Biosynthesis of Δ-aminolevulinate in greening barley leaves IV. Isolation of three soluble enzymes required for the conversion of glutamate to Δ-aminolevulinate

The role of the 5' sensing function of ribonuclease E in cyanobacteria

RNA degradation is critical for synchronising gene expression with changing conditions in prokaryotic and eukaryotic organisms. In bacteria, the preference of the central ribonucleases RNase E, RNase J and RNase Y for 5'-monophosphorylated RNAs is considered important for RNA degradation. For RNase E, the underlying mechanism is termed 5' sensing, contrasting to the alternative 'direct entry' mode, which is independent of monophosphorylated 5' ends. Cyanobacteria, such as Synechocystis sp. PCC 6803 (Synechocystis), encode RNase E and RNase J homologues. Here, we constructed a Synechocystis strain lacking the 5' sensing function of RNase E and mapped on a transcriptome-wide level 283 5'-sensing-dependent cleavage sites. These included so far unknown targets such as mRNAs encoding proteins related to energy metabolism and carbon fixation. The 5' sensing function of cyanobacterial RNase E is important for the maturation of rRNA and several tRNAs, including tRNAGluUUC. This tRNA activates glutamate for tetrapyrrole biosynthesis in plant chloroplasts and in most prokaryotes. Furthermore, we found that increased RNase activities lead to a higher copy number of the major Synechocystis plasmids pSYSA and pSYSM. These results provide a first step towards understanding the importance of the different target mechanisms of RNase E outside Escherichia coli.

Phycobilisome breakdown effector NblD is required to maintain the cellular amino acid composition during nitrogen starvation

Small proteins are critically involved in the acclimation response of photosynthetic cyanobacteria to nitrogen starvation. NblD is the 66-amino-acid effector of nitrogen-limitation-induced phycobilisome breakdown, which is believed to replenish the cellular amino acid pools. To address the physiological functions of NblD, the concentrations of amino acids, intermediates of the arginine catabolism pathway and several organic acids were measured during the response to nitrogen starvation in the cyanobacterium Synechocystis sp. PCC 6803 wild type and in an nblD deletion strain. A characteristic signature of metabolite pool composition was identified, which shows that NblD-mediated phycobilisome degradation is required to maintain the cellular amino acid and organic acid pools during nitrogen starvation. Specific deviations from the wild type suggest wider-reaching effects that also affect such processes as redox homeostasis via glutathione and tetrapyrrole biosynthesis, both of which are linked to the strongly decreased glutamate pool, and transcriptional reprogramming via an enhanced concentration of 2-oxoglutarate, the metabolite co-regulator of the NtcA transcription factor. The essential role played by NblD in metabolic homeostasis is consistent with the widespread occurrence of NblD throughout the cyanobacterial radiation and the previously observed strong positive selection for the nblD gene under fluctuating nitrogen supply. Importance Cyanobacteria play important roles in the global carbon and nitrogen cycles. In their natural environment, these organisms are exposed to fluctuating nutrient conditions. Nitrogen starvation induces a coordinated nitrogen-saving program that includes the breakdown of nitrogen-rich photosynthetic pigments, particularly phycobiliproteins. The small protein NblD was recently identified as an effector of phycobilisome breakdown in cyanobacteria. In this study, we demonstrate that the NblD-mediated degradation of phycobiliproteins is needed to sustain cellular pools of soluble amino acids and other crucial metabolites. The essential role played by NblD in metabolic homeostasis explains why genes encoding this small protein are conserved in almost all members of cyanobacterial radiation.

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.

Isolation of the Staphylococcus aureus hemCDBL gene cluster coding for early steps in heme biosynthesis

We have recently reported [Kafala, B., Sasarman, A., 1994. Can. J. Microbiol. 40, 651 657] the cloning and sequencing of the Staphylococcus aureus hemB gene. This gene purportedly encodes the delta-aminolevulinic acid dehydratase of the heme pathway. In this present communication, we report the sequences and identities of three putative hem genes. Two of these genes are located immediately upstream from hemB. Complementation analysis of Escherichia coli and Salmonella typhimurium hemC and hemD mutants and the comparison of the Sa nucleotide sequences with those of Bacillus subtilis and Ec showed that these two open reading frames, ORF1 and ORF2, are likely to be the hemC gene coding for porphobilinogen deaminase and the hemD gene coding for uroporphyrinogen III synthase, respectively. The third hem gene, hemL, is located immediately downstream of hemB, and encodes glutamate 1-semialdehyde 2,1-aminotransferase. Sequencing of the region which extends past hemL indicates that no further hem genes are located downstream of hemL. In Sa, hemC, hemD, hemB and hemL are proposed to constitute a hem cluster encoding enzymes required for the synthesis of uroporphyrinogen III from glutamate 1-semialdehyde (GSA).

Purification, metal cofactor, N-terminal sequence and subunit composition of a 5-aminolevulinic acid dehydratase from the unicellular green alga Scenedesmus obliquus, mutant C-2A'

5-Aminolevulinic acid dehydratase was purified to apparent homogeneity from Scenedesmus obliquus, mutant C-2A', starting with serial affinity chromatography according to Wang et al., followed by separation on DEAE-Cellulose DE 52, TSKgel Toyopearl HW-55 and FPLC on Mono Q. The enzyme was purified 117-fold compared with the initial crude soluble enzyme preparation and showed a final specific activity of 9.17 microkat/kg protein at pH 8.2 at a total recovery of 7%. Mg2+ was determined to be the metal cofactor of the enzyme. It can, to a certain extent, be substituted by other divalent cations. From the purified enzyme the first 15 amino acids of the N-terminus could be determined, showing a moderate similarity to 5-aminolevulinic acid dehydratases from spinach, pea, Escherichia coli and yeast. The molecular mass of the native protein was determined by gel filtration to be 282+/-5 kDa. 42+/-1 kDa were ascertained for the subunit size by SDS/PAGE. These investigations, supported by electron microscopy, revealed that the enzyme from Scenedesmus consists of six subunits arranged in a six-membered ring. Additionally, there is some evidence that two of the rings form a sandwich-like complex.

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.

The common origins of the pigments of life-early steps of chlorophyll biosynthesis

The complex pathway of tetrapyrrole biosynthesis can be dissected into five sections: the pathways that produce 5-aminolevulinate (the C-4 and the C-5 pathways), the steps that transform ALA to uroporphyrinogen III, which are ubiquitous in the biosynthesis of all tetrapyrroles, and the three branches producing specialized end products. These end products include corrins and siroheme, chlorophylls and hemes and linear tetrapyrroles. These branches have been subjects of recent reviews. This review concentrates on the early steps leading up to uroporphyrinogen III formation which have been investigated intensively in recent years in animals, in plants, and in a wide range of bacteria.

Isolation, sequencing and expression of cDNA sequences encoding uroporphyrinogen decarboxylase from tobacco and barley

We have cloned and sequenced a full-length cDNA for uroporphyrinogen decarboxylase (UROD, EC 4.1.1.37) from tobacco (Nicotiana tabacum L.) and a partial cDNA clone from barley (Hordeum vulgare L.). The cDNA of tobacco encodes a protein of 43 kDa, which has 33% overall similarity to UROD sequences determined from other organisms. We propose that tobacco UROD has an N-terminal extension of 39 amino acid residues. This extension is most likely a chloroplast transit sequence. The in vitro translation product of UROD was imported into pea chloroplasts and processed to ca. 39 kDa. A truncated cDNA, from which the putative transit peptide had been deleted, was used to over-express the mature UROD in Escherichia coli. Purified protein showed UROD activity, thus providing an adequate source for subsequent enzymatic characterization and inhibition studies. Expression of UROD was investigated by northern and western blot analysis during greening of etiolated barley seedlings, and in segments of barley primary leaves grown under day/night cycles. The amount of RNA and protein increased during illumination. Maximum UROD-RNA levels were detected in the basal segments relative to the top of the leaf.

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.

5-Aminolevulinic acid synthesis in Escherichia coli requires expression of hemA

hemA and hemM, which are 213 bp apart and divergently transcribed, were separately cloned. We found that hemA is required for 5-aminolevulinic acid (ALA) synthesis in two ALA- auxotrophs. Overexpression of hemM alone did not produce ALA. More ALA was produced by strains harboring a plasmid with both hemA and hemM than by those with hemA alone. We conclude that hemA alone is required for ALA synthesis but hemA and hemM are required for maximal ALA synthesis.

Conditions for the assay of glutamate semialdehyde aminotransferase that overcome the problem of substrate instability

Investigations into the kinetic properties of glutamate semialdehyde aminotransferase, a key enzyme in the metabolic pathway leading to chlorophyll, are made difficult by the instability of the enzyme's substrate glutamate 1-semialdehyde. The rate of spontaneous disappearance of this compound from solution is shown to vary with the square of its concentration and to be pH-dependent. Thus using conditions appropriate to the assay of the enzyme, half of the substrate is lost from solution in a few minutes. Second-order rate constants for the reaction are determined and conditions are selected whereby the effects of the spontaneous reaction are rendered insignificant. The steady-state kinetic properties of the enzyme determined using these conditions are reported.

Direct identification and quantification of the cofactor in glutamate semialdehyde aminotransferase from pea leaves

Glutamate semialdehyde aminotransferase, a key enzyme in the synthetic pathway leading to chlorophyll was purified from pea (Pisum sativum) leaves. Although the preparation contained a single contaminant the enzyme could be unambiguously identified as a dimer of subunit molar mass 45 kDa having an absorption spectrum consistent with the presence of pyridoxamine phosphate as cofactor. The cofactor was released by treatment with strong phosphate at low pH and was identified and quantified fluorimetrically. The specific activity of the enzyme (1.4 mumol.min-1.mg-1; 23 nkatal.mg-1) is very much higher than previously reported.

The Bacillus subtilis hemAXCDBL gene cluster, which encodes enzymes of the biosynthetic pathway from glutamate to uroporphyrinogen III

We have recently reported (M. Petricek, L. Rutberg, I. Schröder, and L. Hederstedt, J. Bacteriol. 172: 2250-2258, 1990) the cloning and sequence of a Bacillus subtilis chromosomal DNA fragment containing hemA proposed to encode the NAD(P)H-dependent glutamyl-tRNA reductase of the C5 pathway for 5-aminolevulinic acid (ALA) synthesis, hemX encoding a hydrophobic protein of unknown function, and hemC encoding hydroxymethylbilane synthase. In the present communication, we report the sequences and identities of three additional hem genes located immediately downstreatm of hemC, namely, hemD encoding uroporphyrinogen III synthase, hemB encoding porphobilinogen synthase, and hemL encoding glutamate-1-semialdehyde 2,1-aminotransferase. The six genes are proposed to constitute a hem operon encoding enzymes required for the synthesis of uroporphyrinogen III from glutamyl-tRNA. hemA, hemB, hemC, and hemD have all been shown to be essential for heme synthesis. However, deletion of an internal 427-bp fragment of hemL did not create a growth requirement for ALA or heme, indicating that formation of ALA from glutamate-1-semialdehyde can occur spontaneously in vivo or that this reaction may also be catalyzed by other enzymes. An analysis of B. subtilis carrying integrated plasmids or deletions-substitutions in or downstream of hemL indicates that no further genes in heme synthesis are part of the proposed hem operon.

Structural genes of glutamate 1-semialdehyde aminotransferase for porphyrin synthesis in a cyanobacterium and Escherichia coli

In bacteria 5-aminolevulinate, the universal precursor in the biosynthesis of the porphyrin nucleus of hemes, chlorophylls and bilins is synthesised by two different pathways: in non-sulphur purple bacteria (Rhodobacter) or Rhizobium 5-aminolevulinate synthase condenses glycine and succinyl-CoA into 5-aminolevulinate as is the case in mammalian cells and yeast. In cyanobacteria, green and purple sulphur bacteria, as in chloroplasts of higher plants and algae a three step pathway converts glutamate into 5-aminolevulinate. The last step is the conversion of glutamate 1-semialdehyde into 5-aminolevulinate. Using a cDNA clone encoding glutamate 1-semialdehyde aminotransferase from barley, genes for this enzyme were cloned from Synechococcus PCC6301 and Escherichia coli and sequenced. The popC gene of E. coli, previously considered to encode 5-aminolevulinate synthase, appears to be a structural gene for glutamate 1-semialdehyde aminotransferase. Domains with identical amino acid sequences comprise 48% of the primary structure of the barley, cyanobacterial and putative E. coli glutamate 1-semialdehyde aminotransferases. The cyanobacterial and barley enzymes share 72% identical residues. The peptide containing a likely pyridoxamine phosphate binding lysine is conserved in all three protein sequences.

Biosynthesis of phycobilins. Formation of the chromophore of phytochrome, phycocyanin and phycoerythrin

Phycobiliproteins play important roles in photomorphogenesis and photosynthesis. The light-absorbing chromophores of the phycobiliproteins are linear tetrapyrroles (bilins) very similar in structure to the mammalian bile pigments. 5-Aminolaevulinate (5-ALA) is the first committed intermediate in phycobilin synthesis. The biosynthesis of 5-ALA, destined for phycobilins, occurs via the five-carbon pathway, now well established for tetrapyrrole synthesis in plants and distinct from the mammalian pathway. The phycobilins are formed by reduction of biliverdin which results from the synthesis and degradation of haem. This haem is an essential intermediate in the biosynthesis of phycobilins. Phycocyanobilin, the blue-green pigment found in certain algae and cyanobacteria, is formed from biliverdin via phytochromobilin, the chromophore of phytochrome. This leads to the likelihood that phytochromobilin is formed as an end product, or intermediate, in the synthesis of all phycobilins.

Cloning and structure of the hem A gene of Escherichia coli K-12

An Escherichia coli gene, which complements two independent hemA mutants of E. coli, has been cloned onto a multi-copy plasmid and both its strands have been sequenced. Both complemented mutants produce 5-aminolevulinic acid (ALA) and display fluorescence after 24h. The cloned sequence appears to encode a 46-kDa protein, which when produced in the maxicell procedure is processed to a 41-kDa protein as determined by sodium dodecyl sulfate-polyacrylamide-gel electrophoresis. The amino acid sequence of the cloned gene product shows no significant homologies with any cloned ALA synthase, nor with any protein, in two E. coli databanks. A second cloned gene fragment, which has its coding region 34 bp away from the coding region of the gene that complements hemA, has been identified as part of protein release factor 1(RF1), thus confirming the location of hemA at min 26.7 and mapping it precisely near RF1. We have shown that E. coli utilizes the intact five-carbon chain of glutamate for the synthesis of ALA [Li et al., J Bacteriol. 171 (1989b) 2547-2552].

5-Aminolevulinic acid synthesis in Escherichia coli

A hemA mutant of Escherichia coli containing a multicopy plasmid which complemented the mutation excreted 5-aminolevulinic acid (ALA) into the medium. [1-14C]glutamate was substantially incorporated into ALA by this strain, whereas [2-14C]glycine was not. Periodate degradation of labeled ALA showed that C-5 of ALA was derived from C-1 of glutamate. The synthesis of ALA by two sonicate fractions which had been processed by gel filtration and dialysis, respectively, was dependent on glutamate, ATP, NADPH, tRNA(Glu), and pyridoxal phosphate. tRNA(Glu) stimulated ALA synthesis in a concentration-dependent manner. Pretreatment with RNase reduced this stimulation. The amino acid sequence of the cloned insert, derived from the nucleotide sequence (J.-M. Li, C. S. Russell, and S. D. Cosloy, J. Cell Biol. 107:617a, 1988), showed no homology with any ALA synthase sequenced to date. These results suggest that E. coli synthesizes ALA by the C5 pathway from the intact five-carbon chain of glutamate.

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.

Formation of 5-aminolevulinate via glutamate-1-semialdehyde and 4,5-dioxovalerate with participation of an RNA component in Scenedesmus obliquus mutant C-2A'

In the yellow mutant C-2A' of the unicellular green alga Scenedesmus obliquus the participation of an RNA species in the conversion of glutamate to 5-aminolevulinate is clearly demonstrated by the fact that RNAase treatment of a soluble enzyme preparation drastically decreases the formation of 5-aminolevulinate. The involvement of 4,5-dioxovalerate in the C5 pathway is demonstrated by the decrease of label in enzymatically formed 5-aminolevulinate from [14C]glutamate by providing an increased unlabelled pool of 4,5-dioxovalerate. Evidence supporting the role of glutamate-1-semialdehyde as an additional intermediate in the reaction sequence is also presented. We propose a new reaction scheme, consistent with the results reported here, for the formation of 5-aminolevulinate via the C5 pathway.

5-Aminolevulinic acid formation from glutamate via the C5 pathway in Clostridium thermoaceticum

A cell-free extract of the anaerobic eubacterium, Clostridium thermoaceticum, catalyzes the synthesis of 5-aminolevulinic acid (ALA) from glutamate via the C5 pathway. The enzyme reaction resembles that of higher plants and algae in cofactor requirements and sensitivity to ribonuclease. From the phylogenetic distribution it is proposed that the C5 pathway evolved earlier than the ALA synthase pathway.

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.

Identification of an intermediate of delta-aminolevulinate biosynthesis in Chlamydomonas by high-performance liquid chromatography

The first committed intermediate of the chlorophyll biosynthetic pathway is delta-aminolevulinic acid (ALA). In plant cells, ALA is formed from glutamate by a pathway not yet clearly defined. One of the proposed pathways involves the reduction of glutamate to glutamate-1-semialdehyde (GSA) via a glutamyl-tRNA intermediate. GSA is then converted to ALA by an aminotransferase. We are studying this pathway using partially purified components from Chlamydomonas reinhardtii in in vitro reactions with [3H]L-glutamate as the substrate and analysis of the radioactive reaction products via HPLC. In reactions either lacking GSA-aminotransferase or containing gabaculine (an inhibitor of aminotransferase), a radioactive intermediate is formed which cochromatographs with synthetic GSA. As observed previously for ALA synthesis, the synthesis of this intermediate has an absolute requirement for RNA, ATP, and active enzymes, while the requirement for NADPH is less stringent. Both the accumulated intermediate and the synthetic GSA can be converted to ALA by the aminotransferase without any additional substrates or cofactors. These results support previous observations that GSA or a very similar compound is an intermediate of ALA synthesis.

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.

Inhibition of chlorophyll synthesis in Hordeum vulgare by 3-amino 2,3-dihydrobenzoic acid (gabaculin)

Gabaculin (3-amino 2,3-dihydrobenzoic acid) is shown to be a very potent inhibitor of chlorophyll formation in Hordeum vulgare. Exposure of leaf segments to 30 microM gabaculin results in an 80% inhibition of chlorophyll synthesis, and this is paralleled by a decrease in carotenoid. Dual-inhibitor studies with dioxoheptanoic acid, which is an inhibitor of delta-amino-laevulinic acid dehydratase, show that gabaculin inhibits an earlier step than dioxoheptanoic acid and affects delta-amino-laevulinic acid synthesis rather than its subsequent metabolism.

RNA is required for enzymatic conversion of glutamate to delta-aminolevulinate by extracts of Chlorella vulgaris

Formation of delta-aminolevulinic acid (ALA) from glutamete catalyzed by a soluble extract from the unicellular green alga, Chlorella vulgaris, was abolished after incubation of the cell extract with bovine pancreatic ribonuclease A (RNase). Cell extract was prepared for the ALA formation assay by high-speed centrifugation and gel-filtration through Sephadex G-25 to remove insoluble and endogenous low-molecular-weight components. RNA hydrolysis products did not affect ALA formation, and RNase did not affect the ability of ATP and NADPH to serve as reaction substrates, indicating that the effect of RNase cannot be attributed to degradation of reaction substrates or transformation of a substrate or cofactor into an inhibitor. The effect of RNase was blocked by prior addition of placental RNase inhibitor (RNasin) to the cell extract, but RNasin did not reverse the effect of prior incubation of the cell extract with RNase, indicating that RNase does not act by degrading a component generated during the ALA-forming reaction, but instead degrades an essential component already present in active cell extract at the time the ALA-forming reaction is initiated. After inactivation of the cell extract by incubation with RNase, followed by administration of RNasin to block further RNase action, ALA-forming activity could be restored to a higher level than originally present by addition of a C. vulgaris tRNA-containing fraction isolated from an active ALA-forming preparation by phenol extraction and DEAE-cellulose chromatography. Baker's yeast tRNA, wheat germ tRNA, Escherichia coli tRNA, and E. coli tRNAglu type II were unable to reconstitute ALA-forming activity in RNase-treated cell extract, even though the cell extract was capable of catalyzing the charging of some of these RNAs with glutamate.

Enzymatic conversion of glutamate to delta-aminolevulinate in soluble extracts of the unicellular green alga, Chlorella vulgaris

Cell-free preparations from the unicellular green alga, Chlorella vulgaris, catalyze the conversion of glutamate to delta-aminolevulinate, which is the first committed step in heme and chlorophyll biosynthesis. Most activity remains in the supernatant fraction after centrifugation at 264,000g. Additional activity can be solubilized from the high-speed pellet by treatment with 0.5 M NaCl. After gel filtration through Sephadex G-25, the reaction catalyzed by the high-speed supernatant requires glutamate, ATP, Mg2+, and NADPH. Boiled extract is inactive. The pH optimum is between 7.8 and 7.9 and the temperature optimum is 30 degrees C. Concentrations required for half-maximal activity are 0.05 mM glutamate, 0.4 mM ATP, 6 mM MgCl2, and 0.4 mM NADPH or 0.7 mM NADH. The reaction requires no additional amino donor. Involvement of pyridoxal phosphate in the catalytic mechanism is suggested by sensitivity to pyridoxal antagonists; 50% inhibition is achieved with 5 microM gabaculine or 0.4 mM aminooxyacetate. Involvement of two or more enzymes is suggested by the nonlinear reaction rate dependence on protein concentration. Evidence for the involvement of an activated glutamate intermediate was obtained by product formation after sequential addition and removal of substrates, and by inhibition (80%) with 1 mM hydroxylamine. Protoheme inhibits the activity by 50% at 1.2 microM. Preincubation of the extract with ATP causes stimulation and/or stabilization of the activity compared to preincubation without ATP or no preincubation. In preparations obtained from C. vulgaris strain C-10, which requires light for greening, dark-grown cells yield one-third as much activity as 4-h-greened cells.

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.