Uroporphyrinogen III cosynthase-deficient mutant of Salmonella typhimurium LT2

Coproporphyrin excretion and low thiol levels caused by point mutation in the Rhodobacter sphaeroides S-adenosylmethionine synthetase gene

A spontaneous mutant of Rhodobacter sphaeroides f. sp. denitrificans IL-106 was found to excrete a large amount of a red compound identified as coproporphyrin III, an intermediate in bacteriochlorophyll and heme synthesis. The mutant, named PORF, is able to grow under phototrophic conditions but has low levels of intracellular cysteine and glutathione and overexpresses the cysteine synthase CysK. The expression of molybdoenzymes such as dimethyl sulfoxide (DMSO) and nitrate reductases is also affected under certain growth conditions. Excretion of coproporphyrin and overexpression of CysK are not directly related but were both found to be consequences of a diminished synthesis of the key metabolite S-adenosylmethionine (SAM). The wild-type phenotype is restored when the gene metK encoding SAM synthetase is supplied in trans. The metK gene in the mutant strain has a mutation leading to a single amino acid change (H145Y) in the encoded protein. This point mutation is responsible for a 70% decrease in intracellular SAM content which probably affects the activities of numerous SAM-dependent enzymes such as coproporphyrinogen oxidase (HemN); uroporphyrinogen III methyltransferase (CobA), which is involved in siroheme synthesis; and molybdenum cofactor biosynthesis protein A (MoaA). We propose a model showing that the attenuation of the activities of SAM-dependent enzymes in the mutant could be responsible for the coproporphyrin excretion, the low cysteine and glutathione contents, and the decrease in DMSO and nitrate reductase activities.

Biosynthesis of Hemes

This review is concerned specifically with the structures and biosynthesis of hemes in E. coli and serovar Typhimurium. However, inasmuch as all tetrapyrroles share a common biosynthetic pathway, much of the material covered here is applicable to tetrapyrrole biosynthesis in other organisms. Conversely, much of the available information about tetrapyrrole biosynthesis has been gained from studies of other organisms, such as plants, algae, cyanobacteria, and anoxygenic phototrophs, which synthesize large quantities of these compounds. This information is applicable to E. coli and serovar Typhimurium. Hemes play important roles as enzyme prosthetic groups in mineral nutrition, redox metabolism, and gas-and redox-modulated signal transduction. The biosynthetic steps from the earliest universal precursor, 5-aminolevulinic acid (ALA), to protoporphyrin IX-based hemes constitute the major, common portion of the pathway, and other steps leading to specific groups of products can be considered branches off the main axis. Porphobilinogen (PBG) synthase (PBGS; also known as ALA dehydratase) catalyzes the asymmetric condensation of two ALA molecules to form PBG, with the release of two molecules of H2O. Protoporphyrinogen IX oxidase (PPX) catalyzes the removal of six electrons from the tetrapyrrole macrocycle to form protoporphyrin IX in the last biosynthetic step that is common to hemes and chlorophylls. Several lines of evidence converge to support a regulatory model in which the cellular level of available or free protoheme controls the rate of heme synthesis at the level of the first step unique to heme synthesis, the formation of GSA by the action of GTR.

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.

Cloning and characterisation of genes for tetrapyrrole biosynthesis from the cyanobacterium Anacystis nidulans R2

The genes for 5-aminolevulinic acid dehydratase (ALAD) and uroporphyrinogen III synthase (UROS), two enzymes in the biosynthetic pathway for tetrapyrroles, were independently isolated from a plasmid-based genomic library of Anacystis nidulans R2 (also called Synechococcus sp. PCC7942), by their ability to complement Escherichia coli strains carrying mutations in the equivalent genes (hemB and hemD respectively). The identity of the genes was confirmed by comparing the appropriate enzyme activities in complemented and mutant strains. Subclones of the original plasmids that were also capable of complementing the mutants were sequenced. The inferred amino acid sequence of the cyanobacterial HemB protein indicates a significant difference in the metal cofactor requirement from the higher-plant enzymes, which was confirmed by overexpression and biochemical analysis. The organisation of the cyanobacterial hemD locus differs markedly from other prokaryotes. Two open reading frames were found immediately upstream of hemD. The product of one shows considerable similarity to published sequences from other organisms for uroporphyrinogen III methylase (UROM), an enzyme involved in the production of sirohaem and cobalamins (including vitamin B-12). The product of the other shows motifs which are similar to those found in proteins responsible for metabolic regulation in yeast and indicates that this family of transcription control proteins, which has previously been reported only from eukaryotes, is also represented in prokaryotes.

The Pseudomonas aeruginosa homologs of hemC and hemD are linked to the gene encoding the regulator of mucoidy AlgR

The algD gene encodes NAD-linked GDPmannose dehydrogenase, which is essential for the mucoid phenotype, an important virulence factor expressed by Pseudomonas aeruginosa in cystic fibrosis patients. AlgR, a response regulator controlling mucoidy, is required for high level expression of algD. Inactivation of algR completely abrogates algD expression while mutations immediately downstream of algR affect induction of the algD promoter. In order to examine the nature of genetic elements located downstream of algR, the complete nucleotide sequence of this region was determined. This analysis revealed the presence of two newly identified P. aeruginosa genes with predicted gene products homologous to known porphobilinogen deaminases (HemC) from other organisms, and uroporphyrinogen III cosynthase (HemD) from Escherichia coli. The concerted action of both of these enzymes is essential for the synthesis of heme precursors. Mutations within the region containing the P. aeruginosa homologs of hemC and hemD affect algD promoter activity during growth on nitrate. Furthermore, transcriptional analyses indicated that hemC was cotranscribed with algR at detectable levels in mucoid cells. These results suggest a link between physiological processes dependent on heme and conditions conductive to algD expression and mucoidy.

The genes required for heme synthesis in Salmonella typhimurium include those encoding alternative functions for aerobic and anaerobic coproporphyrinogen oxidation

Insertion mutagenesis has been used to isolate Salmonella typhimurium strains that are blocked in the conversion of 5-aminolevulinic acid (ALA) to heme. These mutants define the steps of the heme biosynthetic pathway after ALA. Insertions were recovered at five unlinked loci: hemB, hemCD, and hemE, which have been mapped previously in S. typhimurium, and hemG and hemH, which have been described only for Escherichia coli. No other simple hem mutants were found. However, double mutants are described that are auxotrophic for heme during aerobic growth and fail to convert coproporphyrinogen III to protoporphyrinogen IX. These mutant strains are defective in two genes, hemN and hemF. Single mutants defective only in hemN require heme for anaerobic growth on glycerol plus nitrate but not for aerobic growth on glycerol. Mutants defective only in hemF have no apparent growth defect. We suggest that these two genes encode alternative forms of coproporphyrinogen oxidase. Anaerobic heme synthesis requires hemN function, while either hemN or hemF is sufficient for aerobic heme synthesis. These phenotypes are consistent with the requirement of a well-characterized class of coproporphyrinogen oxidase for molecular oxygen.

Bacterial heme synthesis is required for expression of the leghemoglobin holoprotein but not the apoprotein in soybean root nodules

In Bradyrhizobium japonicum/soybean symbiosis, the leghemoglobin (legume hemoglobin) apoprotein is a plant product, but the origin of the heme prosthetic group is not known. B. japonicum strain LO505 is a transposon Tn5-induced cytochrome-deficient mutant; it excreted the oxidized heme precursor coproporphyrin III into the growth medium. Mutant strain LO505 was specifically deficient in protoporphyrinogen oxidase (protoporphyrinogen-IX:oxygen oxidoreductase, EC 1.3.3.4) activity, and thus it could not catalyze the penultimate step in heme biosynthesis. Soybean root nodules formed from this mutant did not contain leghemoglobin, but the apoprotein was synthesized nevertheless. Data show that bacterial heme synthesis is required for leghemoglobin expression, but the heme moiety is not essential for apoleghemoglobin synthesis by the plant. Soybean leghemoglobin, therefore, is a product of both the plant and bacterial symbionts.

Molecular cloning and sequencing of the hemD gene of Escherichia coli K-12 and preliminary data on the Uro operon

DNA of plasmid pSAS1002TH (F' ilv+ hemD+ hemC+ cya+) was used to clone the hemD gene of Escherichia coli K-12. Due to poor transformability of the heme-deficient mutants, the restriction fragments of the F' plasmid were first cloned into a mobilizable derivative of pBR322, pSAS1211LP, which was then mobilized into a hemD recA mutant (E. coli SASX419AN). One recombinant plasmid, carrying a HindIII fragment of about 5 kilobases (kb), was shown to complement the hemD mutant and also a cya mutant of E. coli K-12, as well as a hemC mutant of Salmonella typhimurium LT2. Further subcloning of the insert enabled us to locate the hemD gene to a BamHI-PstI fragment (approximately 2.3 kb) which also carried the hemC gene. The hemD gene occupies a region close to the PstI end, since the deletion of a 0.6-kb fragment from this end resulted in loss of the ability to complement the hemD mutation. The use of the promoter-probe vector pK01 and the results of complementation showed that the hemD gene was transcribed under physiological conditions from the same promoter as the hemC gene, the direction of transcription being hemC-hemD. This allows us to define a new polycistronic operon of E. coli K-12, for which we propose the designation Uro operon. Sequencing of the hemD gene showed the presence of an open reading frame (ORF) of 738 nucleotides which could code for a protein with a molecular weight of 27,766, which should correspond to the hemD protein; the ORF starts with the last nucleotide of the hemC gene, the two genes having different reading frames. An ORF of at least 480 base pairs follows the hemD gene after a few nucleotides. The corresponding gene X, the function of which is unknown, might represent a third member of the Uro operon.

Mapping the uroporphyrinogen III cosynthase locus in Bacillus subtilis

The gene hemD taking part in the formation of uroporphyrinogen III from porphobilinogen was mapped by two- and three-factor transduction crosses in Bacillus subtilis. This gene codes uroporphyrinogen III cosynthase. The gene hemD is linked to the hemA locus and is located between the hemA and pheA loci.

Mapping of the hemE locus in Salmonella typhimurium

A new type of heme-deficient mutant was isolated in Salmonella typhimurium by neomycin selection. The mutant was deficient in uroporphyrinogen decarboxylase activity, coded by the hemE gene. The hemE gene was located between the genes rif and thi at 128 min on the chromosomal map of S. typhimurium.

A mutant of Escherichia coli which accumulates large amounts of coproporphyrin

A mutant of Escherichia coli which accumulates a large amount of coproporphyrin, presumably because of a block in heme biosynthesis, has been isolated after nitrosoguanidine mutagenesis. On rich media, the mutant forms colonies which give bright orange fluorescence when illuminated with ultraviolet light. The mutant appears to be similar to a Salmonella typhimurium mutant, deficient in uroporphyrinogen III cosynthase, described by Sasarman and Desrochers ((1976) J. Bacteriol. 128, 717--721). A striking property of the mutant is that coproporphyrin is retained within the cells in rich media but is almost totally excreted out of cells in minimal glucose medium.