Iron storage in Mycoplasma capricolum

Atypical iron storage in marine brown algae: a multidisciplinary study of iron transport and storage in Ectocarpus siliculosus

Iron is an essential element for all living organisms due to its ubiquitous role in redox and other enzymes, especially in the context of respiration and photosynthesis. The iron uptake and storage systems of terrestrial/higher plants are now reasonably well understood, with two basic strategies for iron uptake being distinguished: strategy I plants use a mechanism involving induction of Fe(III)-chelate reductase (ferrireductase) and Fe(II) transporter proteins, while strategy II plants utilize high-affinity, iron-specific, binding compounds called phytosiderophores. In contrast, little is known about the corresponding systems in marine, plant-like lineages, particularly those of multicellular algae (seaweeds). Herein the first study of the iron uptake and storage mechanisms in the brown alga Ectocarpus siliculosus is reported. Genomic data suggest that Ectocarpus may use a strategy I approach. Short-term radio-iron uptake studies verified that iron is taken up by Ectocarpus in a time- and concentration-dependent manner consistent with an active transport process. Upon long-term exposure to (57)Fe, two metabolites have been identified using a combination of Mössbauer and X-ray absorption spectroscopies. These include an iron-sulphur cluster accounting for ~26% of the total intracellular iron pool and a second component with spectra typical of a polymeric (Fe(3+)O(6)) system with parameters similar to the amorphous phosphorus-rich mineral core of bacterial and plant ferritins. This iron metabolite accounts for ~74% of the cellular iron pool and suggests that Ectocarpus contains a non-ferritin but mineral-based iron storage pool.

Combined structural and chemical analysis of the anammoxosome: A membrane-bounded intracytoplasmic compartment in anammox bacteria

Anammox bacteria have unique intracellular membranes that divide their cytoplasm into three separate compartments. The largest and innermost cytoplasmic compartment, the anammoxosome, is hypothesized to be the locus of all catabolic reactions in the anammox metabolism. Electron tomography showed that the anammoxosome and its membrane were highly folded. This finding was confirmed by a transmission electron microscopy study using different sample preparation methods. Further, in this study electron-dense particles were observed and electron tomography showed that they were confined to the anammoxosome compartment. Energy dispersive X-ray analysis revealed that these particles contained iron. The functional significance of a highly folded anammoxosome membrane and intracellular iron storage particles are discussed in relation to their possible function in energy generation.

Transcriptional profiling of Mycoplasma hyopneumoniae during iron depletion using microarrays

Mycoplasma hyopneumoniae, the causative agent of swine enzootic pneumonia and a major component of the porcine respiratory disease complex, continues to confound swine producers despite control programmes worldwide. The disease is chronic and self-limiting, but the host is subject to immunopathological changes that potentiate respiratory disease associated with other pathogens. The response of M. hyopneumoniae to environmental stress is of interest because of its relevance to virulence mechanisms in other bacterial pathogens. One of these stressors, iron deprivation, is a prominent feature of the host innate immune response, and most certainly impacts growth of mycoplasmas in vivo. To study this, microarray technology was applied to the transcriptome analysis of M. hyopneumoniae during iron deprivation. An array consisting of 632 of the 698 ORFs in the genome was used to compare the mRNA isolated from organisms grown under normal laboratory conditions with that from organisms subjected to iron deprivation with the chelator 2,2'-dipyridyl. This analysis identified 27 genes that were either up- or down-regulated in response to low-iron growth conditions (P<0.01), with an estimated false discovery rate below 10 %. These included genes encoding transport proteins, enzymes involved in energy metabolism, and components of the translation process. Ten of the 27 identified genes had no assigned function. These studies indicate that M. hyopneumoniae can respond to changes in environmental conditions, but the mechanism employed remains unknown.

The complete sequence of the mucosal pathogen Ureaplasma urealyticum

The comparison of the genomes of two very closely related human mucosal pathogens, Mycoplasma genitalium and Mycoplasma pneumoniae, has helped define the essential functions of a self-replicating minimal cell, as well as what constitutes a mycoplasma. Here we report the complete sequence of a more distant phylogenetic relative of those bacteria, Ureaplasma urealyticum (parvum biovar), which is also a mucosal pathogen of humans. It is the third mycoplasma to be sequenced, and has the smallest sequenced prokaryotic genome except for M. genitalium. Although the U. urealyticum genome is similar to the two sequenced mycoplasma genomes, features make this organism unique among mycoplasmas and all bacteria. Almost all ATP synthesis is the result of urea hydrolysis, which generates an energy-producing electrochemical gradient. Some highly conserved eubacterial enzymes appear not to be encoded by U. urealyticum, including the cell-division protein FtsZ, chaperonins GroES and GroEL, and ribonucleoside-diphosphate reductase. U. urealyticum has six closely related iron transporters, which apparently arose through gene duplication, suggesting that it has a kind of respiration system not present in other small genome bacteria The genome is only 25.5% G+C in nucleotide content, and the G+C content of individual genes may predict how essential those genes are to ureaplasma survival.

Iron metabolism of Escherichia coli studied by Mössbauer spectroscopy and biochemical methods

To date it has barely been recognized that the nature of about 75% of the Escherichia coli iron pool is unknown. Here we report the isolation of two iron species representing major components of iron metabolism in various growth states of E. coli. In vivo Mössbauer spectroscopy was applied to obtain information on the intracellular distribution pattern of iron in E. coli K12 W3110. Only two types of iron could be detected in the cell spectra: hexacoordinated Fe2+ and Fe3+ high-spin complexes. Other iron-requiring compounds are at least one order of magnitude less abundant in E. coli. The Mössbauer parameters of these complexes fit neither cytochromes nor iron-sulfur proteins nor ferric holo-bacterioferritin. They are sensitive to metabolic changes and inhibitors. The ratio of Fe/subunit, Fe2+/Fe3+ interconversion, chromatographic and electrophoretic data exclude bacterioferritin as the main iron metabolite in E. coli. Bacterioferritin can be observed only at very high ferric ion concentrations in the medium. The 55Fe fluorograms of both cytoplasmic and membrane fractions exhibit two exclusive bands with apparent molecular masses of 17 and 15 kDa, respectively. The two bands comprised 70% of the applied radioactivity. In gel filtration the main iron peak elutes at 155 kDa yielding two bands with apparent molecular masses of 17 and 15 kDa on SDS/PAGE. We therefore conclude that the iron species form a protein with an apparent molecular mass of 155 kDa containing 17-kDa and 15-kDa subunits. The iron content of the protein is 44 micrograms Fe/mg protein which corresponds to approximately 13 iron ions/subunit. No iron protein exhibiting the observed features has been described so far. Additional Mössbauer experiments suggest that these novel iron proteins are not restricted to E. coli but that similar components are detectable in several bacterial and fungal systems, thus pointing to a general occurrence.

Role of siderophores in iron storage in spores of Neurospora crassa and Aspergillus ochraceus

Spores of Neurospora crassa 74A are lacking in ferritinlike iron pools, as demonstrated by Mössbauer spectroscopic analysis. The cyclic hexapeptide siderophore ferricrocin constituted 47% of the total iron content in spores. After germination and growth, the ferricrocin iron pool disappeared, indicating that the metal was utilized. In spores of Aspergillus ochraceus, 74% of the total iron content was bound by ferrichrome-type siderophores. Siderophores may function as iron storage forms in fungal systems.

The acquisition of human lactoferrin by Mycoplasma pneumoniae

We tested the in vitro binding of human iron-sequestering lactoferrin and transferrin by the mucosal-surface pathogens, Mycoplasma pneumoniae and Mycoplasma genitalium. Mycoplasma pneumoniae bound lactoferrin, but not transferrin, in a saturable and specific manner. Analysis of binding data indicates less than 10,000 lactoferrin receptors per micro-organism with a moderately high binding affinity of 20 nM. No significant binding of lactoferrin or transferrin by M. genitalium was observed.

Metabolic utilization of 57Fe-labeled coprogen in Neurospora crassa. An in vivo Mössbauer study

Mössbauer spectra of whole cells of Neurospora crassa arg-5 ota aga (a siderophore-free mutant) show that the siderophore coprogen is accumulated inside the cell as an entity. 57Fe from 57Fe-labeled coprogen is slowly removed from the complex (45% in 27 h). The rate of removal depends on the degree of iron starvation of the cells. The distribution of 55Fe from [55Fe]coprogen in vacuoles, membranes, and cytoplasm has been also determined. From this it is clear that coprogen is accumulated in the cytoplasm. In addition to its role as a siderophore, coprogen serves as an iron-storage compound. No holoferritins could be detected. We therefore conclude that this type of iron-storage protein is lacking in N. crassa. Metabolized iron was found predominantly to exist as an envelope of Fe(II) high-spin (delta = 1.2-1.3 mm s-1; delta EQ = 3.0-3.1 mm s-1 at 4.2 K) and fast-relaxing Fe(III) high-spin species (delta approximately equal to 0.25 mm s-1 and 0.45 mm s-1; delta EQ approximately equal to 0.6 mm s-1 and 0.55 mm s-1, respectively, at 4.2 K). An assignment of these major iron metabolites is difficult. The Mössbauer data of the Fe(II) species do not fit those reported for heme, cytochromes and ferredoxins. We therefore assume that this iron metabolite represents a novel internal iron compound. One of the Fe(III) species becomes the dominant component of the cell spectra after 65 h of metabolization and might correspond to an iron-storage compound with iron oxide cores similar to bacterioferritin. After 27 h of growth in mycelia supplied with 57Fe-labeled coprogen, the siderophore ferricrocin was observed in the cell spectra. This is unexpected, since N. crassa arg-5 ota aga is unable to synthesize ornithine. We assume that ferricrocin is synthesized by the use of coprogen degradation products.

Escherichia coli iron enterobactin uptake monitored by Mössbauer spectroscopy

Iron uptake by Escherichia coli under aerobic conditions of iron deficiency is mediated by a highly stable ferric enterobactin [Fe(ent)3-] siderophore complex. Mössbauer spectroscopy has been used to monitor the fate of the iron as 57Fe(ent) was taken up by the cells. Osmotic shock experiments were used to distinguish between the iron present in the periplasmic space and that in the cytoplasm of the cell. Iron delivery by a synthetic analog of enterobactin, 1,3,5-N,N',N''- tris-(2,3-dihydroxybenzoyl)triaminomethylbenzene (MECAM), was also studied. Although Fe-MECAM was transported at the same rate as was Fe(ent) across the outer membrane and was apparently accumulated in the periplasmic space, the subsequent behaviors of Fe(ent) and Fe-MECAM were very different. After more than 30 min, a major fraction of the iron originally absorbed as ferric enterobactin appeared as Fe(II), apparently in the cytoplasm of the cell. However, little iron was delivered to the cytoplasm by the MECAM complex. The differences in specificity of these two stages of iron uptake by E. coli are discussed.

Study of storage iron in cultured chick embryo fibroblasts and rat glioma cells, using Mössbauer spectroscopy

57Fe Mössbauer spectra of normal and Rous sarcoma virus-infected cultured chick embryo fibroblasts and rat glioma cells have been measured between 0.08 and 318 K. Ferritin-like iron and bacterio-ferritin-like iron have been found in these cells, in various relative amounts, indicating a close relationship between the two storage materials. The bacterio-ferritin-like iron was found to be predominantly membrane-bound. Above 260 K very wide lines were observed in the Mössbauer spectra, yielding an effective viscosity of about 1 poise in the normal chick embryo fibroblasts and about 0.5 poise in the virus-infected chick embryo fibroblasts.

Kinetics of biosynthesis of iron-regulated membrane proteins in Escherichia coli

Using biological iron chelators to control specifically iron availability to Escherichia coli K-12 in conjunction with radioactive pulse-labels, we examined the biosynthesis of six iron-regulated membrane proteins. Iron deprivation induced the synthesis of five proteins, which had molecular weights of 83,000 (83K), 81K (Fep), 78K (TonA), 74K (Cir), and 25K. The kinetics of induction were the same in entA and entA(+) strains, but were affected by the initial iron availability in the media. Iron-poor cells induced rapidly (half-time, 10 min), whereas iron-rich cells began induction after a lag and showed a slower induction half-time (30 min). Within this general pattern of induction after iron deprivation, several different kinetic patterns were apparent. The 83K, 81K, and 74K proteins were coordinately controlled under all of the conditions examined. The 78K and 25K proteins were regulated differently. The synthesis of a previously unrecognized 90K inner membrane protein was inhibited by iron deprivation and stimulated by iron repletion. Both ferrichrome and ferric enterobactin completely repressed 81K and 74K synthesis when the siderophores were supplied at concentrations of 5 muM in vivo (half-time, 2.5 min). At concentrations less than 5 muM, however, both siderophores repressed synthesis only temporarily; the duration of repression was proportional to the amount of ferric siderophore added. The half-lives of the 81K and 74K mRNAs, as measured by rifampin treatment, were 1.2 and 1.6 min, respectively. The results of this study suggest that enteric bacteria are capable of instantaneously detecting and reacting to fluctuations in the extracellular iron concentration and that they store iron during periods of iron repletion for utilization during periods of iron stress. Neither iron storage nor iron regulation of envelope protein synthesis is dependent on the ability of the bacteria to form heme.

Respiration-associated components of Mollicutes

No cytochrome pigments were detected by difference (reduced minus oxidized) spectroscopy at liquid nitrogen temperature in whole-cell preparations or membrane fractions of Acholeplasma axanthum S273, Acholeplasma equifetale N93, Acholeplasma granularum BTS39, Acholeplasma laidlawii B-PG9, Acholeplasma modicum PG-49, Acholeplasma oculi 19L, Mycoplasma arginini G230, Mycoplasma arthritidis 07, Mycoplasma pneumoniae FH, and Mycoplasma pulmonis JB. All ten Mollicutes species examined contained iron of unknown function (3.0 to 15.3 nmol of iron per mg of protein). Relatively small amounts of acid-labile sulfide were found in all fractions (0.10 to 1.07 nmol of acid-labile sulfide per mg of protein). The data suggest that, as Mollicutes lack cytochrome pigments, they would synthesize most if not all adenosine triphosphate at the substrate level.