Iron transport in Mycobacterium smegmatis: the role of mycobactin

A genetic selection for Mycobacterium smegmatis mutants tolerant to killing by sodium citrate defines a combined role for cation homeostasis and osmotic stress in cell death

Mycobacteria can colonize environments where the availability of metal ions is limited. Biological or inorganic chelators play an important role in limiting metal availability, and we developed a model to examine Mycobacterium smegmatis survival in the presence of the chelator sodium citrate. We observed that instead of restricting M. smegmatis growth, concentrated sodium citrate killed M. smegmatis. RNAseq analysis during sodium citrate treatment revealed transcriptional signatures of metal starvation and hyperosmotic stress. Notably, metal starvation and hyperosmotic stress, individually, do not kill M. smegmatis under these conditions. A forward genetic transposon selection was conducted to examine why sodium citrate was lethal, and several sodium-citrate-tolerant mutants were isolated. Based on the identity of three tolerant mutants, mgtE, treZ, and fadD6, we propose a dual stress model of killing by sodium citrate, where sodium citrate chelate metals from the cell envelope and then osmotic stress in combination with a weakened cell envelope causes cell lysis. This sodium citrate tolerance screen identified mutants in several other genes with no known function, with most conserved in the pathogen M. tuberculosis. Therefore, this model will serve as a basis to define their functions, potentially in maintaining cell wall integrity, cation homeostasis, or osmotolerance. IMPORTANCE Bacteria require mechanisms to adapt to environments with differing metal availability. When Mycobacterium smegmatis is treated with high concentrations of the metal chelator sodium citrate, the bacteria are killed. To define the mechanisms underlying killing by sodium citrate, we conducted a genetic selection and observed tolerance to killing in mutants of the mgtE magnesium transporter. Further characterization studies support a model where killing by sodium citrate is driven by a weakened cell wall and osmotic stress, that in combination cause cell lysis.

The promoter of Rv0560c is induced by salicylate and structurally-related compounds in Mycobacterium tuberculosis

Mycobacterium tuberculosis, the causative agent of tuberculosis (TB), is a major global health threat. During infection, bacteria are believed to encounter adverse conditions such as iron depletion. Mycobacteria synthesize iron-sequestering mycobactins, which are essential for survival in the host, via the intermediate salicylate. Salicylate is a ubiquitous compound which is known to induce a mild antibiotic resistance phenotype. In M. tuberculosis salicylate highly induces the expression of Rv0560c, a putative methyltransferase. We identified and characterized the promoter and regulatory elements of Rv0560c. P_Rv0560c activity was highly inducible by salicylate in a dose-dependent manner. The induction kinetics of P_Rv0560c were slow, taking several days to reach maximal activity, which was sustained over several weeks. Promoter activity could also be induced by compounds structurally related to salicylate, such as aspirin or para-aminosalicylic acid, but not by benzoate, indicating that induction is specific to a structural motif. The −10 and −35 promoter elements were identified and residues involved in regulation of promoter activity were identified in close proximity to an inverted repeat spanning the −35 promoter element. We conclude that Rv0560c expression is controlled by a yet unknown repressor via a highly-inducible promoter.

Role of porins in iron uptake by Mycobacterium smegmatis

Many bacteria rely on siderophores to extract iron from the environment. However, acquisition of iron-loaded siderophores is dependent on high-affinity uptake systems that are not produced under high-iron conditions. The fact that bacteria are able to maintain iron homeostasis in the absence of siderophores indicates that alternative iron acquisition systems exist. It has been speculated that such low-affinity uptake of iron in Gram-negative bacteria includes diffusion of iron ions or chelates across the outer membrane through porins. The outer membrane of the saprophytic Mycobacterium smegmatis contains the Msp family of porins, which enable the diffusion of small and hydrophilic solutes, such as monosaccharides, amino acids, and phosphate. However, it is unknown how cations cross the outer membrane of mycobacteria. Here, we show that the Msp porins of M. smegmatis are involved in the acquisition of soluble iron under high-iron conditions. Uptake of ferric ions by a triple porin mutant was reduced compared to wild-type (wt) M. smegmatis. An intracellular iron reporter indicated that derepression of iron-responsive genes occurs at higher iron concentrations in the porin mutant. This was consistent with the finding that the porin mutant produced more siderophores under low-iron conditions than wt M. smegmatis. In contrast, uptake of the exochelin MS, the main siderophore of M. smegmatis, was not affected by the lack of porins, indicating that a specific outer membrane siderophore receptor exists. These results provide, to our knowledge, the first experimental evidence that general porins are indeed the outer membrane conduit of low-affinity iron acquisition systems in bacteria.

Utilization of microbial iron assimilation processes for the development of new antibiotics and inspiration for the design of new anticancer agents

Pathogenic microbes rapidly develop resistance to antibiotics. To keep ahead in the "microbial war", extensive interdisciplinary research is needed. A primary cause of drug resistance is the overuse of antibiotics that can result in alteration of microbial permeability, alteration of drug target binding sites, induction of enzymes that destroy antibiotics (ie., beta-lactamase) and even induction of efflux mechanisms. A combination of chemical syntheses, microbiological and biochemical studies demonstrate that the known critical dependence of iron assimilation by microbes for growth and virulence can be exploited for the development of new approaches to antibiotic therapy. Iron recognition and active transport relies on the biosyntheses and use of microbe-selective iron-chelating compounds called siderophores. Our studies, and those of others, demonstrate that siderophores and analogs can be used for iron transport-mediated drug delivery ("Trojan Horse" antibiotics) and induction of iron limitation/starvation (Development of new agents to block iron assimilation). Recent extensions of the use of siderophores for the development of novel potent and selective anticancer agents are also described.

Screening system for xenosiderophores as potential drug delivery agents in mycobacteria

In order to establish a screening system for xenosiderophores which can be utilized by mycobacteria, we generated a set of mutants of Mycobacterium smegmatis that are blocked in different steps of the well-known iron acquisition system. One mutant with a block in mycobactin biosynthesis was generated from strain mc(2)155 by chemical mutagenesis. The exochelin biosynthesis gene fxbA and the ferric exochelin uptake gene fxuA, previously identified by Fiss et al. (E. H. Fiss, S. Yu, and W. R. Jacobs, Jr., Mol. Microbiol. 14:557-559, 1994), were knocked out by gene replacement. Adjacent chromosomal fragments were used for homologous recombination in order to replace wild-type genes by the kanamycin resistance gene from transposon Tn903. Gene replacement was confirmed by PCR. The isolated mutants show the expected phenotype: fxbA mutants are defective in exochelin biosynthesis, whereas fxuA mutants excrete a significantly larger amount of exochelin compared to the amount excreted by the parent strain. This is due to their defectiveness in ferriexochelin uptake, as demonstrated in growth promotion assays. This new set of mutants allows differentiation of siderophores that supply mycobacteria with iron by ligand exchange with exochelin or mycobactin, by the use of separate siderophore uptake routes, or by the use of the exochelin permease. All these types of iron uptake routes were identified with 25 exogenous siderophores as test substances. Siderophores that act without ligand exchange are potential candidates as drug vectors that can be used to overcome permeability-mediated resistance.

Isolation, purification and structure of exochelin MS, the extracellular siderophore from Mycobacterium smegmatis

The extracellular siderophore from Mycobacterium smegmatis, exochelin MS, was isolated from iron-deficiently grown cultures and purified to > 98% by a combination of ion-exchange chromatography and h.p.l.c. The material is unextractable into organic solvents, is basic (pI = 9.3-9.5), has a lambda max at 420 nm and a probable Ks for Fe3+ of between 10(25) and 10(30). Its structure has been determined by examination of desferri- and ferri-exochelin and its gallium complex. The methods used were electrospray-m.s. and one- and two-dimensional (NOESY, DQF-COSY and TOCSY) 1H n.m.r. The constituent amino acids were examined by chiral g.l.c analysis of N-trifluoroacetyl isopropyl and N-pentafluoropropionyl methyl esters after hydrolysis, and reductive HI hydrolysis, of the siderophore. The exochelin is a formylated pentapeptide: N-(delta-N-formyl,delta N-hydroxy-R-ornithyl) -beta-alaninyl-delta N-hydroxy-R-ornithinyl-R-allo-threoninyl-delta N-hydroxy-S-ornithine. The linkages involving the three ornithine residues are via their delta N(OH) and alpha-CO groups leaving three free alpha-NH2 groups. Although there are two peptide bonds, these involve the three R (D)-amino acids. Thus the molecule has no conventional peptide bond, and this suggests that it will be resistant to peptidase hydrolysis. The co-ordination centre with Fe3+ is hexadenate in an octahedral structure involving the three hydroxamic acid groups. Molecular modelling shows it to have similar features to other ferric trihydroxamate siderophores whose three-dimensional structures have been established. The molecule is shown to have little flexibility around the iron chelation centre, although the terminal (Orn-3) residue, which is not involved in iron binding except at its delta N atom, has more motional freedom.

Isolation of a membrane associated iron chelator from Pseudomonas aeruginosa

A membrane associated iron chelator (MAIC) has been extracted with ethanol from the membranes of Pseudomonas aeruginosa, and isolated on thin-layer chromatograms. Also extracted from the membranes is the ferrated form of MAIC, FeMAIC. When cell-bound or in the complete ethanol extract of membranes, MAIC binds iron from exogenous iron sources forming FeMAIC. Methanol solutions of each compound exhibit similar absorption spectra with strong absorption in the ultraviolet, indicating the aromatic structure of the compounds. Colorimetric reactions reveal the presence of a phenolic moiety in these compounds. MAIC and FeMAIC are extracted from the membranes of cells grown in media supplemented with iron or in media containing significant trace levels of iron. Transport studies revealed that neither iron-fed nor iron-starved cells transport detectable levels of radiolabeled iron from exogenous iron sources, yet low amounts of 55FeMAIC are extracted from the membranes of cells incubated with [55Fe]ferric chelators. The MAIC may serve as an iron transporter in these cells, or may serve to bind iron following its transport into the cell via another mechanism.

Siderophore utilization and iron uptake by Rhodopseudomonas sphaeroides

The growth of Rhodopseudomonas sphaeroides in iron-deficient medium did not result in the production of detectable levels of siderophores of either the catechol or hydroxamate type. Iron-limited cultures of R. sphaeroides were not able to remove iron from ferric transferrin unless supplemented with 2,3-dihydroxybenzoic acid. R. sphaeroides was shown to take up 59Fe+3 when it was supplied as ferric chloride, ferric citrate, or ferric parabactin, but not when supplied as ferric rhodotorulate or ferric Desferal. When iron was supplied as ferric citrate, citrate was not taken up by the cells. The growth rate of R. sphaeroides under iron-limiting conditions was decreased by the addition of either Desferal or rhodotorulic acid, while the addition of citrate or parabactin did not affect growth.

Iron-binding compounds of Mycobacterium avium, M. intracellulare, M. scrofulaceum, and mycobactin-dependent M. paratuberculosis and M. avium

Fifty-three strains of M. avium and related species all produced one or more exochelins, the extracellular iron-binding compounds of the mycobacteria, when grown iron deficiently. Only those strains which could grow without the addition of mycobactin (i.e., mycobactin independent) produced mycobactin, the intracellular iron-binding compound of the mycobacteria. Exochelins varied from 20 to 2,000 micrograms per g of cell dry weight; mycobactins were between 1 and 10 mg per g of cell dry weight. M. paratuberculosis (13 strains) and 13 strains of M. avium, both species dependent upon mycobactin for growth, failed to produce spectrophotometrically detectable amounts of mycobactin (less than 0.2 microgram per g of cell dry weight), although mycobactin could be recognized in one strain of M. avium grown with an additional supply of salicylate and examined by a radiolabeling technique. On repeated subculture three of the mycobactin-dependent strains of M. avium, but none of those of M. paratuberculosis, lost their mycobactin dependence and on reexamination were found to produce their own mycobactin at 0.3 mg per g of cell dry weight. It is concluded that mycobactin biosynthesis is probably strongly repressed in the mycobactin-dependent strains rather than being a genetic deletion. The exochelins, when examined by high-pressure thin-layer chromatography were revealed as being multiples of similar compounds, with up to 20 individual iron-binding compounds being recognizable with some strains. It is argued that the exochelins represent the single most important means of iron acquisition in mycobacteria growing in vitro and in vivo, and their elaboration by the fastidious M. paratuberculosis and related species explains how these organisms are able to grow in vivo in the absence of an external supply of mycobactin.

Recent European research activities in mycobacteriology

The subject of mycobacteriology is becoming an increasingly popular and complex one and the contribution by European scientists has been considerable. The important areas of research include taxonomy and the improvement of identification methods; biochemistry, including enzymology, metabolic regulation, lipid chemistry, iron uptake and metabolism, pigment synthesis and DNA chemistry; genetics and bacteriophages; ecology, including the effect of contact with environmental mycobacteria on the mammalian immune response; immunology; and the association of disease and cell-wall-free mycobacteria and studies on the leprosy bacillus. The European Society of Mycobacteriologists has recently been founded to unite workers in these disciplines and to promote and disseminate knowledge in this subject.

Enzymatic release of iron from sideramines in fungi. NADH:sideramine oxidoreductase in Neurospora crassa

Young mycelia of the fungus Neurospora crassa contain a soluble NADH-linked sideramine reductase, which may be responsible for liberating iron in vivo from accumulated sideramines during iron-deficient cultivation. The enzymes can be assayed using a soluble supernatant fraction, EDTA, and an atmosphere of pure nitrogen. The enzyme is stable without loss of activity up to 45 degrees C and has an optimum of activity at pH 7.0. Besides coprogen (Km = 100 micrometer, V=2.8 nmol/min per mg protein), some other ferrichrome-type compounds are reduced. However, ferrichrome, ferrirubin coprogen B and ferrioxamine are poor substrates. When the mucelia were grown in a medium containing 10(-5) M ferri iron, the activity of the reductase was found to be only 30% of that found under low iron conditions. The enzyme is inhibited by oxygen, SH-alkylating agents and partly by some detergents. Unlike the reductase of N. crassa, the corresponding enzyme from Aspergillus fumigatus revealed low reduction of coprogen and high reduction of ferrichrome, indicating genusdependent specificities of sideramine reduction enzymes in fungi. The participation of acids of the citric acid cycle as natural iron acceptors during strong iron deficiency is studied and confirmed by iron uptake measurements on isolated mitochondria.

The effect of p-aminosalicyclic acid on iron transport and assimilation in mycobacteria

p-Aminosalicylic acid inhibits growth of Mycobacterium bovis BCG and Mycobacterium smegmatis more effectively if cells are growing with a sufficiency of iron (more than 1 mu g Fe/ml) in the medium than if cells are deficient in iron (smaller than 0.1 mu g Fe/ml). In iron-deficient cultures formation of mycobactin, an ionophore for iron transport, is strongly inhibited by p-aminosalicylic acid. Uptake of iron into cell suspensions is also inhibited and the activity of several iron-containing enzymes declines in cells exposed to p-aminosalicylic acid during their growth. p-Aminosalicylic acid is about 50 times more effective towards a mutant of M. smegmatis which required mycobactin under iron-deficient growth conditions than towards the wild-type parent. p-Aminosalicylate is taken up into cells by an active process independent of the salicylate uptake system, possibly by the route used for assimilation of p-aminobenzoate. (This could account for why p-aminobenzoic acid, but not salicylic acid, antagonizes the action of p-aminosalicylic acid.) With iron-deficient cells, salicylate assimilation is about 50 times greater than either p-aminosalicylate or p-aminobenzoate but with iron-sufficient cells and with the mycobactin mutant salicylate uptake is negligible whereas p-aminobenzoate and p-aminosalicylate uptakes are unaffected. p-Aminosalicylic acid at 3.3 mM (500 mu g/ml) partially inhibits the uptake of both p-aminobenzoate and, if it is occurring, that of salicylate as well. As p-aminosalicylic acid is always more effective when the intracellular concentration of salicylic acid is low, it probably acts as an anti-metabolite of salicylic acid, not, however, by inhibiting the conversion of salicylic acid to mycobactin, but probably somewhere along the metabolic pathway of iron uptake.

Isolation and structure of nocobactin NA, a lipid-soluble iron-binding compound from Nocardia asteroides

Nocobactin NA, a lipid-soluble iron-chelating product with an unusual and characteristic u.v.-absorption spectrum, was isolated from Nocardia asteroides grown under conditions of iron deficiency. Its structure was determined by physical methods and by synthesis of one of its degradation products. Nocobactin NA was obtained as a homologous mixture of compounds with side chains of differing length, and resembles mycobactin M in structure except that it has an oxazole ring in place of an oxazoline ring, and the side chains in the cobactin fragment are considerably shorter.