutant M21 using low intensity picosecond absorption spectroscopy. The excitation and probing pulses were chosen in the far red wing of the long wavelength absorption band, such that predominantly the minor antenna species B896 was excited. The decay of B896 was studied between 77 and 177K under conditions that the traps were active. In all species the B896 excited state decay is almost temperature independent between 100 and 177K, and probably between 100 and 300 K. In this temperature range the decay rates for the various species are very similar and close to 40 ps. Below 100 K this rate remains temperature independent in Rb. sphaeroides w. t. and M21, while in R. rubrum a steep decrease sets in. An analysis of this data with the theory of nuclear tunneling indicates an activation energy for the final transfer step from B896 to the special pair of 70cm^-1 for R. rubrum and 30cm^-1 or less for Rb. sphaeroides.Abbreviations B880 and B896 the main and long wavelength bacteriochlorophyll's of the LH-1 antenna - RC reaction centre - P special pair in the RC     

Surface charge on thylakoid membranes: regulation of photosynthetic electron transfer in cyanobacteria

The establishment of the steady-state rate of photosynthetic O₂ evolution by cells of Anabaena variabilis and other cyanobacteria was found to be preceded by a lag-phase the duration of which depended on the time of cell preincubation in the dark. Electron acceptors (benzoquinone, N,N,N,N-tetramethyl-p-phenylenediamine, 2,3,5,6-tetramethyl-p-phenylenediamine or 2,6-dichlorophenolindophenol) abolished the lag-phase as well as the inhibitory effect of cyanide on electron transfer. Mono-, di-and trivalent cations added to the cell suspension markedly reduced the lag-phase. As cation concentrations were increased, acceleration and subsequent deceleration of the O₂ evolution rate were observed. The efficient concentrations of cations decreased as their valency increased. The lag-phase and the rate of photosynthetic O₂ evolution by the blue-green algae are suggested to depend on the value of the membrane surface charge governing the electrostatic interaction between unidentified membrane-bound redox components. The combination of valinomycin and nigericin as well as gramicidin D enhanced the duration of the lagphase by deenergization of thylakoid membrane.Abbreviations 9AA 9-aminoacridine - BQ benzoquinone - DAD 2,3,5,6-tetramethyl-p-phenylenediamine - DPIP 2,6-dichlorophenolindophenol - FeCy ferrycyanide - HEPES N-2-hydroxyethylpiperazine-N-2-ethane-sulphonic acid - MES 2(N-morpholino)ethane sulphonic acid - TMPD N,N,NN-tetramethyl-p-phenylenediamine - Tris tris(hydroxymethyl)aminomethane     

Vectorial electron transport in Degulfovibrio vulgaris (Marburg) growing on hydrogen plus sulfate as sole energy source

Desulfovibrio vulgaris (Marburg) was grown on hydrogen plus sulfate as sole energy source in a medium containing excess iron. The topography of electron transport components was investigated. The bacterium contained per mg cells (dry weight) 30U hydrogenase (1U=1 mol/min), 35 g desulfoviridin (= bisulfite reductase), 0.6 U adenosine phosphosulfate reductase, 30 mU thiosulfate reductase, 0.3 nmol cytochrome c ₃ (M _r=13,000), 0.04 nmol cytochrome b, 0.85 nmol menaquinone, and 0.4 nmol ferredoxin. Hydrogenase (>95%) and cytochrome c ₃ (82%) were localized on the periplasmic side and desulfoviridin (95%), adenosine phosphosulfate reductase (87%), thiosulfate reductase (74%), and ferredoxin (71%) on the cytoplasmic side of the cytoplasmic membrane; menaquinone and cytochrome b were exlusively found in the membrane fraction. The location of the oxidoreductases indicate that in D. vulgaris (Marburg) H₂ oxidation and sulfate reduction take place on opposite sides of the cytoplasmic membrane rather than on the same side, as has recently been proposed.     

The pathway of electron transfer in the dimeric QH2: Cytochromec oxidoreductase

The experimental data currently available suggest that QH₂: cytochromec oxidoreductase functions according to a Q-cycle type of mechanism. The molecular weight of the enzyme in a natural or artificial phospholipid bilayer or in solution corresponds to that of a dimer. The pre-steady state kinetics of reduction of the prosthetic groups indicate that the enzyme is functionally dimeric. A double Q cycle is proposed, describing the pathway of electron transfer in the dimeric QH₂: cytochromec oxidoreductase. According to this scheme, the two monomeric halves of the enzyme act in a cooperative fashion to complete the catalytic cycle. It is proposed that high-potential cytochromeb-562 and low-potential cytochromeb-562 act cooperatively, viz. as a functional pair, in the antimycin-sensitive reduction of ubiquinone to ubiquinol.     

Cytochrome b5 reductase–cytochrome b5 as an active P450 redox enzyme system in Phanerochaete chrysosporium: Atypical properties and in vivo evidence of electron transfer capability to CYP63A2

Two central redox enzyme systems exist to reduce eukaryotic P450 enzymes, the P450 oxidoreductase (POR) and the cyt b₅ reductase–cyt b₅. In fungi, limited information is available for the cyt b₅ reductase–cyt b₅ system. Here we characterized the kinetic mechanism of (cyt b₅r)–cyt b₅ redox system from the model white-rot fungus Phanerochaete chrysosporium (Pc) and made a quantitative comparison to the POR system. We determined that Pc-cyt b₅r followed a “ping-pong” mechanism and could directly reduce cytochrome c. However, unlike other cyt b₅ reductases, Pc-cyt b₅r lacked the typical ferricyanide reduction activity, a standard for cyt b₅ reductases. Through co-expression in yeast, we demonstrated that the Pc-cyt b₅r–cyt b₅ complex is capable of transferring electrons to Pc-P450 CYP63A2 for its benzo(a)pyrene monooxygenation activity and that the efficiency was comparable to POR. In fact, both redox systems supported oxidation of an estimated one-third of the added benzo(a)pyrene amount. To our knowledge, this is the first report to indicate that the cyt b₅r–cyt b₅ complex of fungi is capable of transferring electrons to a P450 monooxygenase. Furthermore, this is the first eukaryotic quantitative comparison of the two P450 redox enzyme systems (POR and cyt b₅r–cyt b₅) in terms of supporting a P450 monooxygenase activity.     

Intermonomer electron transfer in the bc1 complex dimer is controlled by the energized state and by impaired electron transfer between low and high potential hemes

The cytochrome bc(1) complex (commonly called Complex III) is the central enzyme of respiratory and photosynthetic electron transfer chains. X-ray structures have revealed the bc(1) complex to be a dimer, and show that the distance between low potential (b(L)) and high potential (b(H)) hemes, is similar to the distance between low potential hemes in different monomers. This suggests that electron transfer between monomers should occur at the level of the b(L) hemes. Here, we show that although the rate constant for b(L)-->b(L) electron transfer is substantial, it is slow compared to the forward rate from b(L) to b(H), and the intermonomer transfer only occurs after equilibration within the first monomer. The effective rate of intermonomer transfer is about 2-orders of magnitude slower than the direct intermonomer electron transfer.     

Control of cytochrome b6f at low and high light intensity and cyclic electron transport in leaves

The light-dependent control of photosynthetic electron transport from plastoquinol (PQH₂) through the cytochrome b₆f complex (Cyt b₆f) to plastocyanin (PC) and P700 (the donor pigment of Photosystem I, PSI) was investigated in laboratory-grown Helianthus annuus L., Nicotiana tabaccum L., and naturally-grown Solidago virgaurea L., Betula pendula Roth, and Tilia cordata P. Mill. leaves. Steady-state illumination was interrupted (light-dark transient) or a high-intensity 10 ms light pulse was applied to reduce PQ and oxidise PC and P700 (pulse-dark transient) and the following re-reduction of P700⁺ and PC⁺ was recorded as leaf transmission measured differentially at 810-950 nm. The signal was deconvoluted into PC⁺ and P700⁺ components by oxidative (far-red) titration (V. Oja et al., Photosynth. Res. 78 (2003) 1-15) and the PSI density was determined by reductive titration using single-turnover flashes (V. Oja et al., Biochim. Biophys. Acta 1658 (2004) 225-234). These innovations allowed the definition of the full light response curves of electron transport rate through Cyt b₆f to the PSI donors. A significant down-regulation of Cyt b₆f maximum turnover rate was discovered at low light intensities, which relaxed at medium light intensities, and strengthened again at saturating irradiances. We explain the low-light regulation of Cyt b₆f in terms of inactivation of carbon reduction cycle enzymes which increases flux resistance. Cyclic electron transport around PSI was measured as the difference between PSI electron transport (determined from the light-dark transient) and PSII electron transport determined from chlorophyll fluorescence. Cyclic e⁻ transport was not detected at limiting light intensities. At saturating light the cyclic electron transport was present in some, but not all, leaves. We explain variations in the magnitude of cyclic electron flow around PSI as resulting from the variable rate of non-photosynthetic ATP-consuming processes in the chloroplast, not as a principle process that corrects imbalances in ATP/NADPH stoichiometry during photosynthesis.     

The effect of venturicidin on light and oxygen-dependent electron transport,proton translocation,membrane potential development and ATP synthesis in intact cells of Rhodopseudomonas capsulata

Venturicidin behaves as an orthodox energy transfer inhibitor in intact cells of Rhodopseudomonas capsulata as judged by the following criteria. 1. It led to inhibition of respiration. Inhibition was relieved by low concentrations of uncoupling agent. 2. It enhanced light-induced and oxygen dependent H⁺ efflux. 3. It stimulated light-induced and oxygen dependent carotenoid band shifts. The rate of decay of the band shifts after short flash excitation was decreased in the presence of venturicidin. 4. It stimulated light-induced and oxygen dependent butyltriphenylphosphonium uptake. 5. It inhibited the rise in cellular ATP concentration accompanying either photosynthesis or respiration.     

The effect of light quality and gibberellic acid (GA3) on photosynthesis and respiration rates of pea seedlings

CO₂ exchange were measured on pea seedlings (Pisum sativum L. var. Bördi) cultivated from seeds imbibed either in water (C-plants) or in gibberellic acid (GA₃) at the concentration of 25 g/1 (GA-plants), and then grown under 17 W/m² blue light (B-plants) or 11 W/m² red light (R-plants).When measured under the same light conditions as during growth the net photosynthesis (APS) rate in B-plants was about twice higher than that in R-plants. Dark respiration (DR) rate was 70% higher in B- than in R-plants. Red light retarded the development of photosynthetic activity, but GA₃ suppressed this effect. The hormone enhanced net photosynthesis and dark respiration to the same extent.When measured under saturating white light net photosynthesis rate of C-plants was also two times higher in B-plants than in R-plants. Growth conditions had only a slight effect on the APS of GA-plants under white light. APS rates of GA-plants grown under red light were higher under white light than those of C-plants, but lower than those of plants grown under blue light.We assume that blue light induced formation of plants that were adapted to higher light intensity: red light had an opposite effect, whereas gibberellic acid induced formation of plants that were adapted to medium light intensity.     

Domain movement of iron sulfur protein in cytochrome bc1 complex is facilitated by the electron transfer from cytochrome b(L) to b(H)

The key step of the "protonmotive Q-cycle" mechanism for cytochrome bc1 complex is the bifurcated oxidation of ubiquinol at the Qp site. ISP is reduced when its head domain is at the b-position and subsequent move to the c1 position, to reduce cytochrome c1, upon protein conformational changes caused by the electron transfer from cytochrome b(L) to b(H). Results of analyses of the inhibitory efficacy and the binding affinity, determined by isothermal titration calorimetry, of Pm and Pf, on different redox states of cytochrome bc1 complexes, confirm this speculation. Pm inhibitor has a higher affinity and better efficacy with the cytochrome b(H) reduced complex and Pf binds better and has a higher efficacy with the ISP reduced complex.     

The effects of chilling in the dark and in the light on photosynthesis of tomato: electron transfer reactions

Exposure of tomato plants (Lycopersicon esculentum Mill. cv. Floramerica) to chilling temperatures in the dark for as little as 12 h resulted in a sizable inhibition in the rate of light- and CO₂-saturated photosynthesis. However, when photosynthesis was measured at low light intensity, the inhibition disappeared and the quantum yield of CO₂ reduction was diminished only slightly. Chilling the tomato plants under strong illumination caused an even more rapid and severe decline in the rate of light- and CO₂-saturated photosynthesis, accompanied by a large decline in the quantum efficiency. Sizeable inhibition of photosystem II activity was observed only after dark exposures to low temperature of grater than 16 h. No inhibition of photosystem I electron transfer capacity was observed even after 40 h of dark chilling. Chilling under high light resulted in a rapid decline in both photosystem I and photosystem II electron transfer capacity as well as in significant reaction center inactivation.Regardless of whether the chilling exposure was in the presence or absence of illumination and regardless of its duration, the electron transfer capacity of thylakoid membranes isolated from the treated plants was always in excess of that necessary to support light- and CO₂-saturated photosynthesis. Thus, in neither case of chilling inhibition of photosynthesis does it appear that impaired electron transfer capacity represents a significant rate limitation to whole plant photosynthesis.Abbreviations BSA bovine serum albumin - DBMIB 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone - DCMU 3-(3,4-Dichlorophenyl)-1,1-dimethylurea - DHQ duroquinol - EDTA ethylene-diamine-tetraacetic acid - HEPES N-2-hydroxylpiperazine-N-2-ethanesulfonic acid - MES 2-(N-Morpholino)ethanesulfonic acid - MV methylviologen - PS I & II photosystems I and II - PD_OX p-phenylenediimine (oxidized) - TMPD N,N,N,N-tetramethyl-p-phenylenediamine     

Ammonia and light effect on nitrogenase activity in nitrogen-limited continuous cultures of Rhodopseudomonas capsulata. Role of glutamine synthetase

Nitrogen-limited continuous cultures of Rhodopseudomonas capsulata were used to investigate some aspects of the regulation of nitrogenase activity. The role of glutamine synthetase (GS) in this regulation was examined by measuring changes of its adenylylation state when the light intensity and the nitrogen source were varied. Maximal nitrogenase activity was observed at a dilution rate corresponding to about one third of the maximum specific growth rate (_max), both in ammonia- and in glutamate-limited cultures. At higher dilution rates, both GS and nitrogenase were inactivated by ammonia. Determination of the kinetics of inhibition of both enzymes indicated that the degree of inactivation of nitrogenase and the adenylylation state of GS were not closely related. Increase of light intensity stimulated nitrogenase activity dramatically. Conversely, a shift-down in light intensity to a limiting value resulted in a decrease of nitrogenase activity suggesting that synthesis was inhibited. On the other hand, the adenylylation state of glutamine synthetase appeared to be unaffected by changes in light intensity, indicating that GS is probably not involved in the regulation of nitrogenase expression by light.Abbreviations GS glutamine synthetase - R Rhodopseudomonas - Rs. Rhodospirillum - CTAB cetyltrimethylammonium bromide Dedicated to Prof. Dr. H. G. Schlegel on the occasion of his 60th birthday