Structural and functional conservation of histidinol dehydrogenase between plants and microbes

Current Promising Therapeutic Targets for Aspergillosis Treatment

Aspergillosis is a fungal disease caused by different species of Aspergillus. They live in soil,dust and decomposed material. Number of Aspergillus species found till now is about 300 and more are still to be identified. Only few Aspergillus species can cause human disease and the most common species for human infection is Aspergillus fumigatus, which is a ubiquitous airborne saprophytic fungus. Severity of the disease ranges from an allergic response to life-threatening generalized infection. They grow optimally at 37°C and can grow upto 50°C. The fungal conidia are being constantly inhaled by humans and animals everyday normally gets eliminated by innate immune mechanism. Due to increasing number of immunocompromised patients, severe and fatal Aspergillosis cases have augmented. Currently, available antifungal drug for the treatment of Aspergillosis act on these three molecular target are 14 alpha demethylase for Azoles, ergosterol for Polyene and β-1,3-glucan synthase for Echinocandin. These antifungal drug show high resistance problem and toxicity. So, it is high time to develop new drugs for treatment with reduced toxicity and drug resistant problem. Synthesis of essential amino acid is absent in human as they obtain it from their diet but fungi synthesis these amino acid. Thus, enzymes in this pathway acts as novel drug target. This article summarizes promising drug targets presents in different metabolic pathway of Aspergillus genome and discusses their molecular functions in detail. This review also list down the inhibitors of these novel target. We present a comprehensive review that will pave way for discovery and development of novel antifungals against these drug targets for Aspergillosis treatment.

The Role of Gene Elongation in the Evolution of Histidine Biosynthetic Genes

Gene elongation is a molecular mechanism consisting of an in-tandem duplication of a gene and divergence and fusion of the two copies, resulting in a gene constituted by two divergent paralogous modules. The aim of this work was to evaluate the importance of gene elongation in the evolution of histidine biosynthetic genes and to propose a possible evolutionary model for some of them. Concerning the genes hisA and hisF, which code for two homologous (β/α)₈-barrels, it has been proposed that the two extant genes could be the result of a cascade of gene elongation/domain shuffling events starting from an ancestor gene coding for just one (β/α) module. A gene elongation event has also been proposed for the evolution of hisB and hisD; structural analyses revealed the possibility of an early elongation event, resulting in the repetition of modules. Furthermore, it is quite possible that the gene elongations responsible for the evolution of the four proteins occurred before the earliest phylogenetic divergence. In conclusion, gene elongation events seem to have played a crucial role in the evolution of the histidine biosynthetic pathway, and they may have shaped the structures of many genes during the first steps of their evolution.

Guarding the gateway to histidine biosynthesis in plants: Medicago truncatula ATP-phosphoribosyltransferase in relaxed and tense states

In the first committed step of histidine biosynthesis, adenosine 5'-triphosphate (ATP) and 5-phosphoribosyl-α1-pyrophosphate (PRPP), in the presence of ATP phosphoribosyltransferase (ATP-PRT, EC 2.4.2.17), yield phosphoribosyl-ATP. ATP-PRTs are subject to feedback inhibition by histidine that allosterically binds between the regulatory domains. Histidine biosynthetic pathways of bacteria, lower eukaryotes, and plants are considered promising targets for the design of antibiotics, antifungal agents, and herbicides because higher organisms are histidine heterotrophs. Plant ATP-PRTs are similar to one of the two types of their bacterial counterparts, the long-type ATP-PRTs. A biochemical and structural study of ATP-PRT from the model legume plant, Medicago truncatula (MedtrATP-PRT1) is reported herein. Two crystal structures, presenting homohexameric MedtrATP-PRT1 in its relaxed (R-) and histidine-bound, tense (T-) states allowed to observe key features of the enzyme and provided the first structural insights into an ATP-PRT from a eukaryotic organism. In particular, they show pronounced conformational reorganizations during R-state to T-state transition that involves substantial movements of domains. This rearrangement requires a trans- to cis- switch of a peptide backbone within the hinge region of MedtrATP-PRT1. A C-terminal α-helix, absent in bacteria, reinforces the hinge that is constituted by two peptide strands. As a result, conformations of the R- and T-states are significantly different from the corresponding states of prokaryotic enzymes with known 3-D structures. Finally, adenosine 5'-monophosphate (AMP) bound at the active site is consistent with a competitive (and synergistic with histidine) nature of AMP inhibition.

Differential effects of zinc exposure on male and female oysters (Crassostrea angulata) as revealed by label-free quantitative proteomics

Oysters accumulate Zn as an adaptation to Zn exposure; however, it is not known whether male and female oysters respond differently to Zn exposure. Proteomic and real-time polymerase chain reaction analyses were used to investigate differential responses of male and female oysters (Crassostrea angulata) to Zn exposure. After exposure to 50 μg L^-1 or 500 μg L^-1 Zn for 30 d, gonads of female oysters accumulated more Zn than those of males, and gonadal development was accelerated in females but was abnormal in males. Differentially expressed proteins after exposure to Zn were identified and shown to function in Zn transport, Ca transport, phosphate metabolism, energy metabolism, immune regulation, oxidative stress responses, gene expression regulation, and fat metabolism. Proteins with functions in Zn transportation and storage, and multifunctional proteins, such as hemicentin-1 and histidinol dehydrogenase, were expressed at significantly higher levels in the gonads of female than male oysters after Zn exposure. Environ Toxicol Chem 2017;36:2602-2613. © 2017 SETAC.

Structures of Medicago truncatula L-Histidinol Dehydrogenase Show Rearrangements Required for NAD+ Binding and the Cofactor Positioned to Accept a Hydride

Plants, lower eukaryotes, bacteria, and archaebacteria synthesise L-histidine (His) in a similar, multistep pathway that is absent in mammals. This makes the His biosynthetic route a promising target for herbicides, antifungal agents, and antibiotics. The last enzyme of the pathway, bifunctional L-histidinol dehydrogenase (HDH, EC 1.1.1.23), catalyses two oxidation reactions: from L-histidinol (HOL) to L-histidinaldehyde and from L-histidinaldehyde to His. Over the course of the reaction, HDH utilises two molecules of NAD⁺ as the hydride acceptor. The object of this study was the HDH enzyme from the model legume plant, Medicago truncatula (MtHDH). Three crystal structures complexed with imidazole, HOL, and His with NAD⁺ provided in-depth insights into the enzyme architecture, its active site, and the cofactor binding mode. The overall structure of MtHDH is similar to the two bacterial orthologues whose three-dimensional structures have been determined. The three snapshots, with the MtHDH enzyme captured in different states, visualise structural rearrangements that allow for NAD⁺ binding for the first time. Furthermore, the MtHDH complex with His and NAD⁺ displays the cofactor molecule situated in a way that would allow for a hydride transfer.

Structural Studies of Medicago truncatula Histidinol Phosphate Phosphatase from Inositol Monophosphatase Superfamily Reveal Details of Penultimate Step of Histidine Biosynthesis in Plants

The penultimate enzyme in the histidine biosynthetic pathway catalyzes dephosphorylation of l-histidinol 1-phosphate (HOLP) into l-histidinol. The recently discovered in Arabidopsis thaliana plant-type histidinol phosphate phosphatase (HPP) shares no homology with the two other HPP superfamilies known previously in prokaryotes and resembles myo-inositol monophosphatases (IMPases). In this work, identification of an HPP enzyme from a model legume, Medicago truncatula (MtHPP) was based on the highest sequence identity to A. thaliana enzyme. Biochemical assays confirmed that MtHPP was able to cleave inorganic phosphate from HOLP but not from d-myo-inositol-1-phosphate, the main substrate of IMPases. Dimers of MtHPP, determined by size exclusion chromatography, in the presence of CO2 or formaldehyde form mutual, methylene-bridged cross-links between Lys(158) and Cys(245) residues. Four high resolution crystal structures, namely complexes with HOLP (substrate), l-histidinol (product), and PO4 (3-) (by-product) as well as the structure showing the cross-linking between two MtHPP molecules, provide detailed structural information on the enzyme. Based on the crystal structures, the enzymatic reaction mechanism of IMPases is accustomed to fit the data for MtHPP. The enzymatic reaction, which requires Mg(2+) cations, is catalyzed mainly by amino acid residues from the N-terminal domain. The C-terminal domain, sharing little identity with IMPases, is responsible for the substrate specificity (i.e. allows the enzyme to distinguish between HOLP and d-myo-inositol-1-phosphate). Structural features, mainly the presence of a conserved Asp(246), allow MtHPP to bind HOLP specifically.

Biosynthesis of Histidine

The biosynthesis of histidine in Escherichia coli and Salmonella typhimurium has been an important model system for the study of relationships between the flow of intermediates through a biosynthetic pathway and the control of the genes encoding the enzymes that catalyze the steps in a pathway. This article provides a comprehensive review of the histidine biosynthetic pathway and enzymes, including regulation of the flow of intermediates through the pathway and mechanisms that regulate the amounts of the histidine biosynthetic enzymes. In addition, this article reviews the structure and regulation of the histidine (his) biosynthetic operon, including transcript processing, Rho-factor-dependent "classical" polarity, and the current model of his operon attenuation control. Emphasis is placed on areas of recent progress. Notably, most of the enzymes that catalyze histidine biosynthesis have recently been crystallized, and their structures have been determined. Many of the histidine biosynthetic intermediates are unstable, and the histidine biosynthetic enzymes catalyze some chemically unusual reactions. Therefore, these studies have led to considerable mechanistic insight into the pathway itself and have provided deep biochemical understanding of several fundamental processes, such as feedback control, allosteric interactions, and metabolite channeling. Considerable recent progress has also been made on aspects of his operon regulation, including the mechanism of pp(p)Gpp stimulation of his operon transcription, the molecular basis for transcriptional pausing by RNA polymerase, and pathway evolution. The progress in these areas will continue as sophisticated new genomic, metabolomic, proteomic, and structural approaches converge in studies of the histidine biosynthetic pathway and mechanisms of control of his biosynthetic genes in other bacterial species.

Histidine biosynthesis

Histidine (His) is one of the standard amino acids in proteins, and plays a critical role in plant growth and development. The chemical properties of the imidazole side group allow His to participate in acid-base catalysis, and in the co-ordination of metal ions. Despite the biological importance of this molecule, His biosynthesis has been somewhat neglected in plants, in stark contrast to micro-organisms where the study of this pathway was fundamental in the discovery of operon structure and regulation by attenuation. With the recent isolation of histidinol-phosphate phosphatase, all the enzymes of His biosynthesis have now been identified in Arabidopsis, and several lines of evidence have implicated ATP-phosphoribosyl transferase (which catalyses the first committed step of the pathway) as playing an important role in the regulation of this pathway. However, little is known about the transcriptional regulation of the His biosynthetic genes, nor how demand for this amino acid is balanced with other metabolic requirements in plants. Similarly, the pathway of His catabolism has yet to be determined.

Biosynthesis of Histidine

The biosynthesis of histidine in Escherichia coli and Salmonella typhimurium has been an important model system for the study of relationships between the flow of intermediates through a biosynthetic pathway and the control of the genes encoding the enzymes that catalyze the steps in a pathway. This article provides a comprehensive review of the histidine biosynthetic pathway and enzymes, including regulation of the flow of intermediates through the pathway and mechanisms that regulate the amounts of the histidine biosynthetic enzymes. In addition, this article reviews the structure and regulation of the histidine (his) biosynthetic operon, including transcript processing, Rho-factor-dependent "classical" polarity, and the current model of his operon attenuation control. Emphasis is placed on areas of recent progress. Notably, most of the enzymes that catalyze histidine biosynthesis have recently been crystallized, and their structures have been determined. Many of the histidine biosynthetic intermediates are unstable, and the histidine biosynthetic enzymes catalyze some chemically unusual reactions. Therefore, these studies have led to considerable mechanistic insight into the pathway itself and have provided deep biochemical understanding of several fundamental processes, such as feedback control, allosteric interactions, and metabolite channeling. Considerable recent progress has also been made on aspects of his operon regulation, including the mechanism of pp(p)Gpp stimulation of his operon transcription, the molecular basis for transcriptional pausing by RNA polymerase, and pathway evolution. The progress in these areas will continue as sophisticated new genomic, metabolomic, proteomic, and structural approaches converge in studies of the histidine biosynthetic pathway and mechanisms of control of his biosynthetic genes in other bacterial species.

Depiction of metabolome changes in histidine-starved Escherichia coli by CE-TOFMS

Metabolic changes in response to histidine starvation were observed in histidine-auxotrophic Escherichia coli using a capillary electrophoresis time-of-flight mass spectrometry (CE-TOFMS)-based metabolomics technique. Prior to the analysis, we prepared an E. coli metabolome list of 727 metabolites reported in the literature. An improved method for metabolite extraction was developed, which resulted in higher extraction efficiency in phosphate-rich metabolites, e.g., ATP and GTP. Based on the results, 375 charged, hydrophilic intermediates in primary metabolisms were analysed simultaneously, providing quantitative data of 198 metabolites. We confirmed that the intracellular levels of intermediates in histidine biosynthesis are rapidly accumulated in response to a drop in histidine level under histidine-starved conditions. Simultaneously, disciplined responses were observed in the glycolysis, tricarboxylic acid cycle, and amino acid and nucleotide biosynthesis pathways as regulated by amino acid starvation.

Targeting of the Brucella suis virulence factor histidinol dehydrogenase by histidinol analogues results in inhibition of intramacrophagic multiplication of the pathogen

Brucella suis histidinol dehydrogenase (HDH) can be efficiently targeted by substrate analogues. The growth of this pathogen in minimal medium was inhibited and the multiplication in human macrophages was totally abolished in the presence of the drugs. These effects have been shown to be correlated with the previously described inhibition of Brucella HDH activity.

Diversity of the trifunctional histidine biosynthesis gene (his) in cereal Phaeosphaeria species

Phaeosphaeria species are important causal agents of Stagonospora leaf blotch diseases in cereals. In this study, the nucleotide sequence and deduced polypeptide of the trifunctional histidine biosynthesis gene (his) are used to investigate the phylogenetic relationships and provide molecular identification among cereal Phaeosphaeria species. The full-length sequences of the his gene were obtained by PCR amplification and compared among cereal Phaeosphaeria species. The coding sequence of the his gene in wheat-biotype P. nodorum (PN-w) was 2697 bp. The his genes in barley-biotype P. nodorum (PN-b), two P. avenaria f. sp. triticea isolates (homothallic Pat1 and Pat3), and Phaeosphaeria species from Polish rye and dallis grass were 2694 bp. The his gene in heterothallic isolate Pat2, however, was 2693 bp because the intron had one fewer base. In P. avenaria f. sp. avenaria (Paa), the his gene was only 2670 bp long. The differences in the size of the his gene contributed to the variation in amino acid sequences in the gap region located between the phosphoribosyl-ATP pyrophosphohydrolase and histidinol dehydrogenase sub-domains. Based on nucleotide and deduced amino acid sequences of the his gene, Pat1 was not closely related to either PN-w or the Paa clade. It appears that rates of evolution of the his gene were fast in cereal Phaeosphaeria species. The possible involvement of meiotic recombination in genetic diversity of the his gene in P. nodorum is discussed.

Genetic dissection of histidine biosynthesis in Arabidopsis

The biosynthesis of histidine (His) in microorganisms, long studied through the isolation and characterization of auxotrophic mutants, has emerged as a paradigm for the regulation of metabolism and gene expression. Much less is known about His biosynthesis in flowering plants. One limiting factor has been the absence of large collections of informative auxotrophs. We describe here the results of a systematic screen for His auxotrophs of Arabidopsis (Arabidopsis thaliana). Ten insertion mutants disrupted in four different biosynthetic genes (HISN2, HISN3, HISN4, HISN6A) were identified through a combination of forward and reverse genetics and were shown to exhibit an embryo-defective phenotype that could be rescued by watering heterozygous plants with His. Male transmission of the mutant allele was in several cases reduced. Knockouts of two redundant genes (HISN1B and HISN5A) had no visible phenotype. Another mutant blocked in the final step of His biosynthesis (hisn8) and a double mutant altered in the redundant first step of the pathway (hisn1a hisn1b) exhibited a combination of gametophytic and embryonic lethality in heterozygotes. Homozygous mutant seedlings and callus tissue produced from rescued seeds appeared normal when grown in the presence of His but typically senesced after continued growth in the absence of His. These knockout mutants document the importance of His biosynthesis for plant growth and development, provide valuable insights into amino acid transport and source-sink relationships during seed development, and represent a significant addition to the limited collection of well-characterized auxotrophs in flowering plants.

ALBINO AND PALE GREEN 10 Encodes BBMII Isomerase Involved in Histidine Biosynthesis in Arabidopsis thaliana

We isolated an Arabidopsis albino and pale green 10 (apg10) mutant which exhibits pale green cotyledons and true leaves at the juvenile stage. We identified a valine to leucine change in BBMII (N'-[(5'-phosphoribosyl)-formimino]-5-aminoimidazole-4-carboxamide ribonucleotide) isomerase involved in histidine biosynthesis. The morphological abnormality of apg10 was recovered by histidine supplementation. The histidine limitation induced by apg10 mutation causes dynamic changes of the free amino acid profile, suggesting the existence of a cross-pathway regulatory mechanism of amino acid biosynthesis in plants. We also revealed that the APG10 knockout mutant exhibited embryo lethality, indicating the essential role of the Arabidopsis BBMII isomerase for plant growth.

Enhancing the first enzymatic step in the histidine biosynthesis pathway increases the free histidine pool and nickel tolerance inArabidopsis thaliana

Naturally selected nickel (Ni) tolerance in Alyssum lesbiacum has been proposed to involve constitutively high levels of endogenous free histidine. Transgenic Arabidopsis thaliana expressing a Salmonella typhimurium ATP phosphoribosyl transferase enzyme (StHisG) resistant to feedback inhibition by histidine contained approximately 2-fold higher histidine concentrations than wild type plants. Under exposure to a toxic Ni concentration, biomass production in StHisG expressing lines was between 14- and 40-fold higher than in wild-type plants. This suggested that enhancing the first step in the histidine biosynthesis pathway is sufficient to increase the endogenous free histidine pool and Ni tolerance in A. thaliana.

Molecular characterization and expression study of a histidine auxotrophic mutant (his1-) of Nicotiana plumbaginifolia

The histidine auxotroph mutant his 1(-) isolated from Nicotiana plumbaginifolia haploid protoplasts was first characterized to be deficient for the enzyme histidinol phosphate aminotransferase that is responsible for one of the last steps of histidine biosynthesis. Expression of the mutated gene at the RNA level was assessed by northern analysis of various tissues. Transcriptional activity was unimpaired by the mutation and, in contrast, a higher level of expression was obtained when compared to the wild-type. The cDNA sequence encoding the mutated gene was isolated by RT-PCR and compared to the wild-type gene. A single point mutation corresponding to the substitution of a G nucleotide by A was identified at position 1212 starting from the translation site. The alignment of the deduced amino acid sequences from the mutated and wild-type gene showed that this mutation resulted in the substitution of an Arg by a His residue at position 381. This Arg residue is a conserved amino acid for histidinol phosphate aminotransferase of many species. These results indicate that the identified mutation results in an altered histidinol phosphate aminotransferase enzyme that is unable to convert the substrate imidazole acetol phosphate to histidinol phosphate and thereby leads to the blockage of histidine biosynthesis. Possible consequences of this blockage on the expression of other amino acid biosynthesis genes were evaluated by analysing the expression of the dhdps gene encoding dihydrodipicolinate synthase, the first key enzyme of the lysine pathway.

Molecular dissection of the role of histidine in nickel hyperaccumulation in Thlaspi goesingense (Hálácsy)

To understand the role of free histidine (His) in Ni hyperaccumulation in Thlaspi goesingense, we investigated the regulation of His biosynthesis at both the molecular and biochemical levels. Three T. goesingense cDNAs encoding the following His biosynthetic enzymes, ATP phosphoribosyltransferase (THG1, GenBank accession no. AF003347), imidazoleglycerol phosphate dehydratase (THB1, GenBank accession no. AF023140), and histidinol dehydrogenase (THD1, GenBank accession no. AF023141) were isolated by functional complementation of Escherichia coli His auxotrophs. Northern analysis of THG1, THD1, and THB1 gene expression revealed that each gene is expressed in both roots and shoots, but at the concentrations and dosage times of Ni treatment used in this study, these genes failed to show any regulation by Ni. We were also unable to observe any increases in the concentration of free His in root, shoot, or xylem sap of T. goesingense in response to Ni exposure. X-ray absorption spectroscopy of root and shoot tissue from T. goesingense and the non-accumulator species Thlaspi arvense revealed no major differences in the coordination of Ni by His in these tissues. We therefore conclude that the Ni hyperaccumulation phenotype in T. goesingense is not determined by the overproduction of His in response to Ni.

UV-responsive genes of arabidopsis revealed by similarity to the Gcn4-mediated UV response in yeast

A UV response that involves the Ras proteins and AP-1 transcription factors has recently been described in mammals and yeast. To test whether an equivalent response exists in plants, we monitored the expression of Arabidopsis histidinol dehydrogenase gene (HDH), a homologue of the yeast HIS4 gene, which is strongly induced by UV light and is a target of the transcriptional activator Gcn4. We show that HDH mRNA levels increase specifically in response to UV-B light. Only small increases were detected upon exposure to other wavelengths. To isolate plant genes involved in this UV response, a gcn4 mutant was transfected with an Arabidopsis thaliana cDNA library. A new type of nucleotide diphosphate kinase (NDPK Ia) with a significant homology to the human tumor suppressor protein Nm23 rescued the gcn4 phenotype. NDPK Ia specifically binds to the HIS4 promoter in vitro and induces HIS4 transcription in yeast. In Arabidopsis, the NDPK Ia protein is located in the nucleus and cytosol. Expression studies in seedlings revealed that the level of NDPK Ia mRNA, like that of HDH, increases in response to UV-B light. It appears that NDPK Ia and HDH are components of a novel UV-responsive pathway in A. thaliana.

Isolation and characterization of a histidine biosynthetic gene in Arabidopsis encoding a polypeptide with two separate domains for phosphoribosyl-ATP pyrophosphohydrolase and phosphoribosyl-AMP cyclohydrolase

Phosphoribosyl-ATP pyrophosphohydrolase (PRA-PH) and phosphoribosyl-AMP cyclohydrolase (PRA-CH) are encoded by HIS4 in yeast and by hisIE in bacteria and catalyze the second and the third step, respectively, in the histidine biosynthetic pathway. By complementing a hisI mutation of Escherichia coli with an Arabidopsis cDNA library, we isolated an Arabidopsis cDNA (At-IE) that possesses these two enzyme activities. The At-IE cDNA encodes a bifunctional protein of 281 amino acids with a calculated molecular mass of 31,666 D. Genomic DNA-blot analysis with the At-IE cDNA as a probe revealed a single-copy gene in Arabidopsis, and RNA-blot analysis showed that the At-IE gene was expressed ubiquitously throughout development. Sequence comparison suggested that the At-IE protein has an N-terminal extension of about 50 amino acids with the properties of a chloroplast transit peptide. We demonstrated through heterologous expression studies in E. coli that the functional domains for the PRA-CH (hisI) and PRA-PH (hisE) resided in the N-terminal and the C-terminal halves, respectively, of the At-IE protein.

Molecular cloning and expression of a cDNA sequence encoding histidinol phosphate aminotransferase from Nicotiana tabacum

A Nicotiana tabacum cDNA sequence encoding histidinol phosphate aminotransferase (HPA) was isolated by functional complementation of an Escherichia coli histidine auxotroph (UTH780). The enzymatic assay has confirmed that the isolated cDNA encodes a functional HPA protein. Amino acid sequence alignment of the HPA protein from N. tabacum, Saccharomyces cerevisiae and E. coli revealed that, despite the low degree of identity, some residues were found to be highly conserved. The predicted protein contains a transit peptide sequence at the amino-terminal end, suggesting a chloroplastic localization of the HPA enzyme. Western blot analysis demonstrated that the deduced HPA protein and the mature HPA protein have an apparent molecular mass of about 45 kDa and 40 kDa respectively. Gene copy number estimation by Southern analysis indicates the presence of at least two genes per haploid genome coding for this protein in Nicotiana sp. From northern analysis results, the gene seems to be highly expressed in green tissues and the detected transcript showed a single band of expected molecular size.

An Arabidopsis cDNA encoding a bifunctional glutamine amidotransferase/cyclase suppresses the histidine auxotrophy of a Saccharomyces cerevisiae his7 mutant

A cDNA encoding a glutamine amidotransferase and cyclase catalyzing the fifth and sixth steps of the histidine (His) biosynthetic pathway has been isolated from Arabidopsis thaliana. The N- and C-terminal domains of the primary structure deduced from a full-length Arabidopsis hisHF (At-HF) cDNA showed significant homology to the glutamine amidotransferase and cyclase of microorganisms, respectively. Effective suppression of the His auxotrophy of a Saccharomyces cerevisiae his7 mutant with the At-HF cDNA confirmed that the At-HF protein has bifunctional glutamine amidotransferase (HisH) and cyclase (HisF) activities.

The HIS4 gene from the yeast Kluyveromyces lactis

The Kluyveromyces lactis HIS4 gene was cloned by complementation of a Saccharomyces cerevisiae his4 mutant. Sequence analysis revealed a 2388 bp open reading frame encoding a single polypeptide predicted to encompass three distinct enzymatic activities (phosphoribosyl-AMP cyclohydrolase, phosphoribosyl-ATP pyrophosphohydrolase and histidinol dehydrogenase). This structural organization is strikingly similar to that of the His4 proteins from S. cerevisiae and Pichia pastoris. Transcript analysis detected a single mRNA species of 2.5 kb.

Molecular evolution of the histidine biosynthetic pathway

The available sequences of genes encoding the enzymes associated with histidine biosynthesis suggest that this is an ancient metabolic pathway that was assembled prior to the diversification of the Bacteria, Archaea, and Eucarya. Paralogous duplications, gene elongation, and fusion events involving different his genes have played a major role in shaping this biosynthetic route. Evidence that the hisA and the hisF genes and their homologous are the result of two successive duplication events that apparently took place before the separation of the three cellular lineages is extended. These two successive gene duplication events as well as the homology between the hisH genes and the sequences encoding the TrpG-type amidotransferases support the idea that during the early stages of metabolic evolution at least parts of the histidine biosynthetic pathway were mediated by enzymes of broader substrate specificities. Maximum likelihood trees calculated for the available sequences of genes encoding these enzymes have been obtained. Their topologies support the possibility of an evolutionary proximity of archaebacteria with low GC Gram-positive bacteria. This observation is consistent with those detected by other workers using the sequences of heat-shock proteins (HSP70), glutamine synthetases, glutamate dehydrogenases, and carbamoylphosphate synthetases.

Evidence for cross-pathway regulation of metabolic gene expression in plants

In Arabidopsis thaliana, blocking histidine biosynthesis with a specific inhibitor of imidazoleglycerol-phosphate dehydratase caused increased expression of eight genes involved in the biosynthesis of aromatic amino acids, histidine, lysine, and purines. A decrease in expression of glutamine synthetase was also observed. Addition of histidine eliminated the gene-regulating effects of the inhibitor, demonstrating that the changes in gene expression resulted from histidine-pathway blockage. These results show that plants are capable of cross-pathway metabolic regulation.

Cloning, primary structure, and regulation of the HIS7 gene encoding a bifunctional glutamine amidotransferase: cyclase from Saccharomyces cerevisiae

The Saccharomyces cerevisiae HIS7 gene was cloned by its location immediately downstream of the previously isolated and characterized ARO4 gene. The two genes have the same orientation with a distance of only 416 bp between the two open reading frames. The yeast HIS7 gene represents the first isolated eukaryotic gene encoding the enzymatic activities which catalyze the fifth and sixth step in histidine biosynthesis. The open reading frame of the HIS7 gene has a length of 1,656 bp resulting in a gene product of 552 amino acids with a calculated molecular weight of 61,082. Two findings implicate a bifunctional nature of the HIS7 gene product. First, the N-terminal and C-terminal segments of the deduced HIS7 amino acid sequence show significant homology to prokaryotic monofunctional glutamine amidotransferases and cyclases, respectively, involved in histidine biosynthesis. Second, the yeast HIS7 gene is able to suppress His auxotrophy of corresponding Escherichia coli hisH and hisF mutants. HIS7 gene expression is regulated by the general control system of amino acid biosynthesis. GCN4-dependent and GCN4-independent (basal) transcription use different initiator elements in the HIS7 promoter.

Histidine biosynthesis genes in Lactococcus lactis subsp. lactis

The genes of Lactococcus lactis subsp. lactis involved in histidine biosynthesis were cloned and characterized by complementation of Escherichia coli and Bacillus subtilis mutants and DNA sequencing. Complementation of E. coli hisA, hisB, hisC, hisD, hisF, hisG, and hisIE genes and the B. subtilis hisH gene (the E. coli hisC equivalent) allowed localization of the corresponding lactococcal genes. Nucleotide sequence analysis of the 11.5-kb lactococcal region revealed 14 open reading frames (ORFs), 12 of which might form an operon. The putative operon includes eight ORFs which encode proteins homologous to enzymes involved in histidine biosynthesis. The operon also contains (i) an ORF encoding a protein homologous to the histidyl-tRNA synthetases but lacking a motif implicated in synthetase activity, which suggests that it has a role different from tRNA aminoacylation, and (ii) an ORF encoding a protein that is homologous to the 3'-aminoglycoside phosphotransferases but does not confer antibiotic resistance. The remaining ORFs specify products which have no homology with proteins in the EMBL and GenBank data bases.