Porphyrin Biosynthesis in Cell-free Homogenates from Higher Plants

The porphyrin and phorbin biosynthetic activity of etiolated cucumber (Cucumis sativus, L.) cotyledons was compared to that of cotyledonary homogenates. Etiolated cotyledons incubated with δ-aminolevulinic acid accumulate protoporphyrin, coproporphyrin, small amounts of Mg protoporphyrin monoester, and trace amounts of uroporphyrin. They also incorporate 4-¹⁴C-δ-aminolevulinic acid into free porphyrins, protochlorophyllide, protochlorophyllide phytyl ester, and Mg protoporphyrin monoester. Homogenates incubated with δ-aminolevulinic acid likewise accumulate coproporphyrin, uroporphyrin, Mg coproporphyrin, and trace amounts of protoporphyrin. They also incorporate 4-¹⁴C-δ-aminolevulinic acid into Mg protoporphyrin monoester, Mg coproporphyrin, and free porphyrins. However, the capacity to synthesize protochlorophyllide and protochlorophyllide phytyl ester is lost and the endogenous protochlorophylls gradually disappear. Mg protoporphyrin monoester represents the terminal biosynthetic step in this cell-free system.     

Investigation of the porphyrins accumulation in barley leaves incubated with Mn cations and δ-aminolevulinic acid

In greening etiolated primary leaves of barley (Hordeum vulgare L.), Mn²⁺ ions have been shown to inhibit chlorophyll (Chl) accumulation in a dose dependent manner and to lead to an accumulation of protoporphyrin IX (Proto) and Mg-protoporphyrin IX monomethyl ester (MgPE). The amount of MgPE that accumulated, was 2 times higher than Proto. In the dark, Proto and MgPE were observed to have accumulated to high levels in seven-day old green and etiolated leaves in the presence of 5 mmol/L Mn²⁺, but only if 5 mmol/L δ-aminolevulinic acid (ALA) was present. The 24 hours of irradiation of the green barley leaves treated in this way, resulted in a photodynamic destruction of Proto and MgPE as well as of Chl and carotenoids (Car). The observed porphyrin accumulation caused by the Mn²⁺ ions was reversed in the presence of active iron (Fe²⁺). This effect was observed when the iron concentration in incubation solutions was half the Mn²⁺ concentration, most effective for porphyrin synthesis, i.e. 5 mmol/L. The action of Mn²⁺ on porphyrin accumulation is also discussed.     

Induction of porphyrin synthesis in etiolated bean leaves by chelators of iron

Primary leaves of 7- to 9-day-old etiolated seedlings of Phaseolus vulgaris L. var. Red Kidney infiltrated in darkness with aqueous solutions of alpha, alpha'-dipyridyl, o-phenanthroline, pyridine-2-aldoxime, pyridine-2-aldehyde, 8-hydroxyquinoline, or picolinic acid synthesize large amounts of magnesium protoporphyrin monomethyl ester and lesser amounts of magnesium protoporphyrin, protoporphyrin, and protochlorophyllide. Pigment formation proceeds in a linear manner for up to 21 hours after vacuum infiltration with 10 mm alpha, alpha'-dipyridyl. Etiolated tissues of Zea mays L., Cucumis sativus L., and Pisum sativum L. respond in the same way to dipyridyl treatment. Compounds active in eliciting this response are aromatic heterocyclic nitrogenous bases which also act as bidentate chelators and form extremely stable complexes with iron; other metal ion chelators, such as ethylenediaminetetraacetic acid, salicylaldoxime, and sodium diethyldithiocarbamate, do not elicit any pigment synthesis. The ferrous, ferric, cobaltous, and zinc chelates of alpha, alpha'-dipyridyl are similarly ineffective. If levulinic acid is supplied to etiolated bean leaves together with alpha, alpha'-dipyridyl, porphyrin production is inhibited and delta-aminolevulinic acid accumulates in the tissue. Synthesis of porphyrins proceeds in the presence of 450 micrograms per milliliter chloramphenicol or 50 micrograms per milliliter cycloheximide with only partial diminution. We propose that heme or an iron-protein complex blocks the action of the enzyme(s) governing the synthesis of delta-aminolevulinic acid in etiolated leaves in the dark and that iron chelators antagonize this inhibition, leading to the biosynthesis of delta-aminolevulinic acid and porphyrins.     

Accumulation of photodynamic tetrapyrroles induced by acifluorfen-methyl

Treatment with acifluorfen-methyl (AFM), methyl 5-(2-chloro-4-[tri-fluoromethyl] phenoxy)-2-nitrobenzoate, inhibited protochlorophyllide synthesis in dark-held, δ-amino levulinic acid-fed, excised cotyledons of cucumber (Cucumis sativus L.). Protochlorophyllide and protoporphyrin IX levels in AFM-treated cotyledons were inversely related and dependent on AFM concentration; as the herbicide dose increased, protoporphyrin IX levels also increased with a concomitant loss of protochlorophyllide. Significant protoporphyrin IX accumulation was induced by concentrations of AFM from the linear region of the membrane disruption dose response curve. The pattern of precursor accumulation seen in HPLC chromatograms from extracts of AFM-treated tissue indicate that Mg insertion into the tetrapyrrole ring is inhibited, suggesting interference with Mg-chelatase. An inhibitor of δ-amino levulinic acid synthesis, gabaculine (3-amino-2,3-dihydrobenzoic acid), completely blocked the membrane disruption activity of AFM in illuminated cotyledons. Protoporphyrin IX accumulating in AFM-treated tissues may serve as the primary photosensitizer for initiating lipid peroxidation.     

Ferrochelatase of Rhodopseudomonas spheroides

Extracts of Rhodopseudomonas spheroides contain two ferrochelatases: one is soluble and forms metalloporphyrins from deuteroporphyrin and haematoporphyrin; the other is particulate and forms metalloporphyrins from protoporphyrin, mesoporphyrin, deuteroporphyrin and haematoporphyrin. Neither enzyme incorporates Mg²⁺ into porphyrins or Fe²⁺ into porphyrin cytochrome c. By using the particulate enzyme, plots of 1/v versus 1/s when one substrate was varied and the other kept constant showed that neither substrate affected the K_m of the other. The suggested sequential mechanism for the reaction is supported by derivative plots of slopes and intercepts. The K_m for deuteroporphyrin was 21.3μm and that for Co²⁺ was 6.13μm. The enzyme incorporated Co²⁺, Fe²⁺, Zn²⁺, Ni²⁺ and Mn²⁺; Cd²⁺ was not incorporated and was an inhibitor, competitive with respect to Co²⁺, non-competitive with respect to deuteroporphyrin. The K_i for Cd²⁺ was 0.73μm. Ferrochelatase was inhibited by protohaem, non-competitively with respect to Co²⁺ or with respect to deuteroporphyrin. Inhibition by magnesium protoporphyrin was non-competitive with respect to deuteroporphyrin, uncompetitive with respect to Co²⁺. The inhibitory concentrations of the metalloporphyrins are lower than those required for the inhibition of δ-aminolaevulate synthetase by protohaem. Fe²⁺ is not incorporated aerobically into porphyrins unless an electron donor, succinate or NADH, is supplied; the low aerobic rate of metalloporphyrin synthesis obtained is insensitive to rotenone and antimycin. The rate of Fe³⁺ incorporation increases as anaerobic conditions are achieved.     

Amyloid β-Mediated Zn2+ Influx into Dentate Granule Cells Transiently Induces a Short-Term Cognitive Deficit

We examined an idea that short-term cognition is transiently affected by a state of confusion in Zn²⁺ transport system due to a local increase in amyloid-β (Aβ) concentration. A single injection of Aβ (25 pmol) into the dentate gyrus affected dentate gyrus long-term potentiation (LTP) 1 h after the injection, but not 4 h after the injection. Simultaneously, 1-h memory of object recognition was affected when the training was performed 1 h after the injection, but not 4 h after the injection. Aβ-mediated impairments of LTP and memory were rescued in the presence of zinc chelators, suggesting that Zn²⁺ is involved in Aβ action. When Aβ was injected into the dentate gyrus, intracellular Zn²⁺ levels were increased only in the injected area in the dentate gyrus, suggesting that Aβ induces the influx of Zn²⁺ into cells in the injected area. When Aβ was added to hippocampal slices, Aβ did not increase intracellular Zn²⁺ levels in the dentate granule cell layer in ACSF without Zn²⁺, but in ACSF containing Zn²⁺. The increase in intracellular Zn²⁺ levels was inhibited in the presence of CaEDTA, an extracellular zinc chelator, but not in the presence of CNQX, an AMPA receptor antagonist. The present study indicates that Aβ-mediated Zn²⁺ influx into dentate granule cells, which may occur without AMPA receptor activation, transiently induces a short-term cognitive deficit. Extracellular Zn²⁺ may play a key role for transiently Aβ-induced cognition deficits.     

Chloroplast Biogenesis: XXIV. Intrachloroplastic Localization of the Biosynthesis and Accumulation of Protoporphyrin IX, Magnesium-Protoporphyrin Monoester, and Longer Wavelength Metalloporphyrins during Greening

The intraplastidic localization of the endogenous metabolic pools from protoporphyrin to protochlorophyll was determined in Cucumis sativus. The endogenous protoporphyrin, Mg-protoporphyrin monoester + longer wavelength metalloporphyrins, protochlorophyllide and protochlorophyllide ester were membrane-bound. Protoporphyrin was synthesized in the stroma and subsequently became associated with the membranes. The membrane-associated protoporphyrin was then converted into Mg-protoporphyrin monoester + longer wavelength metalloporphyrins by membrane-bound enzymes. Although lysed plastids were capable of converting exogenous δ-aminolevulinic acid to protochlorophyllide, the net synthesis of protochlorophyllide from exogenous δ-aminolevulinic acid was lost upon segregating the lysed plastids into stromal and membrane fractions and then recombining the stromal and membrane fraction prior to incubation. The segregated membrane fraction was still capable of converting protoporphyrin into Mg-protoporphyrin monoester + longer wavelength metalloporphyrins in the presence or absence of the stromal fraction. These results indicated that although the reactions from protoporphyrin to Mg-protoporphyrin monoester and longer wavelength metalloporphyrins could survive a considerable degree of plastid disruption, the reactions from Mg-protoporphyrin monoester and longer wavelength metalloporphyrins to protochlorophyllide were more sensitive to structural disorganization.     

Leaf magnesium alters photosynthetic response to low water potentials in sunflower

We grew sunflower (Helianthus annuus L.) plants in nutrient solutions having nutritionally adequate but low or high Mg²⁺ concentrations and determined whether photosynthesis was effected as leaf water potentials (ψ_w) decreased. Leaf Mg contents were 3- to 4-fold higher in the plants grown in high Mg²⁺ concentrations (10 millimolar) than in those grown in low concentrations (0.25 millimolar). These contents were sufficient to support maximum growth, plant dry weight, and photosynthesis, and the plants appeared normal. As low ψ_w developed, photosynthesis was inhibited but moreso in high Mg leaves than in low Mg leaves. The effect was particularly apparent under conditions of light- and CO₂-saturation, indicating that the chloroplast capacity to fix CO₂ was altered. The differential inhibition observed in leaves of differing Mg contents was not observed in leaves having differing K contents, suggesting that the effect may have been specific for Mg. Because Mg²⁺ inhibits photophosphorylation and coupling factor activities at concentrations likely to occur as leaves dehydrate, Mg may play a role in the inhibition of chloroplast reactions at low ψ_w, especially in leaves such as sunflower that markedly decrease in water content as ψ_w decreases.     

Magnetic isotope and magnetic field effects on the DNA synthesis

Magnetic isotope and magnetic field effects on the rate of DNA synthesis catalysed by polymerases β with isotopic ions ²⁴Mg²⁺, ²⁵Mg²⁺ and ²⁶Mg²⁺ in the catalytic sites were detected. No difference in enzymatic activity was found between polymerases β carrying ²⁴Mg²⁺ and ²⁶Mg²⁺ ions with spinless, non-magnetic nuclei ²⁴Mg and ²⁶Mg. However, ²⁵Mg²⁺ ions with magnetic nucleus ²⁵Mg were shown to suppress enzymatic activity by two to three times with respect to the enzymatic activity of polymerases β with ²⁴Mg²⁺ and ²⁶Mg²⁺ ions. Such an isotopic dependence directly indicates that in the DNA synthesis magnetic mass-independent isotope effect functions. Similar effect is exhibited by polymerases β with Zn²⁺ ions carrying magnetic ⁶⁷Zn and non-magnetic ⁶⁴Zn nuclei, respectively. A new, ion–radical mechanism of the DNA synthesis is suggested to explain these effects. Magnetic field dependence of the magnesium-catalysed DNA synthesis is in a perfect agreement with the proposed ion–radical mechanism. It is pointed out that the magnetic isotope and magnetic field effects may be used for medicinal purposes (trans-cranial magnetic treatment of cognitive deceases, cell proliferation, control of the cancer cells, etc).     

Probing the Structural Basis of Zn2+ Regulation of the Epithelial Na+ Channel

Extracellular Zn²⁺ activates the epithelial Na⁺ channel (ENaC) by relieving Na⁺ self-inhibition. However, a biphasic Zn²⁺ dose response was observed, suggesting that Zn²⁺ has dual effects on the channel (i.e. activating and inhibitory). To investigate the structural basis for this biphasic effect of Zn²⁺, we examined the effects of mutating the 10 extracellular His residues of mouse γENaC. Four mutations within the finger subdomain (γH193A, γH200A, γH202A, and γH239A) significantly reduced the maximal Zn²⁺ activation of the channel. Whereas γH193A, γH200A, and γH202A reduced the apparent affinity of the Zn²⁺ activating site, γH239A diminished Na⁺ self-inhibition and thus concealed the activating effects of Zn²⁺. Mutation of a His residue within the palm subdomain (γH88A) abolished the low-affinity Zn²⁺ inhibitory effect. Based on structural homology with acid-sensing ion channel 1, γAsp⁵¹⁶ was predicted to be in close proximity to γHis⁸⁸. Ala substitution of the residue (γD516A) blunted the inhibitory effect of Zn²⁺. Our results suggest that external Zn²⁺ regulates ENaC activity by binding to multiple extracellular sites within the γ-subunit, including (i) a high-affinity stimulatory site within the finger subdomain involving His¹⁹³, His²⁰⁰, and His²⁰² and (ii) a low-affinity Zn²⁺ inhibitory site within the palm subdomain that includes His⁸⁸ and Asp⁵¹⁶.     

Ferrochelatase activity inAzospirillum brasilense with reference to the influence of metal cations

Summary Ferrochelatase in membrane preparations fromAzospirillum brasilense displayed an activity of 2.17 mol protoheme formed · h^–1 · mg protein^–1 which is 10-fold greater than previous reports for other bacteria. This ferrochelatase showed an apparentK _m of 20.9 M for Fe²⁺, a pH optimum of 6.0–6.5, and stimulation by oleic or stearic acids. Co²⁺, Cu²⁺ and Zn²⁺ inhibited the incorporation of Fe²⁺ into protoporphyrin IX while Ni² and Mg²⁺ had no effect on protoheme synthesis. Activity with Fe²⁺ and mesoporphyrin IX was less than with protoporphyrin IX but deuteroporphyrin IX produced the highest rate of protoheme synthesis. The membrane fraction containing ferrochelatase activity was found to insert Cu²⁺, Ni²⁺, Zn²⁺ and Co²⁺ enzymatically into protoporphyrin IX to produce metalloporphyrins. Cu²⁺ incorporation into protoporphyrin IX proceeded at a rate greater than with Fe²⁺ and theK _m for Cu²⁺ was 21.9 M.     

The sites of photoconversion of protochlorophyllide to chlorophyllide in barley seedlings

The photoreduction of protochlorophyllide a to chlorophyllide a in intact 6-day-old seedlings of etiolated barley (Hordeum vulgare) exhibits a small initial phase, followed by an induction period of about 1 hour before a rapid phase of additional chlorophyll formation begins. Cycloheximide, an inhibitor of protein synthesis, has no effect on the initial phase of conversion of preformed protochlorophyllide, but it either abolishes or severely inhibits the subsequent phase of rapid chlorophyll synthesis within 45 minutes of its application to the seedlings. An analysis of the biphasic inhibition process suggests that the lifetime of the enzyme controlling protochlorophyllide synthesis (probably δ-amino-levulinic acid synthetase) is not longer than 10 minutes.     

Studies on the regeneration of protochlorophyllide after brief illumination of etiolated bean leaves

The effects of various inhibitors of nucleic acid and protein synthesis on protochlorophyllide synthesis in dark-grown Phaseolus vulgaris var. Red Kidney have been studied. Actinomycin D, chloramphenicol, and puromycin inhibit the regeneration of protochlorophyllide holochrome (detected as a 650 mμ absorption peak) in vivo in darkness after photoconversion of endogenous protochlorophyllide a to chlorophyllide a; this inhibition does not occur in similarly treated leaves supplied with δ-aminolevulinic acid.

These data suggest that the regeneration of protochlorophyllide results from the synthesis of RNA and enzymes required for the production of δ-aminolevulinate.

     

Chloroplast biogenesis. Biosynthesis and accumulation of protochlorophyll by isolated etioplasts and developing chloroplasts.

Etioplasts and developing chloroplasts were isolated from etiolated Cucumis cotyledons that were irradiated with white fluorescent light for various periods of time. The endogenous porphyrins and phorbins of the isolated plastids were partitioned between hexane, hexane-extracted aqueous acetone and a lipoprotein precipitate. Spectrofluorometric determinations were performed on these fractions without further fractionation. For quantitative determinations, the fluorescence amplitudes of the various fluorescent components were corrected for fluorescence emission overlap by sets of simultaneous equations. Developing chloroplasts contained endogenous amounts of the following metabolites: Protochlorophyllide, protochlorophyllide ester, Mg-protoporphyrin monoester + longer-wavelength metalloporphyrins and protoporphyrin. The protochlorophyll pool consisted mainly of protochlorophyllide. The latter was heterogeneous and consisted of at least two chemically related protochlorophyllides. In contrast to developing chloroplasts, irradiated etioplasts contained mostly protochlorophyllide ester and smaller amounts of protochlorophyllide. Upon incubation of developing chloroplasts and irradiated etioplasts with δ-aminolevulinic acid and cofactors (coenzyme A, glutathione, adenosine triphosphate, nicotinamide adenine dinucleotide, methyl alcohol, magnesium, potassium and phosphate), a net synthesis and accumulation of protochlorophyllide, Mg-protoporphyrin monoester + longer-wavelength metalloporphyrins, protoporphyrin, coproporphyrin and uroporphyrin were observed. Small amounts of zinc-coproporphyrin and zinc-uroporphyrin were also formed. In some experiments a net synthesis of protochlorophyllide ester was also observed. This report represents the first account of the unambiguous net synthesis of protochlorophyll in vitro.     

Glucose and δ-Aminolevulinic Acid Stimulate the Dark Chlorophyll Synthesis of Rice Seedlings

This research was to examine if rice (Oryza sativa L.), a monocotyledon of angiosperm, was able to synthesize chlorophyll (Chl) in complete darkness. Five-cm-tall etiolated seedlings of rice were used as starting materials and treated with or without various concentrations of glucose and/or δ-aminolevulinic acid (ALA) in the dark. Leaves harvested at the indicated time were determined for their contents of Chl, protoporphyrin Ⅸ(Proto), Mg-protoporphyrin Ⅸ(Mg-Proto) and protochlorophyllide (Pchlide). The mole percentage of porphyrin was calculated. The Chl content in the etiolated rice seedlings slightly increased from about 2.5 μg/g to 7.5 μg/g within 12 d in the dark, but the total Chl of dark-grown rice increased from 0.36 μg/g to 3.6 μg/g. While the mole percentages of Proto, Mg-Proto and Pchlide in the dark-grown seedlings without any treatment were about 65%, 27.5% and 7.5% at the beginning, respectively, those in the light-grown seedlings were about 42.5%, 35% and 22.5%, respectively. The mole percentage of porphyrin of etiolated seedlings resumed its normal ratio within 2 d after treatment with glucose. While the Chl content of etiolated seedlings grown in culture solution with 3% and 6% glucose increased 2.5 and 4.0 folds, respectively, those with 3% and 6% glucose and 1 mmol/L ALA increased 22 and 24 folds, respectively. It is concluded that angiosperm might be able to synthesize a small amount of Chl in complete darkness, that either glucose or ALA could stimulate dark Chl synthesis in angiosperm, and that a combination of glucose and ALA exhibited an additional effect. It is still unknown and remains to be further explored what is the mechanism of the effect of glucose and ALA on the Chl synthesis of rice in the dark. Key words: angiosperm; rice; dark chlorophyll synthesis; glucose; δ-aminolevulinic acid; protoporphyrin Ⅸ; Mg-protoporphyrin Ⅸ; protochlorophyllide     

Effect of Cadmium on gamma-Glutamylcysteine Synthesis in Maize Seedlings

Cysteine, γ-glutamylcysteine, and glutathione and the extractable activity of the enzymes of glutathione biosynthesis, γ-glutamylcysteine synthetase (EC 6.3.2.2) and glutathione synthetase (EC 6.3.2.3), were measured in roots and leaves of maize seedlings (Zea mays L. cv LG 9) exposed to CdCl₂ concentrations up to 200 micromolar. At 50 micromolar Cd²⁺, γ-glutamylcysteine contents increased continuously during 4 days up to 21-fold and eightfold of the control in roots and leaves, respectively. Even at 0.5 micromolar Cd²⁺, the concentration of γ-glutamylcysteine in the roots was significantly higher than in the control. At 5 micromolar and higher Cd²⁺ concentrations, a significant increase in γ-glutamylcysteine synthetase activity was measured in the roots, whereas in the leaves this enzyme activity was enhanced only at 200 micromolar Cd²⁺. Labeling of isolated roots with [³⁵S]sulfate showed that both sulfate assimilation and glutathione synthesis were increased by Cd. The accumulation of γ-glutamylcysteine in the roots did not affect the root exudation rate of this compound. Our results indicate that maize roots are at least in part autonomous in providing the additional thiols required for phytochelatin synthesis induced by Cd.     

Induction of delta-Aminolevulinic Acid Formation in Etiolated Maize Leaves Controlled by Two Light Systems

A short illumination of etiolated maize (Zea mays) leaves with red light causes a protochlorophyll(ide)-chlorophyll(ide) conversion and induces the synthesis of δ-aminolevulinic acid (ALA) during a subsequent dark period. In leaves treated with levulinic acid, more ALA is formed in the dark than in control leaves. Far red light does not cause a conversion of protochlorophyll(ide) into chlorophyll(ide) and does not induce accumulation of ALA in the dark. Both red and far red preilluminations cause a significant potentiation of ALA synthesis during a period of white light subsequent to the dark period. The results indicate a dual light control of ALA formation. The possible role of phytochrome and protochlorophyllide as photoreceptors in this control system is discussed.     

Metabolism of Magnesium Protoporphyrin Monomethyl Ester in Chlamydomonas reinhardtii

The y-1 mutant of Chlamydomonas reinhardtii is defective in the conversion of protochlorophyllide (Pchlide) to chlorophyllide in the dark. Aerobic δ-aminolevulinic acid (ALA) feeding of y-1 cells causes protoporphyrin monomethyl ester (PME) to accumulate in addition to increased levels of Pchlide. y-1 cell homogenates are not capable of methylating protoporphyrin (PROTO) to form PME but can methylate magnesium protoporphyrin (MgP) to form magnesium protoporphyrin monomethyl ester (MgPME). Anaerobic ALA feeding of y-1 causes concomitant accumulation of PME and MgPME. y-1 cells treated with α,α′-dipyridyl (DP) accumulate MgPME but not PROTO or PME. A mutant strain (bme) of Chlamydomonas has been isolated which has very little chlorophyll and accumulates PME. bme Cell homogenates can methylate MgP but not PROTO. We propose that: (a) in Chlamydomonas, PME is the initial breakdown product of MgPME; (b) both the breakdown of MgPME to PME and the conversion of MgPME to Pchlide require O₂; (c) the breakdown of MgPME to PME appears to require Fe; and (d) the PME accumulated in the bme mutant is the result of an increased breakdown of MgPME.     

Controls on chlorophyll synthesis in barley

In 7- to 10-day-old leaves of etiolated barley (Hordeum vulgare), all of the enzymes that convert δ-aminolevulinic acid to chlorophyll are nonlimiting during the first 6 to 12 hours of illumination, even in the presence of inhibitors of protein synthesis. The limiting activity for chlorophyll synthesis appears to be a protein (or proteins) related to the synthesis of δ-aminolevulinic acid, presumably δ-aminolevulinic acid synthetase. Protein synthesis in both the cytosol and plastids may be required to produce nonlimiting amounts of δ-aminolevulinic acid. The half-life of a limiting protein controlling the synthesis of δ-aminolevulinic acid appears to be about 1½ hours, when determined with inhibitors of protein synthesis. Acceleration of chlorophyll synthesis by light is not inhibited by inhibitors of nucleic acid synthesis, but is inhibited by inhibitors of protein synthesis. A model for control of chlorophyll synthesis is proposed, based on a light-induced activation at the translational level of the synthesis of proteins forming δ-aminolevulinic acid, as well as the short half-life of these proteins. Evidence is presented confirming the idea that the holochrome on which protochlorophyllide is photoreduced to chlorophyllide functions enzymatically.