Siderophore utilization and iron uptake by Rhodopseudomonas sphaeroides

Prokaryotic Heme Biosynthesis: Multiple Pathways to a Common Essential Product

The advent of heme during evolution allowed organisms possessing this compound to safely and efficiently carry out a variety of chemical reactions that otherwise were difficult or impossible. While it was long assumed that a single heme biosynthetic pathway existed in nature, over the past decade, it has become clear that there are three distinct pathways among prokaryotes, although all three pathways utilize a common initial core of three enzymes to produce the intermediate uroporphyrinogen III. The most ancient pathway and the only one found in the Archaea converts siroheme to protoheme via an oxygen-independent four-enzyme-step process. Bacteria utilize the initial core pathway but then add one additional common step to produce coproporphyrinogen III. Following this step, Gram-positive organisms oxidize coproporphyrinogen III to coproporphyrin III, insert iron to make coproheme, and finally decarboxylate coproheme to protoheme, whereas Gram-negative bacteria first decarboxylate coproporphyrinogen III to protoporphyrinogen IX and then oxidize this to protoporphyrin IX prior to metal insertion to make protoheme. In order to adapt to oxygen-deficient conditions, two steps in the bacterial pathways have multiple forms to accommodate oxidative reactions in an anaerobic environment. The regulation of these pathways reflects the diversity of bacterial metabolism. This diversity, along with the late recognition that three pathways exist, has significantly slowed advances in this field such that no single organism's heme synthesis pathway regulation is currently completely characterized.

Dissimilatory iron(III) reduction by Rhodobacter capsulatus

The photosynthetic proteobacterium Rhodobacter capsulatus was shown to be capable of dissimilatory Fe(III) reduction. Activity was expressed during anaerobic phototrophic and microaerobic growth with malate as the carbon source, but not during equivalent aerobic growth. A variety of Fe(III) complexes were demonstrated to act as substrates for intact cells and membrane fractions of strain N22DNAR+ using a ferrozine assay for Fe(II) formation. Rates of reduction appeared to be influenced by the reduction potentials of the Fe(III) complexes. However, Fe(III) complexed by citrate, which is readily reduced by Shewanella putrefaciens, was a poor substrate for dissimilation by R. capsulatus. The Fe(III)-reducing activity of R. capsulatus was located solely in the membrane fraction. The reduction of Fe(III) complexes by intact cells was inhibited by 2-heptyl-4-hydroxyquinoline-N-oxide (HQNO), suggesting the involvement of ubiquinol: cytochrome c oxidoreductases in the electron transport chain. Lack of sensitivity to myxothiazol plus data from mutant strains implies that the cytochrome bc 1 complex and cytochrome c 2 are not obligatory for dissimilation of Fe(III)(maltol)3. Alternative pathways of electron transfer to Fe(III) must hence operate in R. capsulatus. Using strain N22DNAR+, the reduction rate of Fe(III) complexed by nitrilotriacetic acid (NTA) was elevated compared to that of Fe(III)(maltol)3, and moreover was sensitive to myxothiazol. However, these differences were not observed in the absence of the electron donor malate. The governing factor for the reduction rate of Fe(III)(maltol)3 thus appears to be the limited Fe(III)-reducing activity, whilst the reduction rate of Fe(III) complexed by NTA is controlled by the flux of electrons through the respiratory chain. The use of mutant strains confirmed that the role of the cytochrome bc 1 complex in Fe(III) reduction becomes apparent only with the superior substrate. The energy-conserving nature of Fe(III) reduction by R. capsulatus was demonstrated by electrochromic measurements, with the endogenous carotenoid pigments being employed as indicators of membrane potential generation in intact cells. Using Fe(III)EDTA as electron acceptor, periods of membrane potential generation were directly proportional to the quantity of complex added, and were extended in the presence of HQNO. Fe(III)-dependent carotenoid bandshifts were abolished by addition of the protonophoric uncoupler carbonyl cyanide p-trifluoromethoxy-phenylhydrazone.

Reduction of exogenous ferric iron by a surface-associated ferric reductase of Listeria spp

The reduction of exogenous ferric iron by Listeria monocytogenes, a Gram-positive food-borne pathogen, was investigated. Using an assay incorporating the ferrous iron chelator ferrozine, we showed that intact cells of L. monocytogenes, when exposed to ferric iron, were able to rapidly reduce and solubilize the iron to the ferrous form. Reduction occurred only after direct contact between the bacteria and the iron source. A number of different ferric iron chelates, including transferrin and lactoferrin-bound iron, haemoglobin, ferritin, and iron complexed to siderophores, could be reduced. The ferric reductase activity was expressed by both reference strains and clinical isolates of L. monocytogenes and by all other species of Listeria, although significant quantitative differences were observed. In L. monocytogenes, the expression of ferric reductase was not affected by the growth phase of the bacteria nor by the presence or absence of iron in the growth medium. However, expression was greatly reduced in bacteria grown anaerobically and when cultured in media of reduced pH. In addition, bacteria grown at a cold temperature displayed greater ferric reductase activity than cells grown at higher temperatures. A surface-associated ferric reductase system may be one component of a general iron scavenging mechanism which can be used by Listeria growing in a variety of environments.

Differential Siderophore Utilization and Iron Uptake by Soil and Rhizosphere Bacteria

The differential availabilities of the hydroxamate siderophores ferrioxamine B (FOB) and ferrichrome (FC) and the pseudobactin siderophores St3, 7NSK 2 , and WCS 358 as sources of Fe for soil and rhizosphere bacteria were studied. About 20% of the total bacterial CFU from the rhizospheres of four plant species were able to use FOB as the sole Fe source in an Fe-deficient medium, while about 12, 10, 2, and > 1% were able to use FC and pseudobactins 7NSK 2 , St3, and WCS 358, respectively. Of the 165 colonies isolated from plates containing pseudobactins, 64 were able to use the pseudobactin on which they were isolated as the sole Fe source in pure culture. Cross-feeding tests showed that almost all of these 64 strains were also able to use at least one of the other siderophores studied (pseudobactin, FOB, or FC). Pseudomonas putida StS2, Pseudomonas maltophilia 7NM1, and Vibrio fluvialis WS1, which were originally isolated on pseudobactins St3, 7NSK 2 , and WCS 358, respectively, were selected for their ability to grow with pseudobactin St3 as the sole Fe source. They incorporated 55 Fe 3+ mediated by pseudobactin St3 at various rates (71.5, 4, and 23 pmol/min/mg [dry weight] of cells, respectively). Similarly, P. putida St3 was shown to incorporate 55 Fe 3+ mediated by FOB and FC. We suggest that the ability of bacteria to utilize a large variety of siderophores confers an ecological advantage.

Citrate as a siderophore in Bradyrhizobium japonicum

Under iron-limiting conditions, many bacteria secrete ferric iron-specific ligands, generically termed siderophores, to aid in the sequestering and transport of iron. One strain of the nitrogen-fixing soybean symbiont Bradyrhizobium japonicum, 61A152, was shown to produce a siderophore when 20 B. japonicum strains were screened with all six chemical assays commonly used to detect such production. Production by strain 61A152 was detected via the chrome azurol S assay, a general test for siderophores which is independent of siderophore structure. The iron-chelating compound was neither a catechol nor a hydroxamate and was ninhydrin negative. It was determined to be citric acid via a combination of thin-layer chromatography and high-voltage paper electrophoresis; this identification was verified by a specific enzymatic assay for citric acid. The inverse correlation which was observed between citric acid release and the iron content of the medium suggested that ferric citrate could serve as an iron source. This was confirmed via growth and transport assays. Exogenously added ferric citrate could be used to overcome iron starvation, and iron-deficient cells actively transported radiolabeled ferric citrate. These results, taken together, indicate a role for ferric citrate in the iron nutrition of this strain, which has been shown to be an efficient nitrogen-fixing strain on a variety of soybean cultivars.

Iron-Dependent Production of Hydroxamate by Sodium-Dependent Azotobacter chroococcum

The sodium-dependent strain 184 of Azotobacter chroococcum was unable to grow significantly in iron-limited medium, but did produce iron-repressible outer membrane proteins. Siderophores were not produced under these conditions. Citric acid was excreted, but not in response to iron limitation. This strain, however, was able to grow in insoluble mineral iron sources, and under these conditions the cells produced a hydroxamate. Growth on minerals and hydroxamate production was dependent on a low level of freely exchangeable iron. Optimal hydroxamate production was observed with 0.75 μM ferric citrate, and hydroxamate production was repressed by >5 μM iron. Despite this iron requirement, hyroxamate was only formed during internal iron limitation of the cells. Iron-containing cells were able to grow in iron-limited medium but only produced hydroxamate when their iron-per-cellular-protein content was low. These results, the spectral changes observed upon Fe 3+ addition, and iron-uptake coincident with hydroxamate production suggested that the hydroxamate was a siderophore.

Anthranilate-promoted iron uptake in Rhizobium leguminosarum

Anthranilate promoted the uptake of ferric iron into iron-starved cells of Rhizobium leguminosarum GF160. The uptake system was a saturable function of the concentration of ferric anthranilate. It was characterized by an apparent Km of 6 microM and a Vmax of 1.6 nmol/min/mg cell protein. Uptake was temperature dependent and inhibited by the metabolic poisons arsenate and iodoacetate. The proton motive force may not be involved since no effect was demonstrated by the respiratory inhibitor sodium azide and by various uncouplers. The anthranilate-promoted iron uptake was lower for cells grown in increasing levels of available iron and the addition of anthranilic acid to low-iron cultures resulted in a stimulation of bacterial growth. During growth under iron deficiency, anthranilic acid was not assimilated by the cells.

Ferrochelatase from Rhodopseudomonas sphaeroides: substrate specificity and role of sulfhydryl and arginyl residues

Purified ferrochelatase (protoheme ferrolyase; EC 4.99.1.1) from the bacterium Rhodopseudomonas sphaeroides was examined to determine the roles of cationic and sulfhydryl residues in substrate binding. Reaction of the enzyme sulfhydryl residues with N-ethylmaleimide or monobromobimane resulted in a rapid loss of enzyme activity. Ferrous iron, but not porphyrin substrate, had a protective effect against inactivation by these two reagents. Quantitation with 3H-labeled N-ethylmaleimide revealed that inactivation required one to two sulfhydryl groups to be modified. Modification of arginyl residues with either 2,3-butanedione or camphorquinone 10-sulfonate resulted in a loss of ferrochelatase activity. A kinetic analysis of the modified enzyme showed that the Km for ferrous iron was not altered but that the Km for the porphyrin substrate was increased. These data suggested that arginyl residues may be involved in porphyrin binding, possibly via charge pair interactions between the arginyl residue and the anionic porphyrin propionate side chain. Modification of lysyl residues had no effect on enzyme activity. We also examined the ability of bacterial ferrochelatase to use various 2,4-disubstituted porphyrins as substrates. We found that 2,4-bis-acetal- and 2,4-disulfonate deuteroporphyrins were effective substrates for the purified bacterial enzyme and that N-methylprotoporphyrin was an effective inhibitor of the enzyme. Our data for the ferrochelatase of R. sphaeroides are compared with previously published data for the eucaryotic enzyme.

Ferric iron reductase of Rhodopseudomonas sphaeroides

Ferric iron reductase activity was examined in the facultative photosynthetic bacterium Rhodopseudomonas sphaeroides. The specific activities of extracts from cells grown under phototrophic and aerobic conditions were similar and not affected by the concentration of iron in the growth media. The activity was resolved by ion-exchange column chromatography into two fractions, designated iron reductase A and iron reductase B, with molecular weights of 41,000 and 32,000, respectively. Both of these soluble cytoplasmic enzymes required the presence of flavin mononucleotide for activity and utilized NADH to reduce iron supplied as ferric citrate. Iron reductase B was responsible for the majority of activity in crude extracts and was purified 556-fold by conventional protein purification techniques. The apparent Km values of iron reductase B for NADH, Fe3+, and flavin mononucleotide were determined to be 18.2, 8.3, and 3.2 microM, respectively.

Iron transport and its relation to heme biosynthesis in Rhodopseudomonas sphaeroides

The uptake of iron supplied as ferric citrate or ferric parabactin was examined in aerobically grown whole cells and vesicles of Rhodopseudomonas sphaeroides. Inner and outer membrane fractions from R. sphaeroides contained no membrane proteins which were inducible by growth in low-iron medium. Vesicles composed of the inner membrane and devoid of outer membrane and periplasmic proteins were able to transport iron supplied as ferric citrate and ferric parabactin. This uptake required the presence of NADH. When the electrical component of the proton motive force was depleted in whole cells, the uptake of iron supplied as ferric parabactin was completely inhibited. The uptake of iron supplied as ferric citrate was inhibited by gallium citrate; however, Ga3+ was not transported. The relationship between iron uptake and heme synthesis was examined by treating whole cells with N-methylprotoporphyrin which inhibits ferrochelatase, the enzyme which inserts ferrous iron into protoporphyrin to form heme. This treatment reduced ferrochelatase activity by 82% but had no effect on iron uptake, indicating that iron uptake and heme synthesis are not directly coupled. The fate of transported iron was investigated by measuring intracellular concentrations of heme and nonheme iron. It was determined that newly transported iron exists primarily as nonheme iron.