Nucleases are upregulated in potato tubers afflicted with zebra chip disease

The defense response of potato tubers afflicted with zebra chip disease involves oxidatively mediated upregulation of nucleases that likely modulate localized programmed cell death to restrict the phloem-mobile, CLso bacterial pathogen to the vasculature. Zebra chip (ZC) is a bacterial disease of potato (Solanum tuberosum L.) caused by Candidatus Liberibacter solanacearum (CLso). Tubers from infected plants develop characteristic brown discoloration of the vasculature, a result of localized programmed cell death (PCD). We examined the potential contribution of nucleases in the response of tubers to CLso infection. Specific activities of the major isozymes of dsDNase, ssDNase, and RNase were substantially upregulated in tubers from CLso-infected plants, despite their significantly lower soluble protein content. However, ZC disease had no effect on nuclease isozyme profiles. Activities of the predominant nuclease isoforms from healthy and CLso-infected tubers had similar pH optima, thermotolerance, and responses to metallic co-factors. Nuclease activities were heat stable to 60 °C and resistant to precipitation with 70% (v/v) isopropanol, which constitute effective techniques for partial purification. DNase and RNase isozyme activities were highest at pH 7.2-8.5 and 6.8-7.2, respectively, and profiles were similar for tubers from CLso-infected and non-infected plants. RNase activities were mostly insensitive to inhibition by EDTA, except at pH 8.5 and above. DNase activities were inhibited by EDTA but less sensitive to inhibition at high pH than the RNases. The EDTA-mediated inhibition of DNase (ds/ss) activities was restored with ZnSO₄, but not Ca⁺² or Mg⁺². By contrast, ZnSO₄ inhibited the activities of RNases. DTT and CuSO₄ inhibited the activities of all three nucleases. These results suggest that activation of tuber nucleases is dependent on the oxidation of sulfhydryl groups to disulfide and/or oxidation of Zn to Zn⁺². In light of previous published results that established extensive CLso-induced upregulation of oxidative stress metabolism in tubers, we propose a model to show how increased nuclease activity could result from a glutathione-mediated oxidation of nuclease sulfhydryl groups in diseased tubers. DNases and RNases are likely an integral part of the hypersensitive response and may modulate PCD to isolate the pathogen to the vascular tissues of tubers.

Enzymic and mechanistic studies on the conversion of glutamate to 5-aminolaevulinate

Higher plants, algae, cyanobacteria and several other photosynthetic and non-photosynthetic bacteria synthesize 5-aminolaevulinate by a tRNA(Glu)-mediated pathway. Glutamate is activated at the alpha-carboxyl by ligation to tRNA(Glu) with an aminoacyl-tRNA synthetase. An NADPH-dependent reductase converts glutamyl-tRNA(Glu) to glutamate 1-semialdehyde, which is finally converted to 5-aminolaevulinate by an aminotransferase. These components are soluble and in plants and algae are located in the chloroplast stroma. In plants and algae the tRNA(Glu) is encoded in chloroplast DNA whereas the enzymes are encoded in nuclear DNA. The tRNA(Glu) has a hypermodified 5-methylaminomethyl-2-thiouridine-pseudouridine-C anticodon and probably plays a role in the light-dark regulation of 5-aminolaevulinate synthesis. Ligation of glutamate to tRNA(Glu) requires ATP and Mg2+ and proceeds via a ternary intermediate. Glutamyl-tRNA(Glu) reduction appears to involve formation of a complex. Glutamate 1-semialdehyde non-enzymically synthesized by reductive ozonolysis from gamma-vinyl GABA is used as substrate by the last enzyme. Glutamate-1-semialdehyde aminotransferase contains pyridoxal phosphate as a prosthetic group. The enzyme is converted to spectrally different forms by treatment with 4,5-diaminovalerate or 4,5-dioxovalerate. The pyridoxamine 5'-phosphate form of the enzyme converts (S)-glutamate 1-semialdehyde to 5-aminolaevulinate via 4,5-diaminovalerate through a bi-bi ping-pong mechanism.

Supplements of transgenic malt or grain containing (1,3-1,4)-beta-glucanase increase the nutritive value of barley-based broiler diets to that of maize

1. A diet with addition to normal barley of malt from transgenic barley expressing a protein engineered, thermotolerant Bacillus (1,3-1,4)-beta-glucanase during germination has previously been demonstrated to provide a broiler chicken weight gain comparable to maize diets. It also reduced dramatically the number of birds with adhering sticky droppings, but did not entirely eliminate sticky droppings. One of the objectives of the broiler chicken trials reported here was to determine if higher concentrations of transgenic malt could alleviate the sticky droppings. 2. Another aim was to investigate the feasibility of using mature transgenic grain containing the thermotolerant (1,3-1,4)-beta-glucanase as feed addition and to compare diets containing transgenic grain to a diet with the recommended amount of a commercial beta-glucanase-based product. 3. Inclusion of 75 or 151 g/kg transgenic malt containing 4.7 or 98 mg/kg thermotolerant (1,3-1,4)-beta-glucanase with 545 or 469 g/kg non-transgenic barley instead of maize yielded a weight gain in Cornish Cross broiler chickens indistinguishable from presently used maize diets. The gene encoding the enzyme is expressed in the aleurone with a barley alpha-amylase gene promoter and the enzyme is synthesised with a signal peptide for secretion into the endosperm of the malting grain. 4. Equal weight gain was achieved, when the feed included 39 g/kg transgenic barley grain [containing 66 mg/kg thermotolerant (1,3-1,4)-beta-glucanase] and 581 g/kg non-transgenic barley instead of maize. In this case, the gene encoding the enzyme has been expressed with the D-hordein gene (Hor3-1) promoter during grain maturation. The enzyme is synthesised as a precursor with a signal peptide for transport through the endoplasmic reticulum and targeted into the storage vacuoles. Deposition of the enzyme in the prolamin storage protein bodies of the endosperm protects it from degradation during the programmed cell death of the endosperm in the final stages of grain maturation and provides extraordinary heat stability. The large amount of highly active (1,3-1,4)-beta-glucanase in the mature grain allowed the reduction of the transgenic grain ingredient to 0.2 g/kg diet, thus making the ingredient comparable to that of the trace minerals added to standard diets. 5. A direct comparison using transgenic grain supplement at the level of 1 g/kg of feed with the standard recommended addition of the commercial enzyme preparation Avizyme 1100 at 1 g/kg yielded equal weight gain, feed consumption and feed efficiency in birds fed a barley-based diet. 6. The production of sticky droppings characteristic of broilers fed on barley diets was avoided with all 9 experimental diets and reduced to the level observed with a standard maize diet by supplementation with transgenic barley. 7. The excellent growth and normal survival of the 400 broilers tested on barley diets supplemented with transgenic grain or malt showed the grain and malt not to be toxic. 8. The barley feed with added transgenic grain or malt containing thermotolerant (1,3-1,4)-beta-glucanase provides an environmentally friendly alternative to enzyme additives, as it uses photosynthetic energy for production of the enzyme in the grain and thus avoids use of non-renewable energy for fermentation. The deposition of the enzyme in the protein bodies of the grain in the field makes coating procedures for stabilisation of enzyme activity superfluous. 9. Barley feed with the small amount of transgenic grain as additive to normal barley provides an alternative for broiler feed in areas where grain maize cannot be grown for climatic reasons or because of unsuitable soil and thus has to be imported.

Improved barley broiler feed with transgenic malt containing heat-stable (1,3-1,4)-beta-glucanase

The low nutritional value of barley for poultry is because of the absence of an intestinal enzyme for efficient depolymerization of (1, 3-1,4)-beta-glucan, the major polysaccharide of the endosperm cell walls. This leads to high viscosity in the intestine, limited nutrient uptake, decreased growth rate, and unhygienic sticky droppings adhering to chickens and floors of the production cages. Consequently, the 7.5 billion broiler chickens produced annually in the United States are primarily raised on corn-soybean diets. Here we show that addition to normal barley of 6.2% transgenic malt containing a thermotolerant (1,3-1,4)-beta-glucanase (4.28 microg.g(-1) soluble protein) provides a weight gain equivalent to corn diets. The number of birds with adhering sticky droppings is drastically reduced. Intestines and excrements of chickens fed the barley control diet contained large amounts of soluble (1,3-1,4)-beta-glucan, which was reduced by 75 and 50%, respectively, by adding transgenic malt to the diet. The amount of active recombinant enzyme in the small intestine corresponded to that present in the feed, whereas an 11-fold concentration of the enzyme was observed in the ceca, and a 7.5-fold concentration occurred in the excrement. Glycosylation of the beta-glucanase isolated from the ceca testified to its origin from the transgenic barley. Analysis of the data from this trial demonstrates the possibility of introducing individual recombinant enzymes into various parts of the gastrointestinal tract of chickens with transgenic malt and thereby the possibility of evaluating their effect on the metabolism of a given ingredient targeted by the enzyme.

The production of recombinant proteins in transgenic barley grains

The grain of the self-pollinating diploid barley species offers two modes of producing recombinant enzymes or other proteins. One uses the promoters of genes with aleurone-specific expression during germination and the signal peptide code for export of the protein into the endosperm. The other uses promoters of the structural genes for storage proteins deposited in the developing endosperm. Production of a protein-engineered thermotolerant (1, 3-1, 4)-beta-glucanase with the D hordein gene (Hor3-1) promoter during endosperm development was analyzed in transgenic plants with four different constructs. High expression of the enzyme and its activity in the endosperm of the mature grain required codon optimization to a C+G content of 63% and synthesis as a precursor with a signal peptide for transport through the endoplasmic reticulum and targeting into the storage vacuoles. Synthesis of the recombinant enzyme in the aleurone of germinating transgenic grain with an alpha-amylase promoter and the code for the export signal peptide yielded approximately 1 microgram small middle dotmg(-1) soluble protein, whereas 54 microgram small middle dotmg(-1) soluble protein was produced on average in the maturing grain of 10 transgenic lines with the vector containing the gene for the (1, 3-1, 4)-beta-glucanase under the control of the Hor3-1 promoter.

Predicted structure and fold recognition for the glutamyl tRNA reductase family of proteins

The conserved residues of glutamyl tRNA reductase (GTR) from Hordeum vulgare (GTRhorvu) were found from an alignment/pile-up of 24 homologous sequences found using BLAST searches. A multiple alignment of sequences was used to obtain a prediction of the secondary structure of the GTR's. This secondary structure was submitted to the THREADER program to find possible homologous 3D structures. To help select the template for predicting the fold for GTRhorvu, we employed both molecular-biological and biochemical information about GTRhorvu. After fitting the secondary structure of GTRhorvu to the selected template, the MODELLER program was used to determine the fold for GTRhorvu. This model was built using the B subunit of succinyl CoA synthetase, 1scuB, as a template for the 3D structure of GTRhorvu. From the predicted structure, possible regions were identified for the binding of glutamyl-tRNA, NADPH and a heme inhibitor. The predicted structure was used to propose a detailed biochemical mechanism for the GTR, involving Mg catalyzed thioester formation and reduction by NADPH to glutamate-1-semialdehyde. Sites for these reactions are identified. The predicted structure has been deposited in the Brookhaven database as ID 1b61.

Molecular basis for semidominance of missense mutations in the XANTHA-H (42-kDa) subunit of magnesium chelatase

During biosynthesis of bacteriochlorophyll or chlorophyll, three protein subunits of 140, 70, and 42 kDa interact to insert Mg2+ into protoporphyrin IX. The semidominant Chlorina-125, -157, and -161 mutants in barley are deficient in this step and accumulate protoporphyrin IX after feeding on 5-aminolevulinate. Chlorina-125, -157, and -161 are allelic to the recessive xantha-h mutants and contain G559A, G806A, and C271T mutations, respectively. These mutations cause single amino acid substitutions in residues that are conserved in all known primary structures of the 42-kDa subunit. In vitro complementation and reconstitution of Mg-chelatase activity show that the 42-kDa subunits are defective in the semidominant Chlorina mutants. A mutated protein is maintained in the Chlorina plastids, unlike in the xantha-h plastids. Heterozygous Chlorina seedlings have 25-50% of the Mg-chelatase activity of wild-type seedlings. Codominant expression of active and inactive 42-kDa subunits in heterozygous Chlorina seedlings is likely to produce two types of heterodimers between the strongly interacting 42-kDa and 70-kDa subunits. Reduced Mg-chelatase activity is explained by the capacity of heterodimers consisting of mutated 42-kDa and wild-type 70-kDa protein to bind to the 140-kDa subunit. The 42-kDa subunit is similar to chaperones that refold denatured polypeptides with respect to its ATP-to-ADP exchange activity and its ability to generate ATPase activity with the 70-kDa subunit. We hypothesize that the association of the 42-kDa subunit with the 70-kDa subunit allows them to form a specific complex with the 140-kDa subunit and that this complex inserts Mg2+ into protoporphyrin IX.

Distribution of ATPase and ATP-to-ADP phosphate exchange activities in magnesium chelatase subunits of Chlorobium vibrioforme and Synechocystis PCC6803

Insertion of magnesium into protoporphyrin IX is a complex ATP-dependent reaction catalysed by the enzyme Mg-chelatase. Three separate proteins (Mg-chelatase subunits), designated as D, H and I, are involved in the chelation reaction. The genes encoding the Mg-chelatase subunits of the green sulfur bacterium Chlorobium vibrioforme and of the cyanobacterium Synechocystis strain PCC6803 were expressed in Escherichia coli. The recombinant proteins were purified, tested for ATPase and phosphate exchange activities, and compared with the activities of the corresponding subunits of Rhodobacter sphaeroides. The Synechocystis strain PCC6803 I subunit and the C. vibrioforme H and I subunits hydrolysed ATP at the rates of 2.0, 1.8 and 0.16 nmol (mg protein)-1 min-1, respectively. The ATPase activity of the C. vibrioforme H subunit was similar to that reported for the R. sphaeroides H subunit. The Synechocystis strain PCC6803 H subunit failed to hydrolyse ATP. The I subunit of Synechocystis strain PCC6803 and C. vibrioforme catalysed a transfer of PO4 from ATP to ADP (exchange activity) at the rate of 1.75 +/- 0.15 nmol (mg protein)-1 min-1. This exchange rate was 300-fold lower than that reported for the R. sphaeroides I subunit. The PO4 exchange activities were correlated with the presence of the sequence GXRGTGKSTXVRALA in the primary structure of the three I subunits. Mg-chelatase activity was reconstituted by combining the three subunits of the same bacterium [rates of 41-89 pmol Mg-deuteroporphyrin (mg protein)-1 min-1]. Heterologous subunit combinations resulted in low or no Mg-chelatase activity.

Barley glutamyl tRNAGlu reductase: mutations affecting haem inhibition and enzyme activity

Glutamyl tRNA(Glu) reductase converts glutamate molecules that are ligated at their alpha-carboxyl groups to tRNA(Glu) into glutamate 1-semialdehyde, an intermediate in the synthesis of 5-aminolevulinate, chlorophyll and haem. The mature plant enzymes contain a highly conserved extension of 31-34 amino acids at the N-terminus not present in bacterial enzymes. It is shown that barley glutamyl tRNAGlu reductases with a deletion of the 30 N-terminal amino acids have the same high specific activity as the untruncated enzymes, but are highly resistant to feed-back inhibition by haem. This peptide domain thus interacts directly or indirectly with haem and the toxicity of the 30 amino acid peptide for Escherichia coli experienced in mutant rescue and overexpression experiments can be explained by extensive haem removal from the metabolic pools that cannot be tolerated by the cell. Induced missense mutations identify nine amino acids in the 451 residue long C-terminal part of the barley glutamyl tRNA(Glu) reductase which upon substitution curtail drastically, but do not eliminate entirely the catalytic activity of the enzyme. These amino acids are thus important for the catalytic reaction or tRNA binding.

ATPases and phosphate exchange activities in magnesium chelatase subunits of Rhodobacter sphaeroides

Three separate proteins, BchD, BchH, and BchI, together with ATP, insert magnesium into protoporphyrin IX. An analysis of ATP utilization by the subunits revealed the following: BchH catalyzed ATP hydrolysis at the rate of 0.9 nmol per min per mg of protein. BchI and BchD, tested individually, had no ATPase activity but, when combined, hydrolyzed ATP at the rate of 117.9 nmol/min per mg of protein. Magnesium ions were required for the ATPase activities of both BchH and BchI+D, and these activities were inhibited 50% by 2 mM o-phenanthroline. BchI additionally catalyzed a phosphate exchange reaction from ATP and ADP. We conclude that ATP hydrolysis by BchI+D is required for an activation step in the magnesium chelatase reaction, whereas ATPase activity of BchH and the phosphate exchange activity of BchI participate in subsequent reactions leading to the insertion of Mg2+ into protoporphyrin IX.

Isolated Bacillus subtilis HemY has coproporphyrinogen III to coproporphyrin III oxidase activity

Oxidation of coproporphyrinogen III to coproporphyrin III is found in extracts of Escherichia coli cells containing the Bacillus subtilis HemY protein (M. Hansson and L. Hederstedt, J. Bacteriol. 176, 5962-5970). We have analysed whether this activity is due to the heterologous expression system, since it in vivo would lead to disruption of the heme biosynthetic pathway. B. subtilis hemY was fused in its 3'-end to a polynucleotide encoding six histidine residues and expressed from plasmids in both E. coli and B. subtilis. The His6-tagged HemY protein extracted from membranes using non-ionic detergent was purified by Ni2+ affinity chromatography. Isolated HemY fusion protein synthesised in E. coli and B. subtilis oxidised coproporphyrinogen III to coproporphyrin III. No direct formation of protoporphyrin IX from coproporphyrinogen III could be detected. Our results suggest that the coproporphyrinogen III to coproporphyrin III activity of HemY is either avoided in B. subtilis in vivo or that coproporphyrin III is a heme biosynthetic intermediate in this bacterium.

Magnesium chelatase: association with ribosomes and mutant complementation studies identify barley subunit Xantha-G as a functional counterpart of Rhodobacter subunit BchD

Magnesium chelatase catalyses the insertion of Mg2+ into protoporphyrin and is found exclusively in organisms which synthesise chlorophyll or bacteriochlorophyll. Soluble protein preparations containing >10 mg protein/ml, obtained by gentle lysis of barley plastids and Rhodobacter sphaeroplasts, inserted Mg2+ into deuteroporphyrin IX in the presence of ATP at rates of 40 and 8 pmoles/mg protein per min, respectively. With barley extracts optimal activity was observed with 40 mM Mg2+. The activity was inhibited by micromolar concentrations of chloramphenicol. Mutations in each of three genetic loci, Xantha-f, -g and -h, in barley destroyed the activity. However, Mg-chelatase activity was reconstituted in vitro by combining pairwise the plastid stroma protein preparations from non-leaky xantha-f -g and -h mutants. This establishes that, as in Rhodobacter, three proteins are required for the insertion of magnesium into protoporphyrin IX in barley. These three proteins, Xantha-F, -G and -H, are referred to as Mg-chelatase subunits and they appear to exist separate from each other in vivo. Active preparations from barley and Rhodobacter yielded pellet and supernatant fractions upon centrifugation for 90 min at 272,000 x g. The pellet and the supernatant were inactive when assayed separately, but when they were combined activity was restored. Differential distribution of the Mg-chelatase subunits in the fractions was established by in vitro complementation assays using stroma protein from the xantha-f, -g, and -h mutants. Xantha-G protein was confined to the pellet fraction, while Xantha-H was confined to the supernatant. Reconstitution assays using purified recombinant BchH, BchI and partially purified BchD revealed that the pellet fraction from Rhodobacter contained the BchD subunit. The pellet fractions from both barley and Rhodobacter contained ribosomes and had an A260:A280 ratio of 1.8. On sucrose density gradients both Xantha-G and BchD subunits migrated with the plastid and bacterial ribosomal RNA, respectively.

Expression of catalytically active barley glutamyl tRNAGlu reductase in Escherichia coli as a fusion protein with glutathione S-transferase

delta-Aminolevulinate in plants, algae, cyanobacteria, and several other bacteria such as Escherichia coli and Bacillus subtilis is synthesized from glutamate by means of a tRNA(Glu) mediated pathway. The enzyme glutamyl tRNA(Glu) reductase catalyzes the second step in this pathway, the reduction of tRNA bound glutamate to give glutamate 1-semialdehyde. The hemA gene from barley encoding the glutamyl tRNA(Glu) reductase was expressed in E. coli cells joined at its amino terminal end to Schistosoma japonicum glutathione S-transferase (GST). GST-glutamyl tRNA(Glu) reductase fusion protein and the reductase released from it by thrombin digestion catalyzed the reduction of glutamyl tRNA(Glu) to glutamate 1-semialdehyde. The specific activity of the fusion protein was 120 pmol.micrograms-1.min-1. The fusion protein used tRNA(Glu) from barley chloroplasts preferentially to E. coli tRNA(Glu) and its activity was inhibited by hemin. It migrated as an 82-kDa polypeptide with SDS/PAGE and eluted with an apparent molecular mass of 450 kDa from Superose 12. After removal of the GST by thrombin, the protein migrated as an approximately equal to 60-kDa polypeptide with SDS/PAGE, whereas gel filtration on Superose 12 yielded an apparent molecule mass of 250 kDa. Isolated fusion protein contained heme, which could be reduced by NADPH and oxidized by air.

Structural genes for Mg-chelatase subunits in barley: Xantha-f, -g and -h

Barley mutants in the loci Xantha-f, Xantha-g and Xantha-h, when fed with 5-aminolevulinate in the dark, accumulate protoporphyrin IX. Mutant alleles at these loci that are completely blocked in protochlorophyllide synthesis are also blocked in development of prolamellar bodies in etioplasts. In contrast to wild type, the xan-f, -g and -h mutants had no detectable Mg-chelatase activity, whereas they all had methyltransferase activity for synthesis of Mg-protoporphyrin monomethyl ester. Antibodies recognising the CH42 protein of Arabidopsis thaliana and the OLIVE (OLI) protein of Antirrhinum majus immunoreacted in wild-type barley with 42 and 150 kDa proteins, respectively. The xan-h mutants lacked the protein reacting with antibodies raised against the CH42 protein. Two xan-f mutants lacked the 150 kDa protein recognised by the anti-OLI antibody. Barley genes homologous to the A. majus olive and the A. thaliana Ch-42 genes were cloned using PCR and screening of cDNA and genomic libraries. Probes for these genes were applied to Northern blots of RNA from the xantha mutants and confirmed the results of the Western analysis. The mutants xan-f27, -f40, -h56 and -h57 are defective in transcript accumulation while -h38 is defective in translation. Southern blot analysis established that h38 has a deletion of part of the gene. Mutants xan-f10 and -f41 produce both transcript and protein and it is suggested that these mutations are in the catalytic sites of the protein. It is concluded that X an-f -h genes encode two subunits of the barley Mg-chelatase and that X an-g is likely to encode a third subunit. The XAN-F protein displays 82% amino acid sequence identity to the OLI protein of Antirrhinum, 66% to the Synechocystis homologue and 34% identity to the Rhodobacter BchH subunit of Mg-chelatase. The XAN-H protein has 85% amino acid sequence identity to the Arabidopsis CH42 protein, 69% identity to the Euglena CCS protein, 70% identity to the Cryptomonas BchA and Olisthodiscus CssA proteins, as well as 49% identity to the Rhodobacter BchI subunit of Mg-chelatase. Identification of the barley X an-f and X an-h encoded proteins as subunits required for Mg-chelatase activity supports the notion that the Antirrhinum OLI protein and the Arabidopsis Ch42 protein are subunits of Mg-chelatase in these plants. The expression of both thet X an-f and -h genes in wild-type barley is light induced in leaves of greening seedlings, and in green tissue the genes are under the control of a circadian clock.

Characterization of the different spectral forms of glutamate 1-semialdehyde aminotransferase by mass spectrometry

Glutamate 1-semialdehyde aminotransferase produces delta-aminolevulinate for the synthesis of chlorophyll, heme, and other tetrapyrrole pigments. The native enzyme from Synechococcus is pale yellow and has absorption maxima at 338 and 418 nm from vitamin B6. Yellow, colorless, and pink forms of the protein are obtained by treatment with 4,5-dioxovalerate, 4,5-diaminovalerate, and acetylenic GABA, respectively. Compared to the native enzyme, the 418 nm absorption maximum in the yellow enzyme is enhanced and the 338 nm maximum reduced while the colorless enzyme has a heightened maximum at 338 nm and a barely detectable peak at 418 nm. The pink enzyme has an absorption maximum at 560 nm. When the native and colorless enzymes are repeatedly diluted in 0.5 M Na2HPO4, pH 7.0, and reconcentrated, pyridoxamine 5'-phosphate is released and the 338 nm maximum lost. Thus the 338 nm absorption maximum is associated with noncovalently bound pyridoxamine 5'-phosphate. NaBH4 reduction proved that the absorbance at 418 nm is from pyridoxal 5'-phosphate cofactor bound by a Schiff base to the protein. When the native, colorless, and yellow enzymes were subjected to electrospray ionization mass spectrometry, the B6 cofactor dissociated from the protein and gave a molecular weight of 46,401-46,418. Acetylenic GABA and NaBH4 were used for protein modification, and they reacted with the native and yellow enzymes but had no effect on the colorless enzyme. Pyridoxal 5'-phosphate bound covalently to the protein after NaBH4 reduction. Acetylenic GABA attached covalently to the enzyme produced an additional mass peak, 123-126 mass units higher, in the electrospray ionization spectrum.(ABSTRACT TRUNCATED AT 250 WORDS)

Nucleotides of tRNA (Glu) involved in recognition by barley chloroplast glutamyl-tRNA synthetase and glutamyl-tRNA reductase

The biosynthesis of delta-aminolevulinate (ALA), via the C-5 pathway, requires tRNA(Glu) as a cofactor for the glutamyl tRNA(Glu) synthetase and the glutamyl tRNA(Glu) reductase which are the first two enzymes in this three step pathway. These two enzymes form a ternary complex with the tRNA(Glu) in Chlamydomonas reinhardtii suggesting that the recognition elements on the tRNA cofactor are different for each enzyme. Chemical modification and comparative studies with tRNA(Glu)s from a number of species were used to determine the nucleotides involved in the recognition of the barley chloroplast tRNA(Glu) by the barley enzymes. The barley chloroplast tRNA(Glu) is chemically modified both before and after ligation to glutamate with monobromobimane or CNBr. The chemically modified tRNA(Glu) is a poor substrate for the glutamyl-tRNA synthetase and the chemically modified glutamyl-tRNA(Glu) is used as a substrate for glutamyl-tRNA(Glu) reductase. The tRNA(Glu) from the chloroplasts if barley, Chlamydomonas reinhardtii, tobacco, cucumber, wheat and spinach and tRNA(Glu) from Synechocystis PCC6803, Escherichia coli, barley germ and bakers yeast and the barley chloroplast tRNA(Gln) are all effective substrates for the barley chloroplast glutamyl-tRNA synthetase. A comparison of the sequences of these tRNAs shows 19 conserved bases and five of these bases, G10, A26, U34, U35 and A37 are suggested as recognition elements of barley glutamyl tRNA(Glu) synthetase by assuming a similar binding orientation as in the crystal structure of the E. coli tRNA(Gln) GlnRS complex. The glutamyl-tRNA(Glu) from E. coli, bakers yeast and barley germ and the barley chloroplast glutamyl-tRNA(Gln) are not effective substrates for the barley chloroplast glutamyl-tRNA(Glu) reductase. A comparison of the sequences of these four tRNA species with the sequences of the tRNA(Glu) species that can be used as substrate by the glutamyl-tRNA(Glu) reductase yields seven common differences in the primary sequence. These 7 nucleotides, A7-U66, U29-A41, A53-U61, and U72 are expected to be required for recognition by the barley chloroplast glutamyl-tRNA(Glu) reductase.

Magnesium-protoporphyrin chelatase of Rhodobacter sphaeroides: reconstitution of activity by combining the products of the bchH, -I, and -D genes expressed in Escherichia coli

Magnesium-protoporphyrin chelatase lies at the branch point of the heme and (bacterio)chlorophyll biosynthetic pathways. In this work, the photosynthetic bacterium Rhodobacter sphaeroides has been used as a model system for the study of this reaction. The bchH and the bchI and -D genes from R. sphaeroides were expressed in Escherichia coli. When cell-free extracts from strains expressing BchH, BchI, and BchD were combined, the mixture was able to catalyze the insertion of Mg into protoporphyrin IX in an ATP-dependent manner. This was possible only when all three genes were expressed. The bchH, -I, and -D gene products are therefore assigned to the Mg chelatase step in bacteriochlorophyll biosynthesis. The mechanism of the Mg chelation reaction and the implications for chlorophyll biosynthesis in plants are discussed.

Purification and partial characterisation of barley glutamyl-tRNA(Glu) reductase, the enzyme that directs glutamate to chlorophyll biosynthesis

5-Aminolevulinic acid for chlorophyll synthesis in greening barley is formed from glutamate. One of the steps involved in the conversion of glutamate to 5-aminolevulinic acid involves a reduction of glutamyl-tRNA(Glu) to glutamate 1-semialdehyde and tRNA(Glu). An enzyme catalysing this reduction was purified from the stroma of greening barley chloroplasts. An approximately 270-kDa protein composed of 54-kDa identical subunits was identified as the barley glutamyl-tRNA(Glu) reductase after purification by Sephacryl S-300, Cibacron Blue-Sepharose, 2'-5'-ADP-Sepharose, Mono S, Mini Q and Superose 12 chromatography. The sequence of 18 amino acids from the N-terminus of the reductase is 50% identical to a cDNA-deduced domain of the Arabidopsis thaliana hemA protein and encoded in a barley hemA cDNA sequence. This is an unequivocal demonstration that the glutamyl-tRNA(Glu) reductase subunit of higher plants is encoded in a hemA gene of the nuclear genome. Heme at 4 microM concentration or glutamate 1-semialdehyde at 200 microM caused a 50% inhibition of the reductase activity. Micromolar concentrations of Zn2+, Cu2+ and Cd2+ also inhibited barley glutamyl-tRNA(Glu) reductase.

A visible marker for antisense mRNA expression in plants: inhibition of chlorophyll synthesis with a glutamate-1-semialdehyde aminotransferase antisense gene

Glutamate 1-semialdehyde aminotransferase [(S)-4-amino-5-oxopentanoate 4,5-aminomutase, EC 5.4.3.8] catalyzes the last step in the conversion of glutamate to delta-aminolevulinate of which eight molecules are needed to synthesize a chlorophyll molecule. Two full-length cDNA clones that probably represent the homeologous Gsa genes of the two tobacco (Nicotiana tabacum) genomes have been isolated. The deduced amino acid sequences of the 468-residue-long precursor polypeptides differ by 10 amino acids. The cDNA sequence of isoenzyme 2 was inserted in reverse orientation under the control of a cauliflower mosaic virus 35S promoter derivative in an expression vector and was introduced by Agrobacterium-mediated transformation into tobacco plants. Antisense gene expression decreased the steady-state mRNA level of glutamate 1-semialdehyde aminotransferase, the translation of the enzyme, and chlorophyll synthesis. Remarkably, partial or complete suppression of the aminotransferase mimics in tobacco a wide variety of chlorophyll variegation patterns caused by nuclear or organelle gene mutations in different higher plants. The antisense gene is inherited as a dominant marker.

Glutamate 1-semialdehyde aminotransferase: anomalous enantiomeric reaction and enzyme mechanism

Glutamate 1-semialdehyde aminotransferase (GSA-AT) catalyzes near 50% conversion of the racemic mixture of GSA to 5-aminolevulinate (ALA), indicating quantitative use of the L-glutamate-derived natural (S)-enantiomer as substrate. This enzymic reaction has been extensively studied with (R,S)-GSA because it is readily purified in high yields following ozonolysis of racemic 4-vinyl-4-aminobutyric acid. However upon addition of (R,S)-GSA, GSA-aminotransferase is converted to the pyridoxal-P or internal aldimine form (418 nm) and not rapidly cycled back to the original pyridoxamine-P, as predicted by the rate of product (ALA) accumulation. Addition of the putative intermediate, (R,S)-4,5-diaminovalerate (DAVA), eliminates this rapid conversion of the enzyme by (R,S)-GSA to the internal aldimine and stimulates initial rates of ALA synthesis (2-3-fold) and results in corresponding increases in apparent equilibrium concentrations of ALA. These results indicate that DAVA is rate limiting and suggest anomalous reactivity of (R)-GSA. Steady-state and spectral kinetic experiments with individual purified enantiomers confirm anomalous reactivity of (R)-GSA: in the case of (S)-GSA, spectral changes are lesser in amplitude and at least 1 or 2 orders of magnitude more rapid. Only (S)-GSA yielded significant amounts of ALA. Since (R)-GSA is an apparent substrate in the first half-reaction, the resulting (R)-DAVA is either inactive or a poor substrate in the second half-reaction.

Purification and Characterization of Glutamate 1-Semialdehyde Aminotransferase from Barley Expressed in Escherichia coli

The immediate precursor in the synthesis of tetrapyrroles is Delta-aminolevulinate (ALA). ALA is synthesized from glutamate in higher plants, algae, and certain bacteria. Glutamate 1-semialdehyde aminotransferase (EC 5.4.3.8) (GSA-AT), the third enzyme involved in this metabolic pathway, catalyzes the transamination of GSA to form ALA. The gene encoding this aminotransferase has previously been isolated from barley (Hordeum vulgare) and inserted into an Escherichia coli expression vector. We describe herein the purification of this recombinant barley GSA-AT expressed in Escherichia coli. Coexpression of GroEL and GroES is required for isolation of active aminotransferase from the soluble protein fraction of Escherichia coli. Purified GSA-AT exhibits absorption maxima characteristic of vitamin B(6)-containing enzymes. GSA-AT is primarily in the pyridoxamine form when isolated and can be interconverted between this and the pyridoxal form by addition of 4,5-dioxovalerate and 4,5-diaminovalerate. The conversion of GSA to ALA under steady-state conditions exhibited typical Michaelis-Menten kinetics. Values for K(m) (d,l-GSA) and k(cat) were determined to be 25 micromolar and 0.11 per second, respectively, by nonlinear regression analysis. Stimulation of ALA synthesis by increasing concentrations of d,l-GSA at various fixed concentrations of 4,5-diaminovalerate supports the hypothesis that 4,5-diaminovalerate is the intermediate in the synthesis of ALA.

Glutamyl-tRNA reductase activity in Bacillus subtilis is dependent on the hemA gene product

The Bacillus subtilis hemAXCDBL operon encodes enzymes for the synthesis of 5-aminolaevuline acid via the C5 pathway (hemA and hemL) and uroporphyrinogen III (hemB, hemC and hemD). B. subtilis HemA protein (molecular mass 50 kDa) was overexpressed in hemA mutant of both Escherichia coli and B. subtilis. A mutant B. subtilis HemA protein with a Cys to Tyr change at position 105 was also overexpressed. Both wild-type and mutant HemA proteins migrated as oligomers (molecular mass greater than or equal to 230 kDa) on gel-filtration columns. All column fractions containing wild-type HemA protein had glutamyl-tRNA reductase activity. No glutamyl-tRNA reductase activity was found with the mutant HemA protein. It is concluded that the B. subtilis hemA gene product is identical to, or part of, the glutamyl-tRNA reductase of the C5 pathway.

Characterization of glutamate-1-semialdehyde aminotransferase of Synechococcus. Steady-state kinetic analysis

Synechococcus glutamate-1-semialdehyde aminotransferase was expressed in large amounts in transformed cells of Escherichia coli. The resulting purified enzyme has an absorption spectrum characteristic of B6-containing enzymes and could be converted to the pyridoxal-phosphate form with excess dioxovalerate (O2Val), and back to the pyridoxamine-phosphate form with diaminovalerate (A2Val). Both enzyme forms are similarly active in the conversion of glutamate 1-semialdehyde (GSA) to 5-aminolevulinate (ALev), suggesting that A2Val and O2Val are intermediates. Initial rates of ALev synthesis at various fixed concentrations of GSA followed typical Michaelis-Menten kinetics (Km of GSA for the pyridoxamine-phosphate form of GSA aminotransferase = 12 microM, kcat = 0.23 s-1). In submicromolar amounts A2Val stimulates ALev synthesis, and in a series of concentrations with various fixed concentrations of GSA, gives a family of parallel lines in Lineweaver-Burk plots (Km for A2Val = 1.0 microM). On the other hand, O2Val gives competitive inhibition of the pyridoxamine-phosphate form of GSA-aminotransferase and mixed-type inhibition of the pyridoxal-phosphate form (Ki for O2Val = 1.4 mM). In general the kinetics were typical of ping-pong bi-bi mechanisms in which A2Val is the second substrate (intermediate) and O2Val is an alternative first substrate. There is no compelling evidence that O2Val accepts an amino group at its C5 position resulting in the direct formation of ALev, or the reverse involving the apparent formation of O2Val from ALev. These results are consistent with the hypothesis that the mechanism of GSA aminotransferase mimics that of other aminotransferases and that A2Val is the intermediate.

Spectral kinetics of glutamate-1-semialdehyde aminomutase of Synechococcus

Purified Synechococcus glutamate-1-semialdehyde aminotransferase (GSA-AT; EC 5.4.3.8) has absorption maxima characteristic of vitamin B6-containing enzymes and can be converted to the pyridoxamine 5'-phosphate or pyridoxal 5'-phosphate form by reaction with diaminovalerate or dioxovalerate, respectively, suggesting that these two analogues are intermediates in the conversion of glutamate 1-semialdehyde (GSA) to 5-aminolevulinate (ALA). Values for Km and kmax were calculated for GSA, diaminovalerate, ALA, and gabaculine from absorption change rates during conversion of one coenzyme form of GSA-AT to the other, upon addition of one of these compounds. The substrate specificity (kmax/Km) of diaminovalerate is about 3 orders of magnitude larger than that of dioxovalerate, making the latter an unlikely intermediate in the enzymic conversion of GSA to ALA. GSA reacts with both coenzyme forms, whereas ALA only reacts with the pyridoxamine 5'-phosphate form of the enzyme. However, ALA does form a complex with the pyridoxal 5'-phosphate form of GSA-AT and inhibits reactions between gabaculine and GSA-AT. This relatively stable complex (Ki = 8 M) may have significance in enzyme inhibition. Both L and D enantiomers of GSA react with GSA-AT. Spectral changes observed upon addition of DL-GSA are apparently due to reaction with the less reactive D-isomer. L-GSA is converted to ALA prior to major spectral changes induced by the racemic mixture.

Gabaculine-resistant glutamate 1-semialdehyde aminotransferase of Synechococcus. Deletion of a tripeptide close to the NH2 terminus and internal amino acid substitution

Glutamate 1-semialdehyde aminotransferase (GSA-AT) is the last enzyme in the C5 pathway converting glutamate into the tetrapyrrole precursor delta-aminolevulinate in plants, algae, and several bacteria. Sequence analysis of the genes encoding GSA-AT in barley, Synechococcus, and Escherichia coli revealed 50-70% similarity in the primary structures of the proteins. The enzyme is inhibited rapidly by gabaculine when added in approximately stoichiometric amounts with the enzyme. A gabaculine-tolerant Synechococcus strain, GR6, was found to produce a GSA-AT less sensitive to the inhibitor. Accordingly, the mutant gene was isolated and sequenced. In comparison with the wild-type gene it contains a deletion of nine nucleotides (position 12-20) and a guanine to adenine substitution (position 743). This resulted in the loss of the amino acids serine, proline, and phenylalanine (position 5-7) close to the NH2 terminus of the enzyme and an exchange of Met-248 for isoleucine in the middle of the polypeptide chain. Wild-type and mutant GSA-AT were expressed in E. coli and purified close to homogeneity. Although the specific activity of the mutant GSA-AT was only one-fifth of the wild type, it displayed a 100-fold increased resistance to gabaculine. Peaks in the absorption spectrum of the purified recombinant GSA-ATs at 335 and 417 nm are typical of a transaminase containing a B6 cofactor. Incubation with substrate and with inhibitor induced spectral changes characteristic of other gabaculine-sensitive, B6-requiring enzymes.

Purification and partial amino acid sequence of the glutamate 1-semialdehyde aminotransferase of barley and synechococcus

Glutamate-1-semialdehyde aminotransferase (E.C. 5.4.3.8) was purified from barley and the cyanobacteria Synechococcus PCC 6301. The purification procedure involved serial affinity chromatography and preparative polyacrylamide gel electrophoresis under non-denaturing conditions. The aminotransferase of these two organisms showed different mobilities in non-denaturing gels. In SDS-PAGE the enzyme from both organisms migrated as a single protein with an apparent molecular weight of 46.000 Da. An antibody against the barley enzyme cross-reacted with the cyanobacterial aminotransferase. This antibody also recognized a 17 kDa peptide cleaved from the barley protein with cyanogen bromide. Amino acid sequences of the NH2-termini revealed significant homology between the eucaryotic and cyanobacterial enzyme.

Protein biosynthesis in organelles requires misaminoacylation of tRNA

In the course of our studies on transfer RNA involvement in chlorophyll biosynthesis, we have determined the structure of chloroplast glutamate tRNA species. Barley chloroplasts contain in addition to a tRNA(Glu) species at least two other glutamate-accepting tRNAs. We now show that the sequences of these tRNAs differ significantly: they are differentially modified forms of tRNA(Gln) (as judged by their UUG anticodon). These mischarged Glu-tRNA(Gln) species can be converted in crude chloroplast extracts to Gln-tRNA(Gln). This reaction requires a specific amidotransferase and glutamine or asparagine as amide donors. Aminoacylation studies show that chloroplasts, plant and animal mitochondria, as well as cyanobacteria, lack any detectable glutaminyl-tRNA synthetase activity. Therefore, the requirement for glutamine in protein synthesis in these cells and organelles is provided by the conversion of glutamate attached to an 'incorrectly' charged tRNA. A similar situation has been described for several species of Gram-positive bacteria. Thus, it appears that the occurrence of this pathway of Gln-tRNA(Gln) formation is widespread among organisms and is a function conserved during evolution. These findings raise questions about the origin of organelles and about the evolution of the mechanisms maintaining accuracy in protein biosynthesis.

Biosynthesis of delta-aminolevulinate in greening barley leaves. IX. Structure of the substrate, mode of gabaculine inhibition, and the catalytic mechanism of glutamate 1-semialdehyde aminotransferase

Glutamic acid 1-semialdehyde hydrochloride was synthesized and purified. Its prior structural characterization was extended and confirmed by 1H NMR spectroscopy and chemical analyses. In aqueous solution at pH 1 to 2 glutamic acid 1-semialdehyde exists in a stable hydrated form, but at pH 8.0 it has a half-life of 3 to 4 min. Spontaneous degradation of the material at pH 8.0 generated some undefined condensation products, but coincidentally a significant amount isomerized to 5-aminolevulinate. At pH 6.8 to 7.0, glutamate 1-semialdehyde is sufficiently stable to permit routine and reproducible assay for glutamate 1-semialdehyde aminotransferase activity. Only about 20% of the enzyme extracted from chloroplasts was sensitive to inactivation by gabaculine with no pretreatment. However, when the enzyme was exposed to 5-aminolevulinate, levulinate or 4,5-dioxovalerate in the absence of glutamate 1-semialdehyde, it was completely inactivated by gabaculine; 4,6-dioxoheptanoate had no effect on the enzyme. These results lead to the hypothesis that the aminotransferase exists in the chloroplast in a complex with pyridoxamine phosphate, which must be converted to the pyridoxal form before it can form a stable adduct with gabaculine. We propose that the enzyme catalyzes the conversion of glutamate 1-semialdehyde to 5-aminolevulinate via 4,5-diaminovalerate.

The RNA required in the first step of chlorophyll biosynthesis is a chloroplast glutamate tRNA

A molecule of chlorophyll is synthesized from eight molecules of delta-aminolevulinate (DALA), the universal precursor of porphyrins. The light-regulated conversion of glutamate to delta-aminolevulinate in the stroma of greening plastids involves the reduction of glutamate to glutamate-1-semialdehyde and its subsequent transamination. The components performing this conversion have been isolated from barley and Chlamydomonas and separated into three fractions by serial affinity chromatography on Blue Sepharose and haem- or chlorophyllin-Sepharose. The complete reaction can be performed in vitro in a reconstituted assay by combining all three fractions. An RNA is the essential component of the chlorophyllin-Sepharose-bound fraction. By nucleotide sequence analysis, we have now identified this RNA as a chloroplast glutamate acceptor RNA. Glutamate attached by an aminoacyl bond to the 3'-terminal adenosine of this RNA is a substrate for the enzyme(s) which perform the subsequent reactions. This reaction represents a novel role for transfer RNA: participation in the metabolic conversion of its cognate amino acid into another metabolite of low relative molecular mass which subsequently is not used in peptide bond synthesis.

delta-Aminolevulinic acid-synthesizing enzymes need an RNA moiety for activity

When Chlamydomonas enzymes that convert glutamate to delta-aminolevulinic acid are separated into three fractions, one of the fractions contains RNA, and the RNA moiety is needed for the enzyme activity.

Purification, Characterization, and Fractionation of the delta-Aminolevulinic Acid Synthesizing Enzymes from Light-Grown Chlamydomonas reinhardtii Cells

The synthesis of delta-aminolevulinate from glutamate by Chlamydomonas reinhardtii membrane-free cell homogenates requires Mg(2+), ATP, and NADPH as cofactors. The pH optimum is about 8.3. When analyzed by a Fractogel TSK gel filtration column the delta-aminolevulinate synthesizing enzymes, including glutamate-1-semialdehyde aminotransferase, elute with an apparent molecular weight of about 45,000. The enzymes obtained from the gel filtration column were separated into three fractions by affinity column chromatography. One fraction binds to heme-Sepharose, one to Blue Sepharose, while the enzyme converting the putative glutamate-1-semialdehyde to delta-aminolevulinic acid is retained by neither column. All three fractions are necessary for the conversion of glutamate to delta-aminolevulinate. The delta-aminolevulinate synthesizing enzymes from Chlamydomonas are sensitive to inhibition by heme but not sensitive to inhibition by protoporphyrin.

Biotin carboxyl carrier protein in barley chloroplast membranes

Biotin localized in barley chloroplast lamellae is covalently bound to a single protein with an approximate molecular weight of 21 000. It contains one mole of biotin per mole of protein and functions as a carboxyl carrier in the acetyl-CoA carboxylase reaction. The protein was obtained by solubilization of the lamellae in phenol/acetic acid/8 M urea. Feeding barley seedlings with [14C]-biotin revealed that the vitamin is not degraded into respiratory substrates by the plant, but is specifically incorporated into biotin carboxyl carrier protein.

Fat Metabolism in Higher Plants: LVII. A Comparison of Fatty Acid-Synthesizing Enzymes in Chloroplasts Isolated from Mature and Immature Leaves of Spinach

Chloroplasts isolated from immature leaves of spinach (Spinacia oleracea) differ in enzyme levels from those isolated from mature leaves. On a chlorophyll basis, immature chloroplast preparations had 5- to 6-fold higher capacity to synthesize fatty acids from 2-(14)C-acetate compared to plastids isolated from mature leaves. This difference was correlated with higher activities for the enzymes, acetyl coenzyme A synthetase, malonyl coenzyme A synthetase, acetyl coenzyme A carboxylase, and oleyl coenzyme A transferase in plastid pressates obtained from immature leaves. Disrupted chloroplast preparations from both mature and immature leaves retained the ability to incorporate 2-(14)C-acetate into fatty acids in a pattern similar to that by isolated chloroplasts. 2-(14)C-Acetate, 2-(14)C-acetyl coenzyme A, 2-(14)C-malonate, and 1,3-(14)C malonyl coenzyme A were readily incorporated into a number of fatty acids. Moreover, the synthesis of oleate by chloroplast pressates from these substrates was strongly inhibited by KCN, flavin adenine mononucleotides and dinucleotides, and anaerobic conditions, while linolenic acid synthesis was unaffected by these compounds.

Fat Metabolism in Higher Plants: XLVII. The Effect of Nitrite and Other Anions on the Formation of Unsaturated Fatty Acids by Isolated Chloroplasts

Intact spinach and barley chloroplast normally incorporate (14)C-acetate into palmitate and oleate as the major (14)C fatty acids. Addition of nitrite markedly altered the relative patterns of the products with the appearance of stearate, a drop in oleate, but no marked change in palmitate. Arsenite greatly increased appearance of palmitate with a concomitant decrease in the C(18) fatty acids. The effect of other anions was also examined. Spinach and barley plants grown under different nitrogen nutritional conditions also served as sources of chloroplasts, and their activities suggest a correlation between nitrite reductase activity and stearate accumulation.

Lipid biosynthesis by isolated barley chloroplasts in relation to plastid development

The effect of seedling age and of the time of greening on the incorporation of 1-(14)C-acetate into lipids by isolated barley (Hordeum vulgare cultivar Svalöf's Bonus) plastids was examined. The fatty acid synthesizing capacity of plastids isolated from 5-day-old seedlings did not increase markedly from zero to 36 hours of greening nor was a light stimulation of fatty acid synthesis observed. However, an increasing capacity for fatty acid synthesis and an increasing light stimulation of this process with greening were attained by the plastids isolated from 7-, 9-, and 11-day-old seedlings.Plastids of 7-day-old dark-grown plants, which were illuminated at 2 foot-candles showed increasing capacity of (14)C-acetate incorporation with significant flow into phospholipids and sulfolipid, low flow into digalactosyl diglyceride, and considerable flow into 6-methyl salicylic acid. Exposure of these plants to high light intensity for an hour resulted in chloroplasts that after isolation had a 10-fold increased capacity to incorporate (14)C label into digalactosyl diglyceride, while the flow of (14)C label into phospho- and sulfolipids was unaltered, and that into 6-methyl salicylic acid was drastically curtailed.With plastids from 7-day-old dark-grown plants in early stages of greening, essentially all the (14)C label in the stroma fraction could be accounted for by 6-methyl salicylic acid, while the membrane lipids only contained small amounts of (14)C label. As greening proceeded, the flow of (14)C label into 6-methyl salicylic acid diminished sharply, and the lipids of the lamellar systems became increasingly labeled.Only palmitic and oleic acids were main sites of (14)C label in the membrane lipids.The activity of acetyl CoA carboxylase present in plastids of 5- and 7-day-old dark-grown plants fell sharply as the etioplasts differentiated into chloroplasts.

Localization of ferredoxin in the thylakoid membrane with immunological methods

Antibodies were prepared against ferredoxin isolated from spinach. These antibodies and those against TPN-ferredoxin-reductase, carboxydismutase and coupling factor were used to clarify the localization of these proteins in the thylakoid membrane. Chloroplast preparations obtained by different methods show not the same immunological reactions. Only lamellar systems suspended in the leaf homogenate are agglutinated by anti-ferredoxin. Washed lamellar systems show an agglutination with anti-ferredoxin only if ferredoxin is added. In certain preparations of lamellar systems the presence of ferredoxin inhibit the reaction of anti-TPN-ferredoxin-reductase with the lamellar system. Lamellar systems freed of coupling factor by EDTA treatment are agglutinated by anti-TPN-ferredoxin-reductase. Addition of coupling factor to the EDTA treated lamellar systems inhibit the agglutination but not the adsorption of TPN-ferredoxin-reductase antibodies. By the assumption that the inhibition of the agglutination by the TPN-ferredoxin-reductase antibodies is caused by sterical hindrance, these results indicate, that ferredoxin, coupling factor and TPN-ferredoxin-reductase are localized close to each other in the thylakoid membrane.

Lamellar systems suspended in leaf homogenate are agglutinated by anti-TPN-ferredoxin-reductase. That means, that to a certain extend the molecular structure of the thylakoid membrane changes with the kind of preparations employed.

Immunological evidence for the presence of latent Ca2 dependent ATPase and carboxydismutase on the thylakoid surface

Antibodies were prepared against carboxydismutase and latent Ca2⊕ dependent ATPase (coupling factor) purified from Nicotiana tabacum. The antibodies against carboxydismutase inhibit the enzymatic activity of the purified protein, while those against coupling factor inhibit the Ca2® dependent ATPase activity of the protein as well as the photophosphorylation of the chloroplasts. The antisera against these two proteins agglutinate the isolated lamellar systems of chloroplasts. The lamellar systems after repeated washes with ethylene diamine tetra-acetic acid loose their capacity to agglutinate with the antibodies. So treated lamellar systems regain their capacity to agglutinate when preincubated with purified coupling factor and carboxydismutase. It is concluded that coupling factor and carboxydismutase molecules are present on the outer surface of the thylakoids. The antisera against the proteins isolated from Nicotiana show cross reactions with preparations of Antirrhinum. Coupling factor and carboxydismutase are non-uniformly distributed on the surface of thylakoids. Electron microscopy shows no evidence that coupling factor represent knobs attached to the surface of the thylakoids.

The Formation of Ribulose Diphosphate Carboxylase Protein during Chloroplast Development in Barley

Ribulose 1,5-diphosphate carboxylase is synthesized in barley leaves growing in the dark. Upon illumination there is a marked increase in the rate of synthesis of the enzyme. The specific activity of the enzyme expressed as cpm incorporated into phosphoglyceric acid per mug of fraction I protein, after isolation shows no change either during dark growth or greening. During early stages of illumination of 7 day dark grown leaves with 320 foot-candles the enzymic activity in the water soluble protein fraction of the leaf shows a short term decline after 15 min which lasts for 30 min. Leaves greening at 2 foot-candles show a similar decline which is shifted to a time between the fourth and eighth hr after the onset of illumination.