An exploration of mechanisms underlying Desemzia incerta colonization resistance to methicillin-resistant Staphylococcus aureus on the skin

Colonization of human skin and nares by methicillin-resistant Staphylococcus aureus (MRSA) leads to the community spread of MRSA. This spread is exacerbated by the transfer of MRSA between humans and livestock, particularly swine. Here, we capitalized on the shared features between human and porcine skin, including shared MRSA colonization, to study novel bacterial mediators of MRSA colonization resistance. We focused on the poorly studied bacterial species Desemzia incerta, which we found to exert antimicrobial activity through a secreted product and exhibited colonization resistance against MRSA in an in vivo murine skin model. Using parallel genomic and biochemical investigation, we discovered that D. incerta secretes an antimicrobial protein. Sequential protein purification and proteomics analysis identified 24 candidate inhibitory proteins, including a promising peptidoglycan hydrolase candidate. Aided by transcriptional analysis of D. incerta and MRSA cocultures, we found that exposure to D. incerta leads to decreased MRSA biofilm production. These results emphasize the value of exploring microbial communities across a spectrum of hosts, which can lead to novel therapeutic agents as well as an increased understanding of microbial competition.IMPORTANCEMethicillin-resistant Staphylococcus aureus (MRSA) causes a significant healthcare burden and can be spread to the human population via livestock transmission. Members of the skin microbiome can prevent MRSA colonization via a poorly understood phenomenon known as colonization resistance. Here, we studied the colonization resistance of S. aureus by bacterial inhibitors previously identified from a porcine skin model. We identify a pig skin commensal, Desemzia incerta, that reduced MRSA colonization in a murine model. We employ a combination of genomic, proteomic, and transcriptomic analyses to explore the mechanisms of inhibition between D. incerta and S. aureus. We identify 24 candidate antimicrobial proteins secreted by D. incerta that could be responsible for its antimicrobial activity. We also find that exposure to D. incerta leads to decreased S. aureus biofilm formation. These findings show that the livestock transmission of MRSA can be exploited to uncover novel mechanisms of MRSA colonization resistance.

Complete genome sequence of the bacterium Desemzia incerta isolated from the dorsal skin of a Yucatan pig

We have shown previously that an isolate of Desemzia incerta from porcine skin has antimicrobial activity against methicillin-resistant Staphylococcus aureus. We present here the complete D. incerta genome containing one circular chromosome and five circular plasmids.

Variable staphyloxanthin production by Staphylococcus aureus drives strain-dependent effects on diabetic wound-healing outcomes

Strain-level variation in Staphylococcus aureus is a factor that contributes to disease burden and clinical outcomes in skin disorders and chronic wounds. However, the microbial mechanisms that drive these variable host responses are poorly understood. To identify mechanisms underlying strain-specific outcomes, we perform high-throughput phenotyping screens on S. aureus isolates cultured from diabetic foot ulcers. Isolates from non-healing wounds produce more staphyloxanthin, a cell membrane pigment. In murine diabetic wounds, staphyloxanthin-producing isolates delay wound closure significantly compared with staphyloxanthin-deficient isolates. Staphyloxanthin promotes resistance to oxidative stress and enhances bacterial survival in neutrophils. Comparative genomic and transcriptomic analysis of genetically similar clinical isolates with disparate staphyloxanthin phenotypes reveals a mutation in the sigma B operon, resulting in marked differences in stress response gene expression. Our work illustrates a framework to identify traits that underlie strain-level variation in disease burden and suggests more precise targets for therapeutic intervention in S. aureus-positive wounds.

Harnessing diversity and antagonism within the pig skin microbiota to identify novel mediators of colonization resistance to methicillin-resistant Staphylococcus aureus

The microbiota mediate multiple aspects of skin barrier function, including colonization resistance to pathogens such as Staphylococcus aureus. The endogenous skin microbiota limits S. aureus colonization via competition and direct inhibition. Novel mechanisms of colonization resistance are promising therapeutic targets for drug-resistant infections, such as those caused by methicillin-resistant S. aureus (MRSA). Here, we developed and characterized a swine model of topical microbiome perturbation and MRSA colonization. As in other model systems, topical antimicrobial treatment had a little discernable effect on community diversity though the overall microbial load was sensitive to multiple types of intervention, including swabbing. In parallel, we established a porcine skin culture collection and screened 7,700 isolates for MRSA inhibition. Using genomic and phenotypic criteria, we curated three isolates to investigate whether prophylactic colonization would inhibit MRSA colonization in vivo. The three-member consortium together, but not individually, provided protection against MRSA colonization, suggesting cooperation and/or synergy among the strains. Inhibitory isolates were represented across all major phyla of the pig skin microbiota and did not have a strong preference for inhibiting closely related species, suggesting that relatedness is not a condition of antagonism. These findings reveal the porcine skin as an underexplored reservoir of skin commensal species with the potential to prevent MRSA colonization and infection. IMPORTANCE The skin microbiota is protective against pathogens or opportunists such as S. aureus, the most common cause of skin and soft tissue infections. S. aureus can colonize normal skin and nasal passages, and colonization is a risk factor for infection, especially on breach of the skin barrier. Here, we established a pig model to study the competitive mechanisms of the skin microbiota and their role in preventing colonization by MRSA. This drug-resistant strain is also a livestock pathogen, and swine herds can be reservoirs of MRSA carriage. From 7,700 cultured skin isolates, we identified 37 unique species across three phyla that inhibited MRSA. A synthetic community of three inhibitory isolates provided protection together, but not individually, in vivo in a murine model of MRSA colonization. These findings suggest that antagonism is widespread in the pig skin microbiota, and these competitive interactions may be exploited to prevent MRSA colonization.

Commensal microbiota regulates skin barrier function and repair via signaling through the aryl hydrocarbon receptor

The epidermis forms a barrier that defends the body from desiccation and entry of harmful substances, while also sensing and integrating environmental signals. The tightly orchestrated cellular changes needed for the formation and maintenance of this epidermal barrier occur in the context of the skin microbiome. Using germ-free mice, we demonstrate the microbiota is necessary for proper differentiation and repair of the epidermal barrier. These effects are mediated by microbiota signaling through the aryl hydrocarbon receptor (AHR) in keratinocytes, a xenobiotic receptor also implicated in epidermal differentiation. Mice lacking keratinocyte AHR are more susceptible to barrier damage and infection, during steady-state and epicutaneous sensitization. Colonization with a defined consortium of human skin isolates restored barrier competence in an AHR-dependent manner. We reveal a fundamental mechanism whereby the microbiota regulates skin barrier formation and repair, which has far-reaching implications for the numerous skin disorders characterized by epidermal barrier dysfunction.

Research Techniques Made Simple: Profiling the Skin Microbiota

Skin is colonized by microbial communities (microbiota) that participate in immune homeostasis, development and maintenance of barrier function, and protection from pathogens. The past decade has been marked by an increased interest in the skin microbiota and its role in cutaneous health and disease, in part due to advances in next-generation sequencing platforms that enable high-throughput, culture-independent detection of bacteria, fungi, and viruses. Various approaches, including bacterial 16S ribosomal RNA gene sequencing and metagenomic shotgun sequencing, have been applied to profile microbial communities colonizing healthy skin and diseased skin including atopic dermatitis, psoriasis, and acne, among others. Here, we provide an overview of culture-dependent and -independent approaches to profiling the skin microbiota and the types of questions that may be answered by each approach. We additionally highlight important study design considerations, selection of controls, interpretation of results, and limitations and challenges.

Splitting the functions of Rim2, a mitochondrial iron/pyrimidine carrier

Rim2 is an unusual mitochondrial carrier protein capable of transporting both iron and pyrimidine nucleotides. Here we characterize two point mutations generated in the predicted substrate-binding site, finding that they yield disparate effects on iron and pyrimidine transport. The Rim2 (E248A) mutant was deficient in mitochondrial iron transport activity. By contrast, the Rim2 (K299A) mutant specifically abrogated pyrimidine nucleotide transport and exchange, while leaving iron transport activity largely unaffected. Strikingly, E248A preserved TTP/TTP homoexchange but interfered with TTP/TMP heteroexchange, perhaps because proton coupling was dependent on the E248 acidic residue. Rim2-dependent iron transport was unaffected by pyrimidine nucleotides. Rim2-dependent pyrimidine transport was competed by Zn²⁺ but not by Fe²⁺, Fe³⁺ or Cu²⁺. The iron and pyrimidine nucleotide transport processes displayed different salt requirements; pyrimidine transport was dependent on the salt content of the buffer whereas iron transport was salt independent. In mitochondria containing Rim2 (E248A), iron proteins were decreased, including aconitase (Fe-S), pyruvate dehydrogenase (lipoic acid containing) and cytochrome c (heme protein). Additionally, the rate of Fe-S cluster synthesis in isolated and intact mitochondria was decreased compared with the K299A mutant, consistent with the impairment of iron-dependent functions in that mutant. In summary, mitochondrial iron transport and pyrimidine transport by Rim2 occur separately and independently. Rim2 could be a bifunctional carrier protein.

Cysteine desulfurase is regulated by phosphorylation of Nfs1 in yeast mitochondria

The cysteine desulfurase Nfs1/Isd11 uses the amino acid cysteine as the substrate and its activity is absolutely required for contributing persulfide sulfur to the essential process of iron-sulfur (Fe-S) cluster assembly in mitochondria. Here we describe a novel regulatory process involving phosphorylation of Nfs1 in mitochondria. Phosphorylation enhanced cysteine desulfurase activity, while dephosphorylation decreased its activity. Nfs1 phosphopeptides were identified, and the corresponding phosphosite mutants showed impaired persulfide formation. Nfs1 pull down from mitochondria recovered an associated kinase activity, and Yck2, a kinase present in the pull down, was able to phosphorylate Nfs1 in vitro and stimulate cysteine desulfurase activity. Yck2 exhibited an eclipsed distribution in the mitochondrial matrix, although other cellular localizations have been previously described. Mitochondria lacking the Yck2 protein kinase (∆yck2) showed less phosphorylating activity for Nfs1. Compared with wild-type mitochondria, ∆yck2 mitochondria revealed slower persulfide formation on Nfs1 consistent with a role of Yck2 in regulating mitochondrial cysteine desulfurase activity. We propose that Nfs1 phosphorylation may provide a means of rapid adaptation to increased metabolic demand for sulfur and Fe-S clusters within mitochondria.

Nfs1 cysteine desulfurase protein complexes and phosphorylation sites as assessed by mass spectrometry

Fe-S clusters are cofactors that participate in diverse and essential biological processes. Mitochondria contain a complete machinery for Fe-S cluster assembly. Cysteine desulfurase (Nfs1) is required generation of a form of activated sulfur and is essential for the initial Fe-S cluster assembly step. Using mass-spectometry we identified proteins that were copurified with Nfs1 using a pull-down strategy, including a novel protein kinase. Furthermore, we were able to identify phosphorylation sites on the Nfs1 protein. These data and analyses support the research article “Cysteine desulfurase is regulated by phosphorylation of Nfs1 in yeast mitochondria” by Rocha et al. (in press) [1].

Turning Saccharomyces cerevisiae into a Frataxin-Independent Organism

Frataxin (Yfh1 in yeast) is a conserved protein and deficiency leads to the neurodegenerative disease Friedreich's ataxia. Frataxin is a critical protein for Fe-S cluster assembly in mitochondria, interacting with other components of the Fe-S cluster machinery, including cysteine desulfurase Nfs1, Isd11 and the Isu1 scaffold protein. Yeast Isu1 with the methionine to isoleucine substitution (M141I), in which the E. coli amino acid is inserted at this position, corrected most of the phenotypes that result from lack of Yfh1 in yeast. This suppressor Isu1 behaved as a genetic dominant. Furthermore frataxin-bypass activity required a completely functional Nfs1 and correlated with the presence of efficient scaffold function. A screen of random Isu1 mutations for frataxin-bypass activity identified only M141 substitutions, including Ile, Cys, Leu, or Val. In each case, mitochondrial Nfs1 persulfide formation was enhanced, and mitochondrial Fe-S cluster assembly was improved in the absence of frataxin. Direct targeting of the entire E. coli IscU to ∆yfh1 mitochondria also ameliorated the mutant phenotypes. In contrast, expression of IscU with the reverse substitution i.e. IscU with Ile to Met change led to worsening of the ∆yfh1 phenotypes, including severely compromised growth, increased sensitivity to oxygen, deficiency in Fe-S clusters and heme, and impaired iron homeostasis. A bioinformatic survey of eukaryotic Isu1/prokaryotic IscU database entries sorted on the amino acid utilized at the M141 position identified unique groupings, with virtually all of the eukaryotic scaffolds using Met, and the preponderance of prokaryotic scaffolds using other amino acids. The frataxin-bypassing amino acids Cys, Ile, Leu, or Val, were found predominantly in prokaryotes. This amino acid position 141 is unique in Isu1, and the frataxin-bypass effect likely mimics a conserved and ancient feature of the prokaryotic Fe-S cluster assembly machinery.

Frataxin-bypassing Isu1: characterization of the bypass activity in cells and mitochondria

Frataxin is a conserved mitochondrial protein, and deficiency underlies the neurodegenerative disease Friedreich's ataxia. Frataxin interacts with the core machinery for Fe-S cluster assembly in mitochondria. Recently we reported that in frataxin-deleted yeast strains, a spontaneously occurring mutation in one of two genes encoding redundant Isu scaffold proteins, bypassed the mutant phenotypes. In the present study we created strains expressing a single scaffold protein, either Isu1 or the bypass mutant M107I Isu1. Our results show that in the frataxin-deletion strain expressing the bypass mutant Isu1, cell growth, Fe-S cluster protein activities, haem proteins and iron homoeostasis were restored to normal or close to normal. The bypass effects were not mediated by changes in Isu1 expression level. The persulfide-forming activity of the cysteine desulfurase was diminished in the frataxin deletion (∆yfh1 ISU1) and was improved by expression of the bypass Isu1 (∆yfh1 M107I ISU1). The addition of purified bypass M107I Isu1 protein to a ∆yfh1 lysate conferred similar enhancement of cysteine desulfurase as did frataxin, suggesting that this effect contributed to the bypass mechanism. Fe-S cluster-forming activity in isolated mitochondria was stimulated by the bypass Isu1, albeit at a lower rate. The rescuing effects of the bypass Isu1 point to ways that the core defects in Friedreich's ataxia mitochondria can be restored.

Reduction of 2,3-bis(2-methoxy-4-nitro-5-sulfophenyl)-2H-tetrazolium-5-carboxanilide inner salt (XTT) is dependent on CaFRE10 ferric reductase for Candida albicans grown in unbuffered media

The reduction of 2,3-bis(2-methoxy-4-nitro-5-sulfophenyl)-2H-tetrazolium-5-carboxanilide inner salt (XTT) and other tetrazolium salts is widely used as an assay for bacterial, fungal and mammalian cell viability, but the genes encoding the reductase activities have not been defined. Here, it was shown that XTT and plasma membrane ferric reductase activities were 10-40-fold greater in Candida albicans than in Saccharomyces cerevisiae. XTT reductase activity was induced fivefold in C. albicans grown in low-iron conditions compared with iron-replete conditions, and for cells grown in unbuffered (pH 4.0-4.4) medium, XTT reductase activity was largely dependent on CaFRE10. XTT reductase activity of C. albicans grown in medium buffered to pH 6.8 was independent of CaFRE10 but, nonetheless, was upregulated in cells deprived of iron. Reduction of 2-(4,5-dimethyl-2-thiazolyl)-3,5-diphenyl-2H-tetrazolium bromide (MTT), a membrane-permeable tetrazolium salt, occurred at an intracellular location and was independent of CaFRE10. However, MTT activity was induced by iron deprivation in C. albicans but not in S. cerevisiae. C. albicans possessed multiple iron- and pH-regulated reductase activities capable of reducing tetrazolium salts, but, when grown in unbuffered medium, CaFRE10 was required for XTT reductase activity.

Mrs3p, Mrs4p, and frataxin provide iron for Fe-S cluster synthesis in mitochondria

Yeast Mrs3p and Mrs4p are evolutionarily conserved mitochondrial carrier proteins that transport iron into mitochondria under some conditions. Yeast frataxin (Yfh1p), the homolog of the human protein implicated in Friedreich ataxia, is involved in iron homeostasis. However, its precise functions are controversial. Anaerobically grown triple mutant cells (Deltamrs3/4/Deltayfh1) displayed a severe growth defect corrected by in vivo iron supplementation. Because anaerobically grown cells do not synthesize heme, and they do not experience oxidative stress, this growth defect was most likely due to Fe-S cluster deficiency. Fe-S cluster formation was assessed in anaerobically grown cells shifted to air for a brief period. In isolated mitochondria, Fe-S clusters were detected on newly imported yeast ferredoxin precursor and on endogenous aconitase by means of [35S]cysteine labeling and native gel separation. New cluster formation was dependent on iron addition to mitochondria, and the iron concentration dependence was shifted dramatically upward in the Deltamrs3/4 mutant, indicating a role of Mrs3/4p in iron transport. The frataxin mutant strain lacked protein import capacity because of low mitochondrial membrane potential, although this was partially restored by growth in the presence of high iron. Under these conditions, a kinetic defect in new Fe-S cluster formation was still noted. Import of frataxin into frataxin-minus isolated mitochondria promptly corrected the Fe-S cluster assembly defect without further iron addition. These findings show that Mrs3/4p transports iron into mitochondria, whereas frataxin makes iron already within mitochondria available for Fe-S cluster synthesis.

Iron acquisition from transferrin by Candida albicans depends on the reductive pathway

Host-pathogen interactions that alter virulence are influenced by critical nutrients such as iron. In humans, free iron is unavailable, being present only in high-affinity iron binding proteins such as transferrin. The fungal pathogen Candida albicans grows as a saprophyte on mucosal surfaces. Occasionally it invades systemically, and in this circumstance it will encounter transferrin iron. Here we report that C. albicans is able to acquire iron from transferrin. Iron-loaded transferrin restored growth to cultures arrested by iron deprivation, whereas apotransferrin was unable to promote growth. By using congenic strains, we have been able to show that iron uptake by C. albicans from transferrin was mediated by the reductive pathway (via FTR1). The genetically separate siderophore and heme uptake systems were not involved. FRE10 was required for a surface reductase activity and for efficient transferrin iron uptake activity in unbuffered medium. Other reductase genes were apparently up-regulated in medium buffered at pH 6.3 to 6.4, and the fre10(-/-) mutant had no effect under these conditions. Experiments in which transferrin was sequestered in a dialysis bag demonstrated that cell contact with the substrate was required for iron reduction and release. The requirement of FTR1 for virulence in a systemic infection model and its role in transferrin iron uptake raise the possibility that transferrin is a source of iron during systemic C. albicans infections.

Frataxin and mitochondrial carrier proteins, Mrs3p and Mrs4p, cooperate in providing iron for heme synthesis

Frataxin is a conserved mitochondrial protein implicated in cellular iron metabolism. Deletion of the yeast frataxin homolog (YFH1) was combined with deletions of MRS3 and MRS4, mitochondrial carrier proteins implicated in iron homeostasis. As previously reported, the Deltayfh1 mutant accumulated iron in mitochondria, whereas the triple mutant (DeltaDeltaDelta) did not. When wild-type, Deltamrs3/4, Deltayfh1, and DeltaDeltaDelta strains were incubated anaerobically, all strains were devoid of heme and protected from iron and oxygen toxicity. The cultures were then shifted to air for a short time (4-5 h) or a longer time (15 h), and the evolving mutant phenotypes were analyzed (heme-dependent growth, total heme, cytochromes, heme proteins, and iron levels). A picture emerges from these data of defective heme formation in the mutants, with a markedly more severe defect in the DeltaDeltaDelta than in the individual Deltamrs3/4 or Deltayfh1 mutants (a "synthetic" defect in the genetic sense). The defect(s) in heme formation could be traced to lack of iron. Using a real time assay of heme biosynthesis, porphyrin precursor and iron were presented to permeabilized cells, and the appearance and disappearance of fluorescent porphyrins were followed. The Mrs3/4p carriers were required for rapid iron transport into mitochondria for heme synthesis, whereas there was also evidence for an alternative slower system. A different role for Yfh1p was observed under conditions of low mitochondrial iron and aerobic growth (revealed in the DeltaDeltaDelta), acting to protect bioavailable iron within mitochondria and to facilitate its use for heme synthesis.

Genome-wide screen for genes with effects on distinct iron uptake activities in Saccharomyces cerevisiae

We screened a collection of 4847 haploid knockout strains (EUROSCARF collection) of Saccharomyces cerevisiae for iron uptake from the siderophore ferrioxamine B (FOB). A large number of mutants showed altered uptake activities, and a few turned yellow when grown on agar plates with added FOB, indicating increased intracellular accumulation of undissociated siderophores. A subset consisting of 197 knockouts with altered uptake was examined further for regulated activities that mediate cellular uptake of iron from other siderophores or from iron salts. Hierarchical clustering analysis grouped the data according to iron sources and according to mutant categories. In the first analysis, siderophores grouped together with the exception of enterobactin, which grouped with iron salts, suggesting a reductive pathway of iron uptake for this siderophore. Mutant groupings included three categories: (i) high-FOB uptake, high reductase, low-ferrous transport; (ii) isolated high- or low-FOB transport; and (iii) induction of all activities. Mutants with statistically altered uptake activities included genes encoding proteins with predominant localization in the secretory pathway, nucleus, and mitochondria. Measurements of different iron-uptake activities in the yeast knockout collection make possible distinctions between genes with general effects on iron metabolism and those with pathway-specific effects.

Candida albicans lacking the frataxin homologue: a relevant yeast model for studying the role of frataxin

We cloned the CaYFH1 gene that encodes the yeast frataxin homologue in Candida albicans. CaYFH1 was expressed in Deltayfh1 Saccharomyces cerevisiae cells, where it compensated for all the phenotypes tested except for the lack of cytochromes. Double DeltaCayfh1/DeltaCayfh1 mutant had severe defective growth, accumulated iron in their mitochondria, lacked aconitase and succinate dehydrogenase activity and had defective respiration. The reductive, siderophore and haem uptake systems were constitutively induced and the cells excreted flavins, thus behaving like iron-deprived wild-type cells. Mutant cells accumulated reactive oxygen species and were hypersensitive to oxidative stress, but not to iron. Cytochromes were less abundant in mutants than in wild-type cells, but this did not result from defective haem synthesis. The low cytochrome concentration in mutant cells was comparable to that of iron-deprived wild-type cells. Mitochondrial iron was still available for haem synthesis in DeltaCayfh1/DeltaCayfh1 cells, in contrast to S. cerevisaeDeltayfh1 cells. CaYFH1 transcription was strongly induced by iron, which is consistent with a role of CaYfh1 in iron storage. Iron also regulated transcription of CaHEM14 (encoding protoporphyrinogen oxidase) but not that of CaHEM15 (encoding ferrochelatase). There are thus profound differences between S. cerevisiae and C. albicans in terms of haem synthesis, cytochrome turnover and the role of frataxin in these processes.

Role of YHM1, encoding a mitochondrial carrier protein, in iron distribution of yeast

Mitochondrial carrier proteins are a large protein family, consisting of 35 members in Saccharomyces cerevisiae. Members of this protein family have been shown to transport varied substrates from cytoplasm to mitochondria or mitochondria to cytoplasm, although many family members do not have assigned substrates. We speculated whether one or more of these transporters will play a role in iron metabolism. Haploid yeast strains each deleted for a single mitochondrial carrier protein were analysed for alterations in iron homoeostasis. The strain deleted for YHM1 was characterized by increased and misregulated surface ferric reductase and high-affinity ferrous transport activities. Siderophore uptake from different sources was also increased, and these effects were dependent on the AFT1 iron sensor regulator. Mutants of YHM1 converted into rho degrees, consistent with secondary mitochondrial DNA damage from mitochondrial iron accumulation. In fact, in the Delta yhm1 mutant, iron was found to accumulate in mitochondria. The accumulated iron showed decreased availability for haem synthesis, measured in isolated mitochondria using endogenously available metals and added porphyrins. The phenotypes of Delta yhm1 mutants indicate a role for this mitochondrial transporter in cellular iron homoeostasis.

Iron use for haeme synthesis is under control of the yeast frataxin homologue (Yfh1)

The YFH1 gene is the yeast homologue of the human FRDA gene, which encodes the frataxin protein. Saccharomyces cerevisiae cells lacking the YFH1 gene showed very low cytochrome content. In Deltayfh1 strains, the level of ferrochelatase (Hem15p) was very low, as a result of transcriptional repression of HEM15. However, the low amount of Hem15p was not the cause of haeme deficiency in Deltayfh1 cells. Ferrochelatase, a mitochondrial protein, able to mediate insertion of iron or zinc into the porphyrin precursor, made primarily the zinc protoporphyrin product. Zinc protoporphyrin instead of haeme accumulated during growth of Deltayfh1 mutant cells and, furthermore, preferential formation of zinc protoporphyrin was observed in real time. The method for these studies involved direct presentation of porphyrin to mitochondria and to ferrochelatase of permeabilized cells with intact architecture, thereby specifically testing the iron delivery portion of the haeme biosynthetic pathway. The studies showed that Deltayfh1 mutant cells are defective in iron use by ferrochelatase. Mössbauer spectroscopic analysis showed that iron was present as amorphous nano-particles of ferric phosphate in Deltayfh1 mitochondria, which could explain the unavailability of iron for haeme synthesis. A high frequency of suppressor mutations was observed, and the phenotype of such mutants was characterized by restoration of haeme synthesis in the absence of Yfh1p. Suppressor strains showed a normal cytochrome content, normal respiration, but remained defective in Fe-S proteins and still accumulated iron into mitochondria although to a lesser extent. Yfh1p and Hem15p were shown to interact in vitro by Biacore studies. Our results suggest that Yfh1 mediates iron use by ferrochelatase.

Siderophore uptake by Candida albicans: effect of serum treatment and comparison with Saccharomyces cerevisiae

Iron uptake systems often function as virulence factors in pathogenic organisms. Candida albicans is a fungal pathogen that infects immunocompromised hosts, such as AIDS patients or granulocytopenic bone marrow transplant recipients. Here we show that iron uptake from siderophores occurs in C. albicans and is mediated by one or more high-affinity transport systems. Iron carried on ferrioxamine B, triacethyl-fusarinine, ferrichrome, or ferricrocin was actively taken up via a high-affinity mechanism. The kinetic parameters of uptake were similar to those found in S. cerevisiae. Furthermore, for ferrichrome and ferrioxamine B, cellular uptake of fluorescent analogues was observed. In C. albicans, iron uptake from siderophores was regulated by iron availability, with iron deprivation inducing uptake. Serum exposure, which induces a morphogenic shift from yeast to filamentous forms known to be required for virulence, also resulted in induction of iron transport from ferrichrome-type siderophores. In a tup1/tup1 strain which grows constitutively in the filamentous form, iron transport was derepressed for all siderophores tested. The genes mediating uptake and utilization of iron from siderophores in C. albicans have not been identified; however, the transcript abundance for CaSIT1 was regulated in a manner consistent with the pattern of iron uptake from ferrichrome-type siderophores. Furthermore, CaSIT1 overexpression in S. cerevisiae resulted in inhibited siderophore iron uptake, suggesting that the expressed protein may interact with proteins of S. cerevisiae involved in iron uptake from siderophores. In summary, iron uptake from ferrichrome-type siderophores was induced in filamentous C. albicans, and a potential role of this iron acquisition system in pathogenicity should be considered.

Reductive iron uptake by Candida albicans: role of copper, iron and the TUP1 regulator

High-affinity iron uptake by a ferrous permease in the opportunistic pathogen Candida albicans is required for virulence. Here this iron uptake system has been characterized by investigating three distinct activities: an externally directed surface ferric reductase, a membrane-associated PPD (p-phenylenediamine) oxidase and a cellular ferrous iron transport activity. Copper was required for the PPD oxidase and ferrous transport activities. In contrast, copper was not required for iron uptake from siderophores. Addition of iron to the growth medium repressed ferric reductase and ferrous transport, indicating homeostatic regulation. To identify the genes involved, orthologous mutants of Saccharomyces cerevisiae were transformed with a genomic library of C. albicans. CFL95, a gene with sequence similarity to ferric reductases, restored reductase activity to the orthologous S. cerevisiae mutant. CaFTR2 and CaFTR1, genes with homology to ferrous permeases, conferred ferrous transport activity to the orthologous S. cerevisiae mutant. However, neither a genomic library nor CaFET99, a multicopper oxidase homologue and candidate gene for the PPD oxidase, complemented the S. cerevisiae mutant, possibly because of problems with targeting or assembly. Transcripts for CFL95, CaFTR1 and CaFET99 were strongly repressed by iron, whereas the CaFTR2 transcript was induced by iron. Deletion of the TUP1 regulator perturbed the homeostatic control of reductive iron uptake. Incidentally, iron starvation was noted to induce flavin production and this was misregulated in the absence of TUP1 control. The opposite regulation of two iron permease genes and the role of TUP1 indicate that the process of iron acquisition by C. albicans may be more complex and potentially more adaptable than by S. cerevisiae.