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PubMed2004

Bidirectional electron transfer in photosystem I: accumulation of A0- in A-side or B-side mutants of the axial ligand to chlorophyll A0.

Photosystem I contains two potential electron transfer pathways between P(700) and F(X). These branches are made up of the electron transfer chain components A, A(0), and A(1). The primary electron acceptor A(0) is a chlorophyll a monomer that could be one or both of the two chlorophyll molecules, eC-A(3)/eC-B(3), identified in the 2.5 A resolution structure. The eC-A(3)/eC-B(3) chlorophylls are both coordinated by the sulfur atom of a methionine. This coordination is highly unusual, as interactions between the acid Mg(2+) and the soft base sulfur are weak. The eC-A(3)/eC-B(3) chlorophylls also are located close to one of the connecting chlorophylls that may link the antenna and the electron transfer chain chlorophylls. Due to their location in the structure, the eC-A(3)/eC-B(3) chlorophylls may play a role in both excitation energy transfer and electron transfer. To test the role of the eC-A(3)/eC-B(3) chlorophylls in electron transfer, Met-684 of PsaA and Met-664 of PsaB have been changed to His, Ser, and Leu. Replacement of either M(A684) or M(B664) results in a significant alteration in growth phenotype. The His and Leu mutants are very light sensitive in the presence of oxygen. Growth is impaired to a greater extent in the B-side mutants. However, all of the mutants are able to grow anaerobically at comparable rates. The His and Ser mutants all accumulate PSI at a level similar to that of wild type, whereas the Leu mutants have reduced amounts of PSI. Ultrafast transient absorbance measurements show that the (A(0)(-) - A(0)) difference signal accumulates in the MH(A684) and MH(B664) mutants under neutral conditions, demonstrating that electron transfer between A(0)(-) and A(1) is blocked or significantly slowed. The results show that both the A-branch and the B-branch of the ETC are active in PSI from Chlamydomonas reinhardtii.

Animals↗
PubMed1998

Chlorophyll a availability affects psbA translation and D1 precursor processing in vivo in Synechocystis sp. PCC 6803.

Transcript accumulation and translation of psbA as well as processing of the D1 precursor protein were investigated in relation to chlorophyll availability in vivo in cyanobacterial strains lacking photosystem I (PS I). The psbA transcript level was almost independent of chlorophyll availability and was approximately 3-fold lower in darkness than in continuous light (5 microE m-2 s-1). Upon illumination, it reached a steady-state level within several hours. Upon growth under light-activated heterotrophic growth conditions (LAHG) in the PS I-less strain, D1 synthesis occurred immediately upon illumination. However, in PS I-less/chlL- cells, which lacked the light-independent chlorophyll biosynthesis pathway and had very low chlorophyll levels after LAHG growth, very little D1 synthesis occurred upon illumination, and the synthesis rate increased with time. This result suggests a translational control of D1 biosynthesis related to chlorophyll availability. Upon illumination, initially a high level of the nonprocessed D1 precursor was observed by pulse labeling and immunodetection in LAHG-grown PS I-less/chlL- cells but not in PS I-less cells. A significant amount of the D1 precursor eventually was processed to mature D1, and the half-life of the D1 precursor decreased as the chlorophyll content of the cells increased. The D1 processing enzyme CtpA was found to be present at similar levels regardless illumination or chlorophyll levels. We conclude that, directly or indirectly, chlorophyll availability is needed for D1 translation as well as for efficient processing of the D1 precursor.

Blotting, Western↗
PubMed1997

Light-harvesting chlorophyll a/b-binding protein stably inserts into etioplast membranes supplemented with Zn-pheophytin a/b.

Light-harvesting chlorophyll a/b-binding protein, LHCP, or its precursor, pLHCP, cannot be stably inserted into barley etioplast membranes in vitro. However, when these etioplast membranes are supplemented with the chlorophyll analogs Zn-pheophytin a/b, synthesized in situ from Zn-pheophorbide a/b and digeranyl pyrophosphate, pLHCP is inserted into a protease-resistant state. This proves that chlorophyll is the only component lacking in etioplast membranes that is necessary for stable LHCP insertion. Synthesis of Zn-pheophytin b alone promotes insertion of LHCP in vitro into a protease-resistant state, whereas synthesis of Zn-pheophytin a alone does not. Insertion of pLHCP into etioplast membranes can also be stimulated by adding chlorophyll a and chlorophyll b to the membranes, albeit at a significantly lower efficiency as compared with Zn-pheophytin a/b synthesized in situ. When pLHCP is inserted into chlorophyll- or Zn-pheophytin-supplemented etioplast membranes and then assayed with protease, only the protease digestion product indicative of the monomeric major light-harvesting chlorophyll a/b complex (LHCII) is found but not the one indicating trimeric complexes. In this respect, chlorophyll- or Zn-pheophytin-supplemented etioplast membranes resemble thylakoid membranes at an early greening stage: pLHCP inserted into plastid membranes from greening barley is assembled into trimeric LHCII only after more than 1 h of greening.

Chlorophyll↗
PubMed1991

Complex formation between chlorophyll a and cytochrome c: surface properties at the air-water interface. Absorbance, fluorescence and fluorescence-lifetime in Langmuir-Blodgett films.

The binding of cytochrome c to an insoluble monolayer of chlorophyll a was studied. Surface pressure (II), surface potential (delta V) and [14C]cytochrome c surface-concentration (gamma) isotherms were measured versus molecular area (sigma) in mixed films. Compared to the successive-addition method, this procedure allows the formation of homogeneous mixed films. The cytochrome c is incorporated into a chlorophyll a monolayer, compressed at a surface pressure of 20 mN.m-1. On expansion, the quantity of protein incorporated into the monolayer gradually increases. Subsequent compression-expansion cycles result in similar isotherms, distinct from that measured during the first expansion. All surface properties measured, but more specifically the surface radioactivity of [14C]cytochrome c, indicate the irreversibility of protein incorporation into the chlorophyll a monolayer. In fact, surface properties of the binary film are completely different from the properties of either of the pure components. As a result, calculated values of surface potentials for mixed films using the additivity law deviate from experimentally measured potentials. The absorption and fluorescence spectra of mixed films transferred onto a solid substrate by the Langmuir-Blodgett technique, indicate a dilution effect of chlorophyll a by cytochrome c. However, the dilution effect cannot be detected by the fluorescence lifetimes of pure chlorophyll a and mixed chlorophyll a-cytochrome c films, both shorter than 0.2 ns. This provides support for the existence of an energy-transfer mechanism between chlorophyll a monomer and chlorophyll a aggregates which could serve as an energy trap. The role of the protein could be related to that of the matrix.

Amino Acids↗
PubMed1992

Dodecylpyridinium alkanoates stabilize dispersed chlorophyll.

In the presence of a surfactant that does not ligate Mg, chlorophyll is adsorbed to polyethylene particles swollen with tetradecane principally as the infrared-absorbing, highly polymeric species Chl 740. Examples of such surfactants are quaternary ammonium salts and soaps. However, when surfactants of opposite charge are present together, in this case dodecylpyridinium iodide and Na butyrate or myristate, chlorophyll may exist entirely in a dispersed form absorbing around 666 nm. Absorption and fluorescence spectral data show that much of the dispersed pigment is still associated, but as dimeric and perhaps short oligomeric species. It is concluded from fluorescence lifetime analysis that most of the observed fluorescence comes from a small population of chlorophyll that is probably monomeric and isolated; fluorescence of more highly associated species decays with a wide range of lifetimes. The capacity for photochemical sensitization of these particles with dispersed chlorophyll is similar to that of particles with ligating surfactants examined earlier. Structures are suggested for chlorophyll association in which Mg is ligated by water hydrogen-bonded to a carboxylate group, while the dodecylpyridinium counterion lies near the chlorophyll ring. The effect of the two surfactants is synergistic. Overall, spectra of dispersed chlorophyll adsorbates resemble most closely those of colloidal chlorophyll suspensions prepared by dilution of solutions in polar organic solvents with water.

Chlorophyll↗
PubMed1999

Effect of dietary chlorophyll derivatives on mutagenesis and tumor cell growth.

Much attention in recent years has been given to the antigenotoxicity of chlorophyll. Chlorophyll, however, is known to be converted into pheophytin, pyropheophytin, and pheophorbide in processed vegetable food and following ingestion by humans. Studies were conducted on the antimutagenic and tumoricidal potencies of these compounds. All the chlorophyll derivatives tested exhibit identical antimutagenic effect towards 3-methylcholanthrene (3-MC), suggesting that the porphyrin nucleus may complex directly with the mutagen. It does not exclude, however, another mechanism of activity involving inactivation the enzymatic transformation of 3-MC. In contrast, the action of N'-nitro-N'-nitrosoguanidine (MNNG) depends upon structural differences between the chlorophyll derivatives. It is significantly lower when the phytol-containing pheophytin and pyropheophytin are tested as to that of the phytol-lacking pheophorbide. The higher concentrations of the chlorophyll derivatives were required to reduce the mutagenicity of MNNG than needed for 3-MC. The cytotoxicity of chlorophyll derivatives against tumor cells also was evaluated. The cellular uptake and inhibition of myeloma cell multiplicity were found to be greater for pheophorbide than for pheophytin. Calculated on the amount of cell associated chlorophyll derivative, however, pheophytin was more cytostatic/cytotoxic than pheophorbide. The results presented in this report indicate that food sources that yield chlorophyll derivatives may play a significant role in cancer prevention. Teratogenesis Carcinog. Mutagen. 19:313-322, 1999.

Animals↗
PubMed1975

Energy transfer by chlorophyll a in detergent micelles.

The process of energy transfer was studied in the chlorophyll a-containing detergent micelle, serving as a possible model of the photosynthetic unit. Chlorophyll a was added to aqueous solutions of the detergent Triton X-100 and incorporated into the micelles. The energy transfer process was studied by investigating the concentration depolarization of fluorescence of chlorophyll a. On the basis of the experimental depolarization curves as well as the value of the Förster parameter Ro equals to 56 A calculated from the overlap of absorption and fluorescence spectra it was concluded that energy transfer between chlorophyll a molecules in this model follows the Förster-type mechanism of inductive resonance. Furthermore it was found that the local concentration of chlorophyll a in the micelles is higher by 1-3 orders of magnitude than its overall concentration in the solution and by choosing the appropriate ratio between the consentration of chlorophyll a and the detergent it is possible to reach the in vivo chlorophyll concentration of 0.1 M withing the micelles. Thus the chlorophyll-detergent micelle model may be applied as a model of the separate package-type photosynthetic unit.

Binding Sites↗
PubMed1975

Energy transfer by chlorophyll beta in detergent micelles.

The concentration-dependent depolarization, concentration-dependent quenching, absorption and fluorescence spectra in solutions of chlorophyll beta-containing detergent micelles with Triton X-100 were studied in a concentration range of c equal to 0.4 muM-0.6mM chlorophyll beta and cd equal to 0.4-7.0 mM Triton X-100. The concentration-dependent depolarization obeys Förster's theory of depolarization of fluorescence with a transfer distance parameter R0 equal to 43 plus or minus 2 A. The concentration-dependent quenching is described by an empirical formula for the relative fluorescence yield n/n0 equal to 1/[1+(c/c1/2)-2] given by Kelly and Porter (Kelly A. R. and Porter, G. (1970) Proc. R. Soc. Lond. Ser. A. 315, 149-161). With increasing chlorophyll beta concentration the red absorption band at 650 nm is shifted toward a longer wavelength and its width increases by 10nm, the intensity of the long wave fluorescence band increases about 720 nm. The results analysed in terms of these findings lead to the conclusions that chlorophyll beta molecules are (a) locally concentrated in the micelles up to the concentration range of in vivo conditions, (b) partly in an aggregated state capable for fluorescence, (c) the chlorophyll beta yields chlorophyll beta homotransfer may be about 3-26% of the homotransfer chlorophyll alpha yields chlorophyll-alpha depending on the ratio of their concentrations.

Chlorophyll↗
PubMed1977

Electron transport between plastoquinone and chlorophyll Ai in chloroplasts. II. Reaction kinetics and the function of plastocyanin in situ.

The light-induced reaction kinetics of electron carriers between the two light reactions were studied in spinach chloroplasts. 1. The difference spectrum of the absorbance changes of plastocyanin in situ was separated from superimposing absorbance changes by flash titration described in the preceding paper (Haehnel, W. (1973) Biochim. Biophys. Acta 305, 618-631). Relative amounts of 2 : 1 electron equivalents were observed for plastocyanin and chlorophyll a1 (P-700). 2. A balance of the electron equivalents released from reduced plastoquinone and simultaneously accepted by oxidized plastocyanin, cytochrome f and chlorophyll a1 indicated a quantitative electron transfer. Additional electron carriers between plastoquinone and light reaction I can be excluded with an accuracy of about +/-0.3 electron equivalents per light reaction II. 3. The time course of the absorbance changes of plastocyanin was measured at 584 nm with negligible interference with other absorbance changes. The reduction kinetics show an initial lag followed by a rise with a half time of about 20 ms. The redox states of plastocyanin and chlorophyll a1 during this reduction via the rate-limiting step between the light reactions and during oxidation by weak far-red light suggest a true equilibrium constant of about 20. 4. The simultaneous oxidation and reduction kinetics of plastoquinone, cytochrome f, plastocyanin and chlorophyll a1 induced by two successive groups of saturating flashes after far-red illumination were measured. The oxidation kinetics of plastocyanin and the simultaneous reduction kinetics of chlorophyll a1 after the single flashes indicate a quantitative electron transfer with a half time of 200 mus. 5. The fast reduction of chlorophyll a1 by plastocyanin showed no effect of the inhibitors 3-(3',4'-dichlorophenyl)-1,1-dimethylurea and 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone or of reduced phenazine methosulfate. But it was completed inhibited after KCN incubation. 6. The oxidation kinetics of cytochrome f were reinvestigated with high time resolution from the difference of absorbance changes at 554 minus 540 nm to minimize the disturbing interference with other absorbance changes. Absorbance changes measured 554 nm alone do not reflect kinetics of cytochrome f. The half time of the oxidation was faster than 40 microns. 7. The observed reaction kinetics gave evidence for a function of cytochrome f between plastoquinone and chlorophyll a1 in parallel to plastocyanin. In addition, they indicate that the greater portion of linear electron transport passes plastocyanin. The complex interaction between cytochrome f and chlorophyll a1 is discussed.

Chlorophyll↗
PubMed1977

Effect of linocmycin on the chlorophyll protein complex I content and photosystem I activity of greening leaves.

1. Greening barley and pea leaves treated with lincomycin have a reduced chlorophyll content. Lincomycin dose not alter the proportion of chlorophyll in chlorophyll-protein complex II (CPII) but greatly reduces that in chlorophyll-protein complex I (CPI). 2. Difference spectra show that chloroplasts from lincomycin-treated leaves are deficient in at least two long wavelength forms of chlorophyll ratio of chloroplasts is unaffected by lincomycin but the photochemical P-700/chlorophyll ratio is less than half of that of the control. It is less affected than the chlorophyll-protein complex I content. 4. Photosystem I activity expressed on a chlorophyll basis is unaffected by linocomycin but the light intensity for half saturation is increased 8-fold. 5. Chlorophyll-protein complex I apoprotein content is reduced by lincomycin. No evidence was found for an accumulation of its precursor(s). The relative abundance of major peptides of 18 000, 15 000 and 12 000 daltons in lincomycin-treated chloroplasts is attributed to a general inhibition of greening and associated membrane of formation.

Chlorophyll↗
PubMed1980

The P-700-chlorophyl alpha-protein complex and two major light-harvesting complexes of Acrocarpia paniculata and other brown seaweeds.

Acrocarpia paniculata thylakoids were fragmented with Triton X-100 and the pigment-protein complexes so released were isolated by sucrose density gradient centrifugation. Three main chlorophyll-carotenoid-protein complexes with distinct pigment compositions were isolated. (1) A P-700-chlorophyll a-protein complex, with a ratio of 1 P-700: 38 chlorophyll a: 4 beta-carotene molecules, had similar absorption and fluorescence characteristics to the chlorophyll-protein complex 1 isolated with Triton X-100 from higher plants, green algae and Ecklonia radiata. (2) an orange-brown complex had a chlorophyll a : c2 : fucoxanthin molar ratio of 2 : 1 : 2. this complex had no chlorophyll c1 and contained most of the fucoxanthin present in the chloroplasts. This pigment complex is postulated to be the main light-harvesting complex of brown seaweeds. (3) A green complex had a chlorophyll a : c1 : c2 : violaxanthin molar ratio of 8 : 1 : 1. This also is a light-harvesting complex. the absorption and fluorescence spectral characteristics and other physical properties were consistent with the pigments of these three major complexes being bound to protein. Differential extraction of brown algal thylakoids with Triton X-100 showed that a chlorophyll c2-fucoxanthin-protein complex was a minor pigment complex of these thylakoids.

Carotenoids↗
PubMed2001

Characterization of peroxidase-mediated chlorophyll bleaching in strawberry fruit.

Peroxidase (POX) from strawberry fruits was analyzed for its capacity to bleach chlorophyll. The partially purified enzyme preperation catalyzed the bleaching of chlorophylls and their derivatives in the presence of H(2)O(2) and phenolic compounds. The optimal reaction conditions were 35 degrees C, pH 5.2 and ionic strength equal to 0.2. The maximum activity was observed at 1 mM of H(2)O(2), while higher concentrations inhibited enzyme activity. Compounds with a high affinity to the heme group, radical scavengers and reducing agents, showed an inhibitory effect. Phenolic compounds such as umbelliferone, naringenin and p-substituted monophenols acted as cofactors. Instead, other phenolic compounds tested such as caffeic acid, catechin, ellagic acid, esculin and quercetin inhibited the activity of POX on chlorophylls. Phenolic compounds extracted from strawberry fruits showed an inhibitory effect on POX-chlorophyll bleaching activity, although this effect decreased markedly during ripening. POX showed higher affinity for compounds derived from chlorophyll a than from chlorophyll b, and the enzyme preferentially degraded chlorophyll derivatives with the Mg(2+) ion present and the phytol group removed. The POX-chlorophyll bleaching activity was found in all ripening stages from small green to ripe, the highest activity corresponding to large green fruits.

Chlorophyll↗
PubMed1999

Orientation of chlorophyll transition moments in the higher-plant light-harvesting complex CP29.

The Q(y) transition dipole moment vectors of all eight chlorophylls in the higher-plant antenna protein CP29 were calculated by an original method on the basis of linear dichroism and absorption spectroscopy. The contribution of individual chromophores was determined from difference spectra between wild type and mutant proteins in which a single chlorophyll has been removed by mutating pigment-binding residues. Recombinant proteins were constructed by overexpressing the apoprotein in bacteria and refolding of the pigment-protein complex in vitro [Bassi, R., Croce, R., Cugini, D., and Sandonà, D. (1999) Proc. Natl. Acad. Sci. U.S.A. (in press)]. The spectroscopic data are interpreted on the basis of a protein structural model obtained via the homology with the major antenna complex LHCII [Kuhlbrandt, W., Wang, D. N., and Fujiyoshi, Y. (1994) Nature 367, 614-621]. The results allow us to determine the orientation of six chromophores within the protein structure. The orientations of the two remaining chromophores are inferred by considering the symmetry properties of CP29 and fitting steady state absorption and linear dichroism spectra by independent chlorophyll spectral forms. As a consequence, four "mixed" sites with different chlorophyll a and b binding affinities are identified in CP29. Geometrical data and the Förster mechanism for energy transfer suggest that excitation energy equilibrates rapidly among chlorophyll "pure" sites while energy preferentially flows outward from chlorophyll "mixed" sites. The orientation of the dipole moments of two chlorophyll molecules symmetrically located at the center of the protein and parallel to the carotenoid transition vectors suggests a role in energy transfer from xanthophyll to chlorophyll.

Amino Acid Sequence↗
PubMed1971

Electron spin resonance of chlorophyll and the origin of signal I in photosynthesis.

A comparison has been made between Signal I, the photo-electron spin resonance signal associated with the primary light conversion act in photosynthesis, and free-radical signals generated in various chlorophyll species in vitro. The esr signals obtained from chlorophyll.monomer, (Chl.L)(+.), chlorophyll dimer, (Chl(2))(+.), and chlorophyll oligomer, (Chl(2))(n) (+.), are broader than Signal I, whereas the chlorophyll-water adduct, (Chl.H(2)O)(n) (+.), gives a signal very much narrower than Signal I. The unusually narrow signal from (Chl.H(2)O)(n) (+.) has been ascribed to spin migration, or to unpaired spin delocalization over a large number of chlorophyll molecules. The linewidth of Signal I can be accounted for by a similar delocalization process. A theoretical relationship between the esr linewidth and the number of chlorophyll molecules, N, over which an unpaired spin is delocalized, takes the form DeltaH(N) = 1/ radicalN.DeltaH(M), where DeltaH(M) is the linewidth of monomer (Chl.L)(+.). This relationship for N = 2 accounts well for the linewidths of Signal I in green algae, blue-green algae, and photosynthetic bacteria in both the (1)H- and (2)H-forms. The linewidth of Signal I (as well as the optical properties of reaction-center chlorophyll) are consistent with unpaired spin delocalization over an entity containing two chlorophyll molecules, (Chl.H(2)O.Chl)(+.).

Chlorophyll↗
PubMed2006

Structural role of (bacterio)chlorophyll ligated in the energetically unfavorable beta-position.

Chlorophyll is attached to apoprotein in diastereotopically distinct ways, by beta- and alpha-ligation. Both the beta- and alpha-ligated chlorophylls of photosystem I are shown to have ample contacts to apoprotein within their proteinaceous binding sites, in particular, at C-13 of the isocyclic ring. The H-bonding patterns for the C-13(1) oxo groups, however, are clearly distinct for the beta-ligated and alpha-ligated chlorophylls. The beta-ligated chlorophylls frequently employ their C-13(1) oxo in H-bonds to neighboring helices and subunits. In contrast, the C-13(1) oxo of alpha-ligated chlorophylls are significantly less involved in H-bonding interactions, particularly to neighboring helices. Remarkably, in the peripheral antenna, light harvesting complex (LH2) from Rhodobacter sphaeroides, a single mutation in the alpha-subunit, introduced to eliminate H-bonding to the beta-bacteriochlorophyll-B850, which is ligated in the "beta-position," results in significant thermal destabilization of the LH2 in the membrane. In addition, in comparison with wild type LH2, the expression level of the LH2 lacking this H-bond is significantly reduced. These findings show that H-bonding to the C-13(1) keto group ofbeta-ligated (bacterio)-chlorophyll is a key structural motif and significantly contributes to the stability of bacteriochlorophyll proteins in the native membrane. Our analysis of photosystem I and II suggests that this hitherto unrecognized motif involving H-bonding to beta-ligated chlorophylls may be equally critical for the stable assembly of the inner core antenna of these multicomponent chlorophyll proteins.

Amino Acid Motifs↗
PubMed1952

The fluorescence spectra of red algae and the transfer of energy from phycoerythrin to phycocyanin and chlorophyll.

1. The fluorescence spectra of the alga Porphyridium have been recorded as energy distribution curves for eleven different incident wave lengths of monochromatic incident light between wave lengths 405 and 546 mmicro. 2. In these spectra chlorophyll fluorescence predominates when the incident light is in the blue part of the spectrum which is strongly absorbed by chlorophyll. 3. For blue-green and green light the spectrum excited in Porphyridium contains in addition to chlorophyll fluorescence, the fluorescence bands characteristic of phycoerythrin and of phycocyanin. 4. From these spectra the approximate curves for the fluorescence of the individual pigments phycoerythrin, phycocyanin, and chlorophyll in the living material have been derived and the relative intensity of each of them has been obtained for each of the eleven incident wave lengths. 5. The effectiveness spectrum for the excitation of the fluorescence of these three pigments in vivo has been plotted. 6. From comparisons of the effectiveness spectrum for the excitation of each of these pigments it appears that both phycocyanin and chlorophyll receive energy from light which is absorbed by phycoerythrin. 7. It is suggested that phycocyanin may be an intermediate in the resonance transfer of energy from phycoerythrin to chlorophyll. 8. Since phycoerythrin and phycocyanin transfer energy to chlorophyll, it appears probable that chlorophyll plays a specific chemical role in photosynthesis in addition to acting as a light absorber.

Chlorophyll↗
PubMed1989

Inhibitory effect of chlorophyll on the genotoxicity of 3-amino-1-methyl-5H-pyrido[4,3-b]indole (Trp-P-2).

Genotoxicity of 3-amino-1-methyl-5H-pyrido[4,3-b]indole (Trp-P-2) on Drosophila was suppressed by chlorophyll. Using the wing hair spot test, we found that the formation of mutant hairs in adult flies as a result of feeding them with Trp-P-2 in their larval stage was efficiently inhibited by coadministration of chlorophyll. The decrease in the spot frequencies was dependent on the dose of chlorophyll, and at the highest dose used, where the ratio in weight of Trp-P-2 to chlorophyll was 1:80, a complete prevention of the small single-spot formation was observed. A similar inhibitory effect was detected for chlorophyllin, the chromophore of chlorophyll. In the studies to investigate the mechanism of inhibition, we observed that the mutagenicity of 3-hydroxy-amino-1-methyl-5H-pyrido[4,3-b]indole [Trp-P-2(NHOH)], the metabolically activated form of Trp-P-2, in Salmonella typhimurium TA98 was suppressed effectively with chlorophyll and chlorophyllin. We also found that chlorophyll and chlorophyllin can produce complexes with Trp-P-2 and Trp-P-2(NHOH). A straightforward mechanism of these inhibitions is that Trp-P-2 [and Trp-P-2(NHOH)] becomes no longer available to organisms on forming the chlorophyll complex.

Animals↗
PubMed1997

Phytochrome control of phototropism and chlorophyll accumulation in the apical cells of protonemal filaments of wildtype and an aphototropic mutant of the moss Ceratodon purpureus.

The aphototropic mutant line ptr116 of the moss Ceratodon purpureus shows characteristics of a deficiency in the phytochrome chromophore. Photoreversibility measurements indicate an approximately 20 time lower concentration of spectrally active phytochrome compared to wild-type, whereas normal phytochrome apoprotein levels are found on immunoblots. Feeding with the tetrapyrroles biliverdin, the proposed precursor of the phytochrome chromophore, or phycocyanobilin, which may replace the phytochrome chromophore, resulted in the rescue of ptr116 phototropism. The ptr116 mutant and the phenotypically-related mutant ptr1 contain lower chlorophyll levels than the wild-type. Chlorophyll content of wildtype and mutant tissue grown under different light conditions was estimated using conventional spectrophotometry of extracts and fluorimetrically, on single apical cells. Dark-grown tissue contained about 100 times less chlorophyll than tissue grown under standard white light conditions. Red light given for 24 h to dark adapted filaments induced an increase in the chlorophyll content in the wildtype, but not in ptr116. Blue light induced an increase in chlorophyll both in wildtype and in ptr116. The red light effect on the wildtype was partially reversible with far-red. If ptr116 was grown on phycocyanobilin, an increase in chlorophyll was also found when cells were irradiated with red light. The results indicate that phytochrome as well as a blue light photoreceptor regulate chlorophyll accumulation in C. purpureus protonemata. It can be assumed that in ptr116, the synthesis of the phytochrome chromophore is blocked specifically beyond the synthesis common to chlorophyll and the phytochrome chromophore and affects an enzymatic step between protoporphyrin and biliverdin.

Bryopsida↗