Porphobilinogen deaminase and uroporphyrinogen III synthase: structure, molecular biology, and mechanism

Diverse enzymatic chemistry for propionate side chain cleavages in tetrapyrrole biosynthesis

Tetrapyrroles represent a unique class of natural products that possess diverse chemical architectures and exhibit a broad range of biological functions. Accordingly, they attract keen attention from the natural product community. Many metal-chelating tetrapyrroles serve as enzyme cofactors essential for life, while certain organisms produce metal-free porphyrin metabolites with biological activities potentially beneficial for the producing organisms and for human use. The unique properties of tetrapyrrole natural products derive from their extensively modified and highly conjugated macrocyclic core structures. Most of these various tetrapyrrole natural products biosynthetically originate from a branching point precursor, uroporphyrinogen III, which contains propionate and acetate side chains on its macrocycle. Over the past few decades, many modification enzymes with unique catalytic activities, and the diverse enzymatic chemistries employed to cleave the propionate side chains from the macrocycles, have been identified. In this review, we highlight the tetrapyrrole biosynthetic enzymes required for the propionate side chain removal processes and discuss their various chemical mechanisms.

ONE-SENTENCE SUMMARY

This mini-review describes various enzymes involved in the propionate side chain cleavages during the biosynthesis of tetrapyrrole cofactors and secondary metabolites.

Plasmodium falciparum hydroxymethylbilane synthase does not house any cosynthase activity within the haem biosynthetic pathway

Uroporphyrinogen III, the universal progenitor of macrocyclic, modified tetrapyrroles, is produced from aminolaevulinic acid (ALA) by a conserved pathway involving three enzymes: porphobilinogen synthase (PBGS), hydroxymethylbilane synthase (HmbS) and uroporphyrinogen III synthase (UroS). The gene encoding uroporphyrinogen III synthase has not yet been identified in Plasmodium falciparum, but it has been suggested that this activity is housed inside a bifunctional hybroxymethylbilane synthase (HmbS). Additionally, an unknown protein encoded by PF3D7_1247600 has also been predicted to possess UroS activity. In this study it is demonstrated that neither of these proteins possess UroS activity and the real UroS remains to be identified. This was demonstrated by the failure of codon-optimized genes to complement a defined Escherichia coli hemD⁻ mutant (SASZ31) deficient in UroS activity. Furthermore, HPLC analysis of the oxidized reaction product from recombinant, purified P. falciparum HmbS showed that only uroporphyrin I could be detected (corresponding to hydroxymethylbilane production). No uroporphyrin III was detected, showing that P. falciparum HmbS does not have UroS activity and can only catalyze the formation of hydroxymethylbilane from porphobilinogen.

Porphyrias in the Age of Targeted Therapies

The porphyrias are a group of eight rare genetic disorders, each caused by the deficiency of one of the enzymes in the heme biosynthetic pathway, resulting in the excess accumulation of heme precursors and porphyrins. Depending on the tissue site as well as the chemical characteristics of the accumulating substances, the clinical features of different porphyrias vary substantially. Heme precursors are neurotoxic, and their accumulation results in acute hepatic porphyria, while porphyrins are photoactive, and excess amounts cause cutaneous porphyrias, which present with photosensitivity. These disorders are clinically heterogeneous but can result in severe clinical manifestations, long-term complications and a significantly diminished quality of life. Medical management consists mostly of the avoidance of triggering factors and symptomatic treatment. With an improved understanding of the underlying pathophysiology and disease mechanisms, new treatment approaches have become available, which address the underlying defects at a molecular or cellular level, and promise significant improvement, symptom prevention and more effective treatment of acute and chronic disease manifestations.

Molecular Pathways and Pigments Underlying the Colors of the Pearl Oyster Pinctada margaritifera var. cumingii (Linnaeus 1758)

The shell color of the Mollusca has attracted naturalists and collectors for hundreds of years, while the molecular pathways regulating pigment production and the pigments themselves remain poorly described. In this study, our aim was to identify the main pigments and their molecular pathways in the pearl oyster Pinctada margaritifera—the species displaying the broadest range of colors. Three inner shell colors were investigated—red, yellow, and green. To maximize phenotypic homogeneity, a controlled population approach combined with common garden conditioning was used. Comparative analysis of transcriptomes (RNA-seq) of P. margaritifera with different shell colors revealed the central role of the heme pathway, which is involved in the production of red (uroporphyrin and derivates), yellow (bilirubin), and green (biliverdin and cobalamin forms) pigments. In addition, the Raper–Mason, and purine metabolism pathways were shown to produce yellow pigments (pheomelanin and xanthine) and the black pigment eumelanin. The presence of these pigments in pigmented shell was validated by Raman spectroscopy. This method also highlighted that all the identified pathways and pigments are expressed ubiquitously and that the dominant color of the shell is due to the preferential expression of one pathway compared with another. These pathways could likely be extrapolated to many other organisms presenting broad chromatic variation.

Congenital erythropoietic porphyria: Recent advances

Congenital erythropoietic porphyria (CEP) is a rare autosomal recessive disorder characterized by photosensitivity and by hematologic abnormalities in affected individuals. CEP is caused by mutations in the uroporphyrinogen synthase (UROS) gene. In three reported cases, CEP has been associated with a specific X-linked GATA1 mutation. Disease-causing mutations in either gene result in absent or markedly reduced UROS enzymatic activity. This in turn leads to the accumulation of the non-physiologic and photoreactive porphyrinogens, uroporphyrinogen I and coproporphyrinogen I, which damage erythrocytes and elicit a phototoxic reaction upon light exposure. The clinical spectrum of CEP depends on the level of residual UROS activity, which is determined by the underlying pathogenic loss-of-function UROS mutations. Disease severity ranges from non-immune hydrops fetalis in utero to late-onset disease with only mild cutaneous involvement. The clinical characteristics of CEP include exquisite photosensitivity to visible light resulting in bullous vesicular lesions which, when infected lead to progressive photomutilation of sun-exposed areas such as the face and hands. In addition, patients have erythrodontia (brownish discoloration of teeth) and can develop corneal scarring. Chronic transfusion-dependent hemolytic anemia is common and leads to bone marrow hyperplasia, which further increases porphyrin production. Management of CEP consists of strict avoidance of exposure to visible light with sun-protective clothing, sunglasses, and car and home window filters. Adequate care of ruptured vesicles and use of topical antibiotics is indicated to prevent superinfections and osteolysis. In patients with symptomatic hemolytic anemia, frequent erythrocyte cell transfusions may be necessary to suppress hematopoiesis and decrease marrow production of the phototoxic porphyrins. In severe transfection-dependent cases, bone marrow or hematopoietic stem cell transplantation has been performed, which is curative. Therapeutic approaches including gene therapy, proteasome inhibition, and pharmacologic chaperones are under investigation.

Proteomic Analysis of Arabidopsis pldα1 Mutants Revealed an Important Role of Phospholipase D Alpha 1 in Chloroplast Biogenesis

Phospholipase D alpha 1 (PLDα1) is a phospholipid hydrolyzing enzyme playing multiple regulatory roles in stress responses of plants. Its signaling activity is mediated by phosphatidic acid (PA) production, capacity to bind, and modulate G-protein complexes or by interaction with other proteins. This work presents a quantitative proteomic analysis of two T-DNA insertion pldα1 mutants of Arabidopsis thaliana. Remarkably, PLDα1 knockouts caused differential regulation of many proteins forming protein complexes, while PLDα1 might be required for their stability. Almost one third of differentially abundant proteins (DAPs) in pldα1 mutants are implicated in metabolism and RNA binding. Latter functional class comprises proteins involved in translation, RNA editing, processing, stability, and decay. Many of these proteins, including those regulating chloroplast protein import and protein folding, share common functions in chloroplast biogenesis and leaf variegation. Consistently, pldα1 mutants showed altered level of TIC40 (a major regulator of protein import into chloroplast), differential accumulation of photosynthetic protein complexes and changed chloroplast sizes as revealed by immunoblotting, blue-native electrophoresis, and microscopic analyses, respectively. Our proteomic analysis also revealed that genetic depletion of PLDα1 also affected proteins involved in cell wall architecture, redox homeostasis, and abscisic acid signaling. Taking together, PLDα1 appears as a protein integrating cytosolic and plastidic protein translations, plastid protein degradation, and protein import into chloroplast in order to regulate chloroplast biogenesis in Arabidopsis.

Facilitation of Endosomal Recycling by an IRG Protein Homolog Maintains Apical Tubule Structure in Caenorhabditis elegans

Determination of luminal diameter is critical to the function of small single-celled tubes. A series of EXC proteins, including EXC-1, prevent swelling of the tubular excretory canals in Caenorhabditis elegans In this study, cloning of exc-1 reveals it to encode a homolog of mammalian IRG proteins, which play roles in immune response and autophagy and are associated with Crohn's disease. Mutants in exc-1 accumulate early endosomes, lack recycling endosomes, and exhibit abnormal apical cytoskeletal structure in regions of enlarged tubules. EXC-1 interacts genetically with two other EXC proteins that also affect endosomal trafficking. In yeast two-hybrid assays, wild-type and putative constitutively active EXC-1 binds to the LIM-domain protein EXC-9, whose homolog, cysteine-rich intestinal protein, is enriched in mammalian intestine. These results suggest a model for IRG function in forming and maintaining apical tubule structure via regulation of endosomal recycling.

Synthesis, delivery and regulation of eukaryotic heme and Fe-S cluster cofactors

In humans, the bulk of iron in the body (over 75%) is directed towards heme- or Fe-S cluster cofactor synthesis, and the complex, highly regulated pathways in place to accomplish biosynthesis have evolved to safely assemble and load these cofactors into apoprotein partners. In eukaryotes, heme biosynthesis is both initiated and finalized within the mitochondria, while cellular Fe-S cluster assembly is controlled by correlated pathways both within the mitochondria and within the cytosol. Iron plays a vital role in a wide array of metabolic processes and defects in iron cofactor assembly leads to human diseases. This review describes progress towards our molecular-level understanding of cellular heme and Fe-S cluster biosynthesis, focusing on the regulation and mechanistic details that are essential for understanding human disorders related to the breakdown in these essential pathways.

Recent advances in the biosynthesis of modified tetrapyrroles: the discovery of an alternative pathway for the formation of heme and heme d 1

Hemes (a, b, c, and o) and heme d 1 belong to the group of modified tetrapyrroles, which also includes chlorophylls, cobalamins, coenzyme F430, and siroheme. These compounds are found throughout all domains of life and are involved in a variety of essential biological processes ranging from photosynthesis to methanogenesis. The biosynthesis of heme b has been well studied in many organisms, but in sulfate-reducing bacteria and archaea, the pathway has remained a mystery, as many of the enzymes involved in these characterized steps are absent. The heme pathway in most organisms proceeds from the cyclic precursor of all modified tetrapyrroles uroporphyrinogen III, to coproporphyrinogen III, which is followed by oxidation of the ring and finally iron insertion. Sulfate-reducing bacteria and some archaea lack the genetic information necessary to convert uroporphyrinogen III to heme along the "classical" route and instead use an "alternative" pathway. Biosynthesis of the isobacteriochlorin heme d 1, a cofactor of the dissimilatory nitrite reductase cytochrome cd 1, has also been a subject of much research, although the biosynthetic pathway and its intermediates have evaded discovery for quite some time. This review focuses on the recent advances in the understanding of these two pathways and their surprisingly close relationship via the unlikely intermediate siroheme, which is also a cofactor of sulfite and nitrite reductases in many organisms. The evolutionary questions raised by this discovery will also be discussed along with the potential regulation required by organisms with overlapping tetrapyrrole biosynthesis pathways.

Crystal structure of uroporphyrinogen III synthase from Pseudomonas syringae pv. tomato DC3000

Uroporphyrinogen III synthase (U3S) is one of the key enzymes in the biosynthesis of tetrapyrrole compounds. It catalyzes the cyclization of the linear hydroxymethylbilane (HMB) to uroporphyrinogen III (uro'gen III). We have determined the crystal structure of U3S from Pseudomonas syringae pv. tomato DC3000 (psU3S) at 2.5Å resolution by the single wavelength anomalous dispersion (SAD) method. Each psU3S molecule consists of two domains interlinked by a two-stranded antiparallel β-sheet. The conformation of psU3S is different from its homologous proteins because of the flexibility of the linker between the two domains, which might be related to this enzyme's catalytic properties. Based on mutation and activity analysis, a key residue, Arg219, was found to be important for the catalytic activity of psU3S. Mutation of Arg219 to Ala caused a decrease in enzymatic activity to about 25% that of the wild type enzyme. Our results provide the structural basis and biochemical evidence to further elucidate the catalytic mechanism of U3S.

A novel pathway for the biosynthesis of heme in Archaea: genome-based bioinformatic predictions and experimental evidence

Heme is an essential prosthetic group for many proteins involved in fundamental biological processes in all three domains of life. In Eukaryota and Bacteria heme is formed via a conserved and well-studied biosynthetic pathway. Surprisingly, in Archaea heme biosynthesis proceeds via an alternative route which is poorly understood. In order to formulate a working hypothesis for this novel pathway, we searched 59 completely sequenced archaeal genomes for the presence of gene clusters consisting of established heme biosynthetic genes and colocalized conserved candidate genes. Within the majority of archaeal genomes it was possible to identify such heme biosynthesis gene clusters. From this analysis we have been able to identify several novel heme biosynthesis genes that are restricted to archaea. Intriguingly, several of the encoded proteins display similarity to enzymes involved in heme d(1) biosynthesis. To initiate an experimental verification of our proposals two Methanosarcina barkeri proteins predicted to catalyze the initial steps of archaeal heme biosynthesis were recombinantly produced, purified, and their predicted enzymatic functions verified.

Structure and function of enzymes in heme biosynthesis

Tetrapyrroles like hemes, chlorophylls, and cobalamin are complex macrocycles which play essential roles in almost all living organisms. Heme serves as prosthetic group of many proteins involved in fundamental biological processes like respiration, photosynthesis, and the metabolism and transport of oxygen. Further, enzymes such as catalases, peroxidases, or cytochromes P450 rely on heme as essential cofactors. Heme is synthesized in most organisms via a highly conserved biosynthetic route. In humans, defects in heme biosynthesis lead to severe metabolic disorders called porphyrias. The elucidation of the 3D structures for all heme biosynthetic enzymes over the last decade provided new insights into their function and elucidated the structural basis of many known diseases. In terms of structure and function several rather unique proteins were revealed such as the V-shaped glutamyl-tRNA reductase, the dipyrromethane cofactor containing porphobilinogen deaminase, or the "Radical SAM enzyme" coproporphyrinogen III dehydrogenase. This review summarizes the current understanding of the structure-function relationship for all heme biosynthetic enzymes and their potential interactions in the cell.

The tetrapyrrole biosynthetic pathway and its regulation in Rhodobacter capsulatus

The purple anoxygenic photosynthetic bacterium Rhodobacter capsulatus is capable of growing in aerobic or anaerobic conditions, in the dark or using light, etc. Achieving versatile metabolic adaptations from respiration to photosynthesis requires the use of tetrapyrroles such as heme and bacteriochlorophyll, in order to carry oxygen, to transfer electrons, and to harvest light energy. A third tetrapyrrole, cobalamin (vitamin B(12)), is synthesized and used as a cofactor for many enzymes. Heme, bacteriochlorophyll, and vitamin B(12) constitute three major end products of the tetrapyrrole biosynthetic pathway in purple bacteria. Their respective synthesis involves a plethora of enzymes, several that have been characterized and several that are uncharacterized, as described in this review. To respond to changes in metabolic requirements, the pathway undergoes complex regulation to direct the flow of tetrapyrrole intermediates into a specific branch(s) at the expense of other branches of the pathway. Transcriptional regulation of the tetrapyrrole synthesizing enzymes by redox conditions and pathway intermediates is reviewed. In addition, we discuss the involvement of several transcription factors (RegA, CrtJ, FnrL, AerR, HbrL, Irr) as well as the role of riboswitches. Finally, the interdependence of the tetrapyrrole branches on each other synthesis is discussed.

Complex formation between protoporphyrinogen IX oxidase and ferrochelatase during haem biosynthesis in Thermosynechococcus elongatus

During haem and chlorophyll biosynthesis, flavin-dependent protoporphyrinogen IX oxidase catalyses the six-electron oxidation of protoporphyrinogen IX to form protoporphyrin IX. In the following step, iron is inserted into protoporphyrin IX by ferrochelatase. Based on the solved crystal structures of these enzymes, an in silico model for a complex between these two enzymes was proposed to protect the highly photoreactive intermediate protoporphyrin IX. The existence of this complex was verified by two independent techniques. First, co-immunoprecipitation experiments using antibodies directed against recombinantly produced and purified Thermosynechococcus elongatus protoporphyrinogen IX oxidase and ferrochelatase demonstrated their physical interaction. Secondly, protein complex formation was visualized by in vivo immunogold labelling and electron microscopy with T. elongatus cells. Finally, oxygen-dependent coproporphyrinogen III oxidase, which catalyses the formation of protoporphyrinogen IX, was not found to be part of this complex when analysed with the same methodology.

Hypoxia decreases the expression of the two enzymes responsible for producing linear and cyclic tetrapyrroles in the heme biosynthetic pathway

Heme is synthesized in all cell types in aerobic organisms. Hydroxymethylbilane synthase (HMBS) and uroporphyrinogen III synthase (UROS) catalyze two consecutive reactions in the heme biosynthetic pathway, generating the first linear and the first cyclic tetrapyrroles, respectively. Each of the HMBS and UROS genes contains the two separate promoters that generate ubiquitous and erythroid-specific mRNAs. Despite the functional significance of HMBS and UROS, regulation of their gene expression remains to be investigated. Here, we showed that hypoxia (1% O(2)) decreased the expression of ubiquitous mRNAs for HMBS and UROS by three- and twofold, respectively, in human hepatic cells (HepG2 and Hep3B), whereas the expression of ubiquitous and erythroid HMBS and UROS mRNAs remained unchanged in erythroid cells (YN-1 and K562). Unexpectedly, hypoxia did not decrease the half-life of HMBS mRNA (8.4 h under normoxia versus 9.1 h under hypoxia) or UROS mRNA (9.0 versus 10.4 h) in hepatic cells. It is therefore unlikely that a change in mRNA stability is responsible for the hypoxia-mediated decrease in the expression levels of these mRNAs. Furthermore, expression levels of HMBS and UROS mRNAs were decreased under normoxia by treatment with deferoxamine or cobalt chloride in hepatic cells, while hypoxia-inducible factor 1alpha was accumulated. Thus, the decrease in the expression of ubiquitous HMBS and UROS mRNAs is associated with accumulation of hypoxia-inducible factor 1alpha protein. In conclusion, the expression of HMBS and UROS mRNAs may be coordinately regulated, which represents a newly identified mechanism that is important for heme homeostasis.

High-performance liquid chromatography/electrospray ionization tandem mass spectrometry of hydroxylated uroporphyrin and urochlorin derivatives formed by photochemical oxidation of uroporphyrinogen I

Hydroxylated uroporphyrin I and urochlorin I derivatives formed by photochemical oxidation of uroporphyrinogen I were separated by high-performance liquid chromatography and fully characterized by electrospray ionization tandem mass spectrometry. The porphyrins and chlorins were identified by analysis of their product ion spectra with each hydroxylated derivative giving a characteristic collision-induced dissociation fragmentation pattern. The porphyrins and chlorins characterized were meso-hydroxyuroporphyrin I, alpha-hydroxypropionic acid uroporphyrin I, beta-hydroxypropionic acid uroporphyrin I, hydroxyacetic acid uroporphyrin I, trans-7-hydroxy-8-spirolactoneurochlorin I, cis-7-hydroxy-8-spirolactoneurochlorin I and trans- and cis-7,8-dihydroxyurochlorins I.

Identification of rate-limiting steps in yeast heme biosynthesis

The heme biosynthesis pathway in the yeast Saccharomyces cerevisiae is a highly regulated system, but the mechanisms accounting for this regulation remain unknown. In an attempt to identify rate-limiting steps in heme synthesis, which may constitute potential regulatory points, we constructed yeast strains overproducing two enzymes of the pathway: the porphobilinogen synthase (PBG-S) and deaminase (PBG-D). Biochemical analysis of the enzyme-overproducing strains revealed intracellular porphobilinogen and porphyrin accumulation. These results indicate that both enzymes play a rate-limiting role in yeast heme biosynthesis.

Acute intermittent porphyria in Sweden. Molecular, functional and clinical consequences of some new mutations found in the porphobilinogen deaminase gene

Acute intermittent porphyria (AIP) is an autosomal dominant disorder caused by a partial deficit of porphobilinogen deaminase (PBGD), the third of eight enzymes in the haem biosynthetic pathway. The overt disease is characterized by neuropsychiatric symptoms that are often triggered by exogenous factors such as certain drugs, stress, and alcohol. The aim of this work has been to identify the underlying genetic defect in each AIP-affected family in order to provide early counselling to assist in the avoidance of precipitating factors. The prevalence of AIP in Sweden is in the order of 1:10 000. The major mutation in Sweden, W198X, is due to a founder effect in the northern part of the country. This mutation, together with a further 11 mutations, have been reported previously. The present communication encompasses the great majority of AIP kindreds in Sweden and includes a further 27 mutations within the PBGD gene. This includes 14 completely new mutations, as well as 11 known mutations detected for the first time in Sweden. The majority of the mutations are located in exons 10 and 12 with fewer in exon 7. The clinical and biochemical outcomes in some patients are described. We also use the three-dimensional structure of the porphobilinogen deaminase enzyme to predict the possible molecular and functional consequences of the new Swedish missense and nonsense mutations.

The W198X and R173W mutations in the porphobilinogen deaminase gene in acute intermittent porphyria have higher clinical penetrance than R167W. A population-based study

In northern Sweden, 468 patients with DNA-verified acute intermittent porphyria (AIP) were registered. A higher prevalence of manifest AIP was found in patients with mutations W198X and R173W when separately compared with mutation R167W, indicating higher clinical penetrance. Signs of increased seriousness of the disease were also found in patients with the W198X and R173W mutations in relation to the number and duration of attacks, impaired renal function and chronic disability. One explanation could be lower PBGD enzyme activity resulting from the W198X and R173W mutations than from the R167W mutation, though other factors might also be the cause.

Insertion of Alu element responsible for acute intermittent porphyria

In this study, we report a large Finnish family in which an Alu element interferes with the coding region of the porphobilinogen deaminase (PBGD) gene resulting in acute intermittent porphyria (AIP). Polymerase chain reaction (PCR) and single-strand conformation polymorphism (SSCP) analysis of exon 5 among patients showed an abnormal band around 350 bp apart from the normal bands. Subcloning and sequencing of the fragment revealed a 333-bp Alu sequence that was directly inserted into exon 5 in antisense orientation. The junction sequences included a 13-bp target site duplication. This Alu cassette belongs to a Ya5 subfamily, one of the youngest and currently most active Alu subfamilies in evolution. The Alu insertion resulted in a dramatically decreased steady-state level of the allelic transcript, as this Alu sequence could not be demonstrated by direct sequencing of the amplified cDNA synthesized from total RNA extracted from the patients' lymphoblast cell lines. A stop codon present in the reading frame causes premature termination of PBGD synthesis. The predicted polypeptide contains 64 of the 361 amino acids of PBGD, followed by 13 amino acids that are not identical to the PBGD polypeptide. To further characterize the consequences of the insertion, the Alu sequence was inserted into exon 5 of the PBGD cDNA and expressed in the eukaryotic COS-1 cell line. The mutated construct expressed no enzyme activity comparable to that of the wild-type PBGD; furthermore, no mutant protein could be detected by Western blot analysis.

Uroporphyrinogen III synthase. An alternative promoter controls erythroid-specific expression in the murine gene

Uroporphyrinogen III synthase (URO-synthase, EC 4.2.1.75) is the fourth enzyme of the heme biosynthetic pathway and is the defective enzyme in congenital erythropoietic porphyria. To investigate the erythroid-specific expression of murine URO-synthase, the cDNA and approximately 24-kilobase genomic sequences were isolated and characterized. Three alternative transcripts were identified containing different 5'-untranslated regions (5'-UTRs), but identical coding exons 2B through 10. Transcripts with 5'-UTR exon 1A alone or fused to exon 1B were ubiquitously expressed (housekeeping), whereas transcripts with 5'-UTR exon 2A were only present in erythroid cells (erythroid-specific). Analysis of the TATA-less housekeeping promoter upstream of exon 1A revealed binding sites for ubiquitously expressed transcription factors Sp1, NF1, AP1, Oct1, and NRF2. The TATA-less erythroid-specific promoter upstream of exon 2A had nine putative GATA1 erythroid enhancer binding sites. Luciferase promoter/reporter constructs transfected into NIH 3T3 and mouse erythroleukemia cells indicated that the housekeeping promoter was active in both cell lines, while the erythroid promoter was active only in erythroid cells. Site-specific mutagenesis of the first GATA1 binding site markedly reduced luciferase activity in K562 cells (<5% of wild type). Thus, housekeeping and erythroid-specific transcripts are expressed from alternative promoters of a single mouse URO-synthase gene.

Purification and characterization of the clostridium josui porphobilinogen deaminase encoded by the hemC gene from a recombinant escherichia coli

The porphobilinogen deaminase encoded by the Clostridium josui hemC gene was purified from a recombinant Escherichia coli strain and its properties were characterized. The optimal temperature and pH of the purified enzyme were 65 degrees C and 7.0, respectively. This enzyme was quite thermostable: it retained 86% of the original activity after incubation at 70 degrees C for 1 h. The Km and Vmax values of the enzyme were 65 microM and 3.3 micromol/h/mg for porphobilinogen, respectively.

Linkage map of Escherichia coli K-12, edition 10: the traditional map

This map is an update of the edition 9 map by Berlyn et al. (M. K. B. Berlyn, K. B. Low, and K. E. Rudd, p. 1715-1902, in F. C. Neidhardt et al., ed., Escherichia coli and Salmonella: cellular and molecular biology, 2nd ed., vol. 2, 1996). It uses coordinates established by the completed sequence, expressed as 100 minutes for the entire circular map, and adds new genes discovered and established since 1996 and eliminates those shown to correspond to other known genes. The latter are included as synonyms. An alphabetical list of genes showing map location, synonyms, the protein or RNA product of the gene, phenotypes of mutants, and reference citations is provided. In addition to genes known to correspond to gene sequences, other genes, often older, that are described by phenotype and older mapping techniques and that have not been correlated with sequences are included.

Update on enzyme and molecular defects in porphyria

Each porphyria results from decreased activity of one of the enzymes of haem biosynthesis. The molecular basis of enzyme deficiencies in acute intermittent porphyria (AIP), variegate porphyria (VP) and congenital erythropoietic porphyria (CEP) is outlined. All three conditions show extensive allelic heterogeneity. In the autosomal dominant disorders, AIP and VP, no genotype/phenotype correlations have been demonstrated, and the explanation for their low clinical penetrance remains uncertain. In AIP and VP, mutational analysis is superior to biochemical methods for screening families for latent porphyria. In the autosomal recessive condition, CEP, there is some genotype/phenotype correlation--one common mutation (C73R) being associated with severe disease in homozygotes. Porphyria cutanea tarda (PCT) is not a simple monogenic disorder. Patients appear to have an inherited susceptibility to inactivation of hepatic uroporphyrinogen decarboxylase (UROD) as part of a response to hepatocyte injury by alcohol, HCV and other agents. Inherited factors that, in combination, may predispose to PCT include mutations in the UROD gene, present in about 20% of patients, and the C282Y mutation in the haemochromatosis (HFE) gene.