Cloning and characterization of the Bacillus subtilis hemEHY gene cluster, which encodes protoheme IX biosynthetic enzymes

Isolation and functional characterization of mutant ferrochelatases in Saccharomyces cerevisiae

Ferrochelatase is a mitochondrial inner membrane-bound enzyme that catalyzes the incorporation of ferrous iron into protoporphyrin, the last step in protoheme biosynthesis. It is encoded by the HEM15 gene in the yeast Saccharomyces cerevisiae. Five hem15 mutants causing defective heme synthesis and protoporphyrin accumulation were investigated. The mutations were identified by sequencing the mutant hem15 alleles amplified in vitro from mutant genomic DNA. A single nucleotide change, causing an amino acid substitution, was found in each mutant. The substitution L62F caused a five-fold increase in Vmax and 32-fold and four-fold increases in the KM's for protoporphyrin and metal. Replacements of the conserved G47 by S and S102 by F increased the KM for protoporphyrin 10-fold without affecting the affinity for metal or enzyme activity. Two amino acid changes, L205P and P221L, produced a thermosensitive phenotype. In vivo heme synthesis, the amount of immunodetected protein, and ferrochelatase activity measured in vitro were more affected in cells grown at 37 degrees C than at 30 degrees C. The effects of these mutations on the enzyme function are discussed with respects to ferrochelatase structure and mechanism of action.

Purification, crystallization, and preliminary X-ray analysis of Bacillus subtilis ferrochelatase

Bacillus subtilis ferrochelatase (EC 4.99.1.1), the final enzyme in protoheme IX biosynthesis, was produced with an inducible T7 RNA polymerase expression system in Escherichia coli and purified from the soluble cell fraction. It was crystallized from polyethylene glycol solution using the microseeding technique. The crystals diffract to a minimum Bragg spacing of 2.1 A. The space group is P4(2) with unit cell dimensions a = b = 50.2 A, c = 120.1 A.

Characterization of the iron-binding site in mammalian ferrochelatase by kinetic and Mössbauer methods

All organisms utilize ferrochelatase (protoheme ferrolyase, EC 4.99.1.1) to catalyze the terminal step of the heme biosynthetic pathway, which involves the insertion of ferrous ion into protoporphyrin IX. Kinetic methods and Mössbauer spectroscopy have been used in an effort to characterize the ferrous ion-binding active site of recombinant murine ferrochelatase. The kinetic studies indicate that dithiothreitol, a reducing agent commonly used in ferrochelatase activity assays, interferes with the enzymatic production of heme. Ferrochelatase specific activity values determined under strictly anaerobic conditions are much greater than those obtained for the same enzyme under aerobic conditions and in the presence of dithiothreitol. Mössbauer spectroscopy conclusively demonstrates that, under the commonly used assay conditions, dithiothreitol chelates ferrous ion and hence competes with the enzyme for binding the ferrous substrate. Mössbauer spectroscopy of ferrous ion incubated with ferrochelatase in the absence of dithiothreitol shows a somewhat broad quadrupole doublet. Spectral analysis indicates that when 0.1 mM Fe(II) is added to 1.75 mM ferrochelatase, the overwhelming majority of the added ferrous ion is bound to the protein. The spectroscopic parameters for this bound species are delta = 1.36 +/- 0.03 mm/s and delta EQ = 3.04 +/- 0.06 mm/s, distinct from the larger delta EQ of a control sample of Fe(II) in buffer only. The parameters for the bound species are consistent with an active site composed of nitrogenous/oxygenous ligands and inconsistent with the presence of sulfur ligands. This finding is in accord with the absence of conserved cysteines among the known ferrochelatase sequences. The implications these results have with regard to the mechanism of ferrochelatase activity are discussed.

Cloning and overexpression of the Rhodobacter capsulatus hemH gene

In photosynthetically grown Rhodobacter capsulatus, heme is a qualitatively minor end product of the common tetrapyrrole pathway, but it may play a significant regulatory role. Heme is synthesized from protoporphyrin by the product of the hemH gene, ferrochelatase. We have cloned the R. capsulatus hemH gene by complementation of an Escherichia coli hemH mutant. When a plasmid carrying the hemH gene is returned to R. capsulatus, ferrochelatase activity increases, aminolevulinate synthase activity decreases, and bacteriochlorophyll levels are dramatically lowered. This is the first in vivo evidence to suggest that heme feedback inhibits aminolevulinate synthase in R. capsulatus, thereby reducing porphyrin synthesis.

Induction of terminal enzymes for heme biosynthesis during differentiation of mouse erythroleukemia cells

To examine the induction of terminal enzymes of the heme-biosynthetic pathway during erythroid differentiation, mouse protoporphyrinogen oxidase (PPO) cDNA has been cloned. The deduced amino acid sequence derived from the nucleotide sequence revealed that mouse PPO consists of 477 amino acid residues, without the leader peptide, which is imported into mitochondria. Comparison of the amino terminus of the deduced amino acid sequence of mouse PPO cDNA with that of purified bovine PPO provided conclusive evidence for lack of the leader peptide in the former. The amino acid sequence has 86% and 28% identity with human PPO and Bacillus subtilis HemY, respectively. When mouse erythroleukemia (MEL) cells were induced with dimethylsulfoxide, PPO mRNA was induced within 12 h of treatment, and with further incubation, reached a plateau. mRNAs for coproporphyrinogen oxidase (CPO) and ferrochelatase (FEC) were induced within 12 h, and continued to increase with time up to 48 h. The activities of CPO and FEC markedly increased with time up to 72 h, while PPO activity increased 1.8-fold within 12 h and remained unchanged thereafter. Immunoblot analysis showed that levels of PPO, CPO and FEC paralleled their corresponding activities. The magnitude of PPO induction was less than that of CPO and FEC. Thus, induction of three terminal enzymes of the heme-biosynthetic pathway is an early event in MEL cell differentiation. The concomitant induction may play an important role in producing large amounts of heme during erythroid differentiation.

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.

Cloning of a human cDNA for protoporphyrinogen oxidase by complementation in vivo of a hemG mutant of Escherichia coli

Protoporphyrinogen oxidase (PPO; EC 1.3.3.4) is the enzyme that catalyzes in the penultimate step in the heme biosynthetic pathway. Hemes are essential components of redox enzymes, such as cytochromes. Thus, a hemG mutant strain of Escherichia coli deficient in PPO is defective in aerobic respiration and grows poorly even in rich medium. By complementation with a human placental cDNA library, we were able to isolate a clone that enhanced the poor growth of such a hemG mutant strain. The clone encoded the gene for human PPO. Sequence analysis revealed that PPO consists of 477 amino acids with a calculated molecular mass of 50.8 kilodaltons. The deduced protein exhibited a high degree of homology over its entire length to the amino acid sequence of PPO encoded by the hemY gene of Bacillus subtilis. The NH2-terminal amino acid sequence of the deduced PPO contains a conserved amino acid sequence that forms the dinucleotide-binding site in many flavin-containing proteins. Northern blot analysis revealed the synthesis of a 1.8-kilobase pair mRNA for PPO. A homogenate of the monkey kidney COS-1 cells that had been transfected with the cDNA had much higher PPO activity than an extract of control cells, and this activity was inhibited by acifluorfen, a specific inhibitor of PPO. Furthermore, the cDNA was expressed in vitro as 51-kilodalton protein, and after incubation with isolated mitochondria the protein was found to be located in the mitochondria, having just the same size as before, an indication that PPO is a mitochondrial enzyme and has no apparent transport-specific leader sequence.

Uroporphyrinogen decarboxylase

Uroporphyrinogen decarboxylase (EC 4.1.1.37) catalyzes the decarboxylation of uroporphyrinogen III to coproporphyrinogen III. The amino acid sequences, kinetic properties, and physicochemical characteristics of enzymes from different sources (mammals, yeast, bacteria) are similar, but little is known about the structure/function relationships of uroporphyrinogen decarboxylases. Halogenated and other aromatic hydrocarbons cause hepatic uroporphyria by decreasing hepatic uroporphyrinogen decarboxylase activity. Two related human porphyrias, porphyria cutanea tarda and hepatoerythropoietic porphyria, also result from deficiency of this enzyme. The roles of inherited and acquired factors, including iron, in the pathogenesis of human and experimental uroporphyrias are reviewed.

Structure and function of ferrochelatase

Ferrochelatase is the terminal enzyme of the heme biosynthetic pathway in all cells. It catalyzes the insertion of ferrous iron into protoporphyrin IX, yielding heme. In eukaryotic cells, ferrochelatase is a mitochondrial inner membrane-associated protein with the active site facing the matrix. Decreased values of ferrochelatase activity in all tissues are a characteristic of patients with protoporphyria. Point-mutations in the ferrochelatase gene have been recently found to be associated with certain cases of erythropoietic protoporphyria. During the past four years, there have been considerable advances in different aspects related to structure and function of ferrochelatase. Genomic and cDNA clones for bacteria, yeast, barley, mouse, and human ferrochelatase have been isolated and sequenced. Functional expression of yeast ferrochelatase in yeast strains deficient in this enzyme, and expression in Escherichia coli and in baculovirus-infected insect cells of different ferrochelatase cDNAs have been accomplished. A recently identified (2Fe-2S) cluster appears to be a structural feature shared among mammalian ferrochelatases. Finally, functional studies of ferrochelatase site-directed mutants, in which key amino acids were replaced with residues identified in some cases of protoporphyria, will be summarized in the context of protein structure.

Cloning, nucleotide sequence, and mutagenesis of the Bacillus subtilis ponA operon, which codes for penicillin-binding protein (PBP) 1 and a PBP-related factor

An oligonucleotide probe designed to hybridize to genes encoding class A high-molecular-weight penicillin-binding proteins (PBPs) was used to identify the ponA gene encoding PBP1a and -1b (PBP1) of Bacillus subtilis. The identity of the ponA product was established by (i) the presence of a sequence coding for a peptide generated from PBP1 and (ii) the disappearance of PBP1 in a ponA mutant. DNA sequence analysis revealed that the amino acid sequence of PBP1 was similar to those of other class A high-molecular-weight PBPs and that ponA appeared to be cotranscribed with an upstream gene (termed prfA) of unknown function. Null mutations in ponA resulted in a slight decrease in growth rate and a change in colony morphology but had no significant effect on cell morphology, cell division, sporulation, spore heat resistance, or spore germination. Mutations in prfA which did not effect ponA expression produced a more significant decrease in growth rate but had no other significant phenotypic effects. Deletion of both prfA and ponA resulted in extremely slow growth and a reduction in sporulation efficiency. Studies of expression of transcriptional fusions of ponA and prfA to lacZ demonstrated that these two genes constitute an operon. Expression of these genes was relatively constant during growth, decreased during sporulation, and was induced approximately 15 min into spore germination. The ponA locus was mapped to the 200 degrees region of the chromosomal physical map.

Point mutations in Staphylococcus aureus PBP 2 gene affect penicillin-binding kinetics and are associated with resistance

In Staphylococcus aureus, penicillin-binding protein 2 (PBP 2) has been implicated in non-PBP 2a-mediated methicillin resistance. The PBP 2 gene (pbpB) was cloned from an expression library of a methicillin-susceptible strain of S. aureus (209P), and its entire sequence was compared with that of the pbpB gene from strains BB255, BB255R, and CDC6. Point mutations that resulted in amino acid substitutions near the conserved penicillin-binding motifs were detected in BB255R and CDC6, two low-level methicillin-resistant strains. Penicillin binding to PBP 2 in both BB255R and CDC6 is altered, and kinetic analysis indicated that altered binding of PBP 2 by penicillin was due to both lower binding affinity and more rapid release of bound drug. These structural and biochemical changes may contribute to the strains' resistance to beta-lactam antibiotics.

Site-directed mutagenesis of human ferrochelatase: identification of histidine-263 as a binding site for metal ions

In nature, ferrochelatase catalyzes the insertion of ferrous ion into the porphyrin macrocycle of protoporphyrin IX to exclude two protons to form protoheme IX: other porphyrin substrates, including mesoporphyrin IX may be used in vitro. Based on the deduced amino-acid sequences, one histidine residue (H263 of human enzyme) is conserved among all ferrochelatases cloned from human to bacterial cells, and three histidine residues (H157, H341 and H388 of human enzyme) are conserved among eukaryotic ferrochelatases; no cysteine residue is conserved. To attempt to clarify the binding site of ferrous ion, we converted four highly conserved histidine residues in human ferrochelatase to alanine, using site-directed mutagenesis. The mutant enzymes were expressed in Escherichia coli, and iron- and zinc-chelating activities were examined. Mutants H157A and H388A lost most of their activities and concomitantly the enzyme became susceptible to proteolytic degradation. Kinetic studies with the residual activities showed no significant change of Km values for metal ions or for mesoporphyrin IX. Mutation at H341 did not alter the enzyme activities. Iron- and zinc-chelating activities of mutant H263A were reduced to 30% and 21% of the activities of the wild type, respectively. Moreover, this mutation resulted in 18- and 3.4-fold increases in Km values toward ferrous and zinc ions, respectively, while the Km value for mesoporphyrin remained unchanged. These results indicate that the binding site for metal ions in ferrochelatase is distinct from that for the porphyrin, and suggest that histidine-263 contributes significantly to the binding of metal ions. Maintenance of the structure of the protein molecule may involve functions related to histidine-157 and -388.

Bacillus subtilis HemY is a peripheral membrane protein essential for protoheme IX synthesis which can oxidize coproporphyrinogen III and protoporphyrinogen IX

The hemY gene of the Bacillus subtilis hemEHY operon is essential for protoheme IX biosynthesis. Two previously isolated hemY mutations were sequenced. Both mutations are deletions affecting the hemY reading frame, and they cause the accumulation of coproporphyrinogen III or coproporphyrin III in the growth medium and the accumulation of trace amounts of other porphyrinogens or porphyrins intracellularly. HemY was found to be a 53-kDa peripheral membrane-bound protein. In agreement with recent findings by Dailey et al. (J. Biol. Chem. 269:813-815, 1994) B. subtilis HemY protein synthesized in Escherichia coli oxidized coproporphyrinogen III and protoporphyrinogen IX to coproporphyrin and protoporphyrin, respectively. The protein is not a general porphyrinogen oxidase since it did not oxidize uroporphyrinogen III. The apparent specificity constant, kcat/Km, for HemY was found to be about 12-fold higher with coproporphyrinogen III as a substrate compared with protoporphyrinogen IX as a substrate. The protoporphyrinogen IX oxidase activity is consistent with the function of HemY in a late step of protoheme IX biosynthesis, i.e., HemY catalyzes the penultimate step of the pathway. However, the efficient coproporphyrinogen III to coproporphyrin oxidase activity is unexplained in the current view of protoheme IX biosynthesis.

Gene dissection demonstrates that the Escherichia coli cysG gene encodes a multifunctional protein

The C-terminus of the Escherichia coli CysG protein, consisting of amino acids 202-457, was expressed as a recombinant protein using gene dissection methodology. Analysis of the activity of this truncated protein, termed CysGA, revealed that it was able to methylate uroporphyrinogen III in the same S-adenosyl-L-methionine (SAM)-dependent manner as the complete CysG protein. However, this truncated protein was not able to complement E. coli cysG cells, thereby suggesting that the first 201 amino acids of the CysG protein had an enzymic activity associated with the conversion of dihydrosirohydrochlorin into sirohaem. Analysis of the N-terminus of the CysG protein revealed the presence of a putative pyridine dinucleotide binding site. When the purified CysG protein was incubated with NADP+, uroporphyrinogen III and SAM the enzyme was found to catalyse a coenzyme-mediated dehydrogenation to form sirohydrochlorin. The CysGA protein on the other hand showed no such coenzyme-dependent activity. Analysis of the porphyrinoid material isolated from strains harbouring plasmids containing the complete and truncated cysG genes suggested that the CysG protein was also involved in ferrochelation. The evidence presented in this paper suggests that the CysG protein is a multifunctional protein involved in SAM-dependent methylation, pyridine dinucleotide dependent dehydrogenation and ferrochelation.

Cytochrome c biogenesis in bacteria: a possible pathway begins to emerge

Cytochrome c biogenesis describes the posttranslational pathway for the conversion of pre-apocytochrome c into the mature holocytochrome c. It involves an unknown number of consecutive biochemical steps, including translocation of the precursor polypeptide and haem into the periplasm and the covalent linkage between these two molecules. Genetic and molecular analysis of several bacterial mutants suggest that at least eight genes contribute to this process. In this review we summarize the present knowledge of the cytochrome c maturation pathway in bacteria and propose a model in which certain genes and their products are attributed to specific functions.

Purification and characterisation of a water-soluble ferrochelatase from Bacillus subtilis

Bacillus subtilis ferrochelatase is encoded by the hemH gene of the hemEHY gene cluster and catalyses the incorporation of Fe2+ into protoporphyrin IX. B. subtilis ferrochelatase produced in Escherichia coli was purified. It was found to be a monomeric, water-soluble enzyme of molecular mass 35 kDa which in addition to Fe2+ can incorporate Zn2+ and Cu2+ into protoporphyrin IX. Chemical modification experiments indicated that the single cysteine residue in the ferrochelatase is required for enzyme activity although it is not a conserved residue compared to other ferrochelatases. In growing B. subtilis, the ferrochelatase constitutes approximately 0.05% (by mass) of the total cell protein, which corresponds to some 600 ferrochelatase molecules/cell. The turnover number of isolated ferrochelatase, 18-29 min-1, was found to be consistent with the rate of haem synthesis in exponentially growing cells (0.2 mol haem formed/min/mol enzyme). It is concluded that the B. subtilis ferrochelatase has enzymic properties which are similar to those of other characterised ferrochelatases of known primary structure, i.e. ferrochelatases of the mitochondrial inner membrane of yeast and mammalian cells. However, in contrast to these enzymes the B. subtilis enzyme is a water-soluble protein and should be more amenable to structural analysis.

Human ferrochelatase is an iron-sulfur protein

Recombinant human ferrochelatase has been expressed in Escherichia coli and purified to homogeneity. Metal analyses revealed approximately 2 mol of non-heme Fe per mol of the purified enzyme (M(r) = 40,000). The UV-visible absorption spectrum of the purified enzyme consists of a protein absorption at 278 nm (epsilon approximately 90,000 M-1 cm-1) and bands at 330 nm (epsilon approximately 24,000 M-1 cm-1), 460 nm (shoulder, epsilon approximately 11,000 M-1 cm-1), and 550 nm (shoulder, epsilon approximately 9000 M-1 cm-1) that are indicative of a [2Fe-2S]2+ cluster. The spectra show an additional band at 415 nm that varied in intensity for different preparations and is attributed, at least in part, to a minor component of enzyme-associated high-spin Fe(III) heme. The presence of a single [2Fe-2S]2+,+ cluster as a redox active component of human ferrochelatase was confirmed by variable-temperature MCD and EPR studies of the dithionite-reduced enzyme which showed the presence of a S = 1/2 [2Fe-2S]+ cluster in addition to residual high spin Fe(II) heme. The reduced enzyme exhibits a S = 1/2 EPR signal, g = 2.00, 1.94, 1.91 accounting for 0.75 +/- 0.25 spins/molecule, that readily saturates at low microwave powers below 10 K but is observable without significant broadening at temperatures up to 100 K. The Fe-S cluster is labile and gradually disappears over period of 24 h, with concomitant loss of enzyme activity, when the enzyme is stored aerobically at 4 degrees C.(ABSTRACT TRUNCATED AT 250 WORDS)

Cloning and sequencing of the cell division gene pbpB, which encodes penicillin-binding protein 2B in Bacillus subtilis

The pbpB gene, which encodes penicillin-binding protein (PBP) 2B of Bacillus subtilis, has been cloned, sequenced, mapped, and mutagenized. The sequence of PBP 2B places it among the class B high-molecular-weight PBPs. It appears to contain three functional domains: an N-terminal domain homologous to the corresponding domain of other class B PBPs, a penicillin-binding domain, and a lengthy carboxy extension. The PBP has a noncleaved signal sequence at its N terminus that presumably serves as its anchor in the cell membrane. Previous studies led to the hypothesis that PBP 2B is required for both vegetative cell division and sporulation septation. Its sequence, map site, and mutant phenotype support this hypothesis. PBP 2B is homologous to PBP 3, the cell division protein encoded by pbpB of Escherichia coli. Moreover, both pbpB genes are located in the same relative position within a cluster of cell division and cell wall genes on their respective chromosomes. However, immediately adjacent to the B. subtilis pbpB gene is spoVD, which appears to be a sporulation-specific homolog of pbpB. Inactivation of SpoVD blocked synthesis of the cortical peptidoglycan in the spore, whereas carboxy truncation of PBP 2B caused cells to grow as filaments. Thus, it appears that a gene duplication has occurred in B. subtilis and that one PBP has evolved to serve a common role in septation during both vegetative growth and sporulation, whereas the other PBP serves a specialized role in sporulation.

Cloning and sequencing of the hemE gene encoding uroporphyrinogen III decarboxylase (UPD) from Escherichia coli K-12

Among the photoresistant revertants of the visA-deleted (hemH-deleted) strain of Escherichia coli K-12, three mutants defective in the hemE gene encoding uroporphyrinogen III decarboxylase (UPD) were identified. Using one of the mutants, we cloned and sequenced the hemE of E. coli. We found an open reading frame of 353 codons, which encoded a predicted amino acid (aa) sequence that exhibited a high degree of homology over its entire length to the aa sequence of UPD from humans and other organisms. This hemE was located at 90.3 min near the hupA gene on the linkage map of the E. coli chromosome.

Bacillus subtilis CtaA and CtaB function in haem A biosynthesis

Haem A, a prosthetic group of many respiratory oxidases, is probably synthesized from haem B (protohaem IX) in a pathway in which haem O is an intermediate. Possible roles of the Bacillus subtilis ctaA and ctaB gene products in haem O and haem A synthesis were studied. Escherichia coli does not contain haem A. The ctaA gene on plasmids in E. coli resulted in haem A accumulation in membranes. The presence of ctaB together with ctaA increased the amount of haem A found in E. coli. Haem O was not detected in wild-type B. subtilis strains. A previously isolated B. subtilis ctaA deletion mutant was found to contain haem B and haem O, but not haem A. B. subtilis ctaB deletion mutants were constructed and found to lack both haem A and haem O. The results with E. coli and B. subtilis strongly suggest that the B. subtilis CtaA protein functions in haem A synthesis. It is tentatively suggested that if functions in the oxygenation/oxidation of the methyl side group of carbon 8 of haem O. B. subtilis CtaB, which is homologous to Saccharomyces cerevisiae COX10 and E. coli CyoE, also has a role in haem A synthesis and seems to be required for both cytochrome a and cytochrome o synthesis.

Analysis of the Bradyrhizobium japonicum hemH gene and its expression in Escherichia coli

Complementation analysis showed that the Bradyrhizobium japonicum hemH gene was both necessary and sufficient to rescue mutant strains I110ek4 and I110bk2 in trans with respect to hemin auxotrophy, protoporphyrin accumulation, and the deficiency in ferrochelatase activity. The B. japonicum hemH gene was expressed in an Escherichia coli T7 expression system and yielded a 39-kDa protein, which was consistent with the predicted size of the deduced product. The overexpressed protein was purified and shown to contain ferrochelatase activity, thereby demonstrating that the hemH gene encodes ferrochelatase. When expressed from the lac promoter, the B. japonicum hemH gene was able to complement the enzyme activity of a ferrochelatase-defective E. coli mutant, and it also conferred hemin prototrophy on those cells. These latter findings confirm the identity of the hemH gene product and demonstrate that B. japonicum ferrochelatase can interact with the E. coli heme synthesis enzymes for heme formation in complemented cells.

Purification and properties of the uroporphyrinogen decarboxylase from Rhodobacter sphaeroides

Uroporphyrinogen decarboxylase (EC 4.1.1.37) was purified 600-fold from Rhodobacter sphaeroides grown anaerobically in the light. The enzyme, under both denaturing and non-denaturing conditions, is a monomer of M(r) 41,000. The Km values are 1.8 microM and 6.0 microM for the conversion of uroporphyrinogen I and III to coproporphyrinogen I and III respectively. The enzyme is susceptible to inhibition by both uroporphyrinogen and uroporphyrin. The pH optimum is 6.8 and the isoelectric point is 4.4. The importance of cysteine and arginine residues is implicated from studies with inhibitors. The sequence of the first 29 amino acids of the N-terminus shows a high degree of similarity to the primary structures of other uroporphyrinogen decarboxylases. Studies on the order of decarboxylation of the four acetic acid side chains of uroporphyrinogen III suggest that at high substrate levels a random route is preferred.

Cloning, nucleotide sequence, and regulation of the Bacillus subtilis pbpF gene, which codes for a putative class A high-molecular-weight penicillin-binding protein

The partial nucleotide sequence of a gene encoding a Bacillus subtilis homolog to the Escherichia coli ponA gene, encoding penicillin-binding protein 1A, was previously reported. The remaining part of this gene, termed pbpF, was isolated, and its nucleotide sequence was completed. Deletion of this gene did not alter the profile of B. subtilis penicillin-binding proteins observed after gel electrophoresis and resulted in no observable phenotype. A transcriptional pbpF-lacZ fusion was weakly expressed during vegetative growth. Expression diminished during the first hours of sporulation but was slightly induced in the forespore compartment during late sporulation. This sporulation expression was dependent on spoIIIG, which encodes the forespore-specific transcription factor sigma G. A single transcription start site which was apparently directly dependent on E sigma A was detected in vegetative cells.

Cloning, nucleotide sequence, and regulation of the Bacillus subtilis pbpE operon, which codes for penicillin-binding protein 4* and an apparent amino acid racemase

Penicillin-binding protein 4* (PBP 4*) was purified from Bacillus subtilis, its N-terminal sequence was determined, and the coding gene, termed pbpE, was cloned and sequenced. The predicted amino acid sequence of PBP 4* exhibited similarity to those of other penicillin-recognizing enzymes. Downstream of pbpE there was a second gene, termed orf2, which exhibited sequence similarity with aspartate racemase. The two genes were found to constitute an operon adjacent to and divergently transcribed from the sacB gene at 296 degrees on the chromosomal map. A weak beta-lactamase activity was associated with PBP 4*, but no enzymatic activity was found for the product of orf2. Mutation of pbpE, orf2, or both genes resulted in no observable effect on growth, sporulation, spore heat resistance, or spore germination. A translational pbpE-lacZ fusion was weakly expressed during vegetative growth and was significantly induced at the onset of sporulation. This induction depended on the activity of the spo0A product in relieving repression by the abrB repressor. A single transcription start site which was apparently dependent on E sigma A was detected upstream of pbpE.