Ferric uptake regulator (Fur) mutants of Pseudomonas aeruginosa demonstrate defective siderophore-mediated iron uptake, altered aerobic growth, and decreased superoxide dismutase and catalase activities

Factors affecting catalase expression in Pseudomonas aeruginosa biofilms and planktonic cells

Previous work with Pseudomonas aeruginosa showed that catalase activity in biofilms was significantly reduced relative to that in planktonic cells. To better understand biofilm physiology, we examined possible explanations for the differential expression of catalase in cells cultured in these two different conditions. For maximal catalase activity, biofilm cells required significantly more iron (25 microM as FeCl(3)) in the medium, whereas planktonic cultures required no addition of iron. However, iron-stimulated catalase activity in biofilms was still only about one-third that in planktonic cells. Oxygen effects on catalase activity were also investigated. Nitrate-respiring planktonic cultures produced approximately twice as much catalase activity as aerobic cultures grown in the presence of nitrate; the nitrate stimulation effect could also be demonstrated in biofilms. Cultures fermenting arginine had reduced catalase levels; however, catalase repression was also observed in aerobic cultures grown in the presence of arginine. It was concluded that iron availability, but not oxygen availability, is a major factor affecting catalase expression in biofilms.

Regulation of ferritin-mediated cytoplasmic iron storage by the ferric uptake regulator homolog (Fur) of Helicobacter pylori

Homologs of the ferric uptake regulator Fur and the iron storage protein ferritin play a central role in maintaining iron homeostasis in bacteria. The gastric pathogen Helicobacter pylori contains an iron-induced prokaryotic ferritin (Pfr) which has been shown to be involved in protection against metal toxicity and a Fur homolog which has not been functionally characterized in H. pylori. Analysis of an isogenic fur-negative mutant revealed that H. pylori Fur is required for metal-dependent regulation of ferritin. Iron starvation, as well as medium supplementation with nickel, zinc, copper, and manganese at nontoxic concentrations, repressed synthesis of ferritin in the wild-type strain but not in the H. pylori fur mutant. Fur-mediated regulation of ferritin synthesis occurs at the mRNA level. With respect to the regulation of ferritin expression, Fur behaves like a global metal-dependent repressor which is activated under iron-restricted conditions but also responds to different metals. Downregulation of ferritin expression by Fur might secure the availability of free iron in the cytoplasm, especially if iron is scarce or titrated out by other metals.

Vanadium interferes with siderophore-mediated iron uptake in Pseudomonas aeruginosa

Vanadium is a metal that under physiological conditions can exist in two oxidation states, V(IV) (vanadyl ion) and V(V) (vanadate ion). Here, it was demonstrated that both ions can form complexes with siderophores. Pseudomonas aeruginosa produces two siderophores under iron-limiting conditions, pyoverdine (PVD) and pyochelin (PCH). Vanadyl sulfate, at a concentration of 1-2 mM, strongly inhibited growth of P. aeruginosa PAO1, especially under conditions of severe iron limitation imposed by the presence of non-utilizable Fe(III) chelators. PVD-deficient mutants were more sensitive to vanadium than the wild-type, but addition of PVD did not stimulate their growth. Conversely, PCH-negative mutants were more resistant to vanadium than the wild-type strain. Both siderophores could bind and form complexes with vanadium after incubation with vanadyl sulfate (1:1, in the case of PVD; 2:1, in the case of PCH). Although only one complex with PVD, V(IV)-PVD, was found, both V(IV)- and V(V)-PCH were detected. V-PCH, but not V-PVD, caused strong growth reduction, resulting in a prolonged lag phase. Exposure of PAO1 cells to vanadium induced resistance to the superoxide-generating compound paraquat, and conversely, exposure to paraquat increased resistance to V(IV). Superoxide dismutase (SOD) activity of cells grown in the presence of V(IV) was augmented by a factor of two. Mutants deficient in the production of Fe-SOD (SodB) were particularly sensitive to vanadium, whilst sodA mutants deficient for Mn-SOD were only marginally affected. In conclusion, it is suggested that V-PCH catalyses a Fenton-type reaction whereby the toxic superoxide anion O(2)- is generated, and that vanadium compromises PVD utilization.

Characterization and mutagenesis of fur gene from Burkholderia pseudomallei

A homolog of the ferric uptake regulator gene (fur) was isolated from Burkholderia pseudomallei (Bp) by a reverse genetic technique. Sequencing of a 2.2kb DNA fragment revealed an open reading frame with extensive homology to bacterial Fur proteins. The cloned gene encodes a 16kDa protein that cross-reacts with a polyclonal anti-Escherichia coli Fur serum. The transcription start site was determined by the primer extension technique. Expression analysis of fur showed no increased fur mRNA levels in response to various stresses and iron conditions. A positive selection procedure involving the isolation of manganese-resistant mutants was used to isolate mutants that produce altered Fur proteins. Sequencing of a fur mutant revealed a nucleotide change (G to A) converting a conserved amino acid arginine-69 to histidine. The fur missense mutant produced an elevated level of siderophore that could be complemented by a multicopy plasmid carrying the Bp fur. Interestingly, Fur was found to play roles as a positive regulator of FeSOD and peroxidase. The mutant showed a decreased activity of FeSOD and peroxidase, which could be important in its pathogenicity and survival in macrophages.

Protective role of the Mycobacterium smegmatis IdeR against reactive oxygen species and isoniazid toxicity

OBJECTIVE

To understand the mechanism by which IdeR is necessary for maintaining wild type levels of KatG and SodA enzyme activity and normal isoniazid (INH) resistance.

DESIGN

To identify the step(s) of SodA and KatG function that were affected by the ideR mutation, quantitative western immunoassays and ribonucleic acid (RNA) hybridizations were performed. To see if the increased INH sensitivity of the ideR mutant was caused by lower SodA activity, the Mycobacterium smegmatis sod gene was inactivated.

RESULTS

The levels of KatG and SodA mRNA and protein in the M. smegmatis IdeR mutant are decreased to approximately 20-40% of those observed in the wild type parent strain. This is quantitatively similar to the decrease in KatG and SodA enzyme activities originally observed in the ideR strain. The M. smegmatis sodA mutant was slightly more sensitive to INH, compared to the wild type strain and was more resistant than the ideR mutant.

CONCLUSION

IdeR is necessary for full expression of the M. smegmatis katG and sodA genes. It is not yet known whether this protein acts directly at the gene level. The lower levels of SodA contribute slightly to the increased susceptibility to INH of the ideR mutant, but cannot explain the magnitude of the INH sensitivity observed when IdeR is not present. These data suggest that IdeR is a regulator of the cellular stress response, as it has a protective role in cells facing environmental stresses, such as increased levels of reactive oxygen species and INH toxic intermediates. These conclusions do not necessarily apply to IdeR's role in M. tuberculosis physiology, since we have not inactivated its gene in this pathogen.

Quorum sensing in Pseudomonas aeruginosa controls expression of catalase and superoxide dismutase genes and mediates biofilm susceptibility to hydrogen peroxide

Quorum sensing (QS) governs the production of virulence factors and the architecture and sodium dodecyl sulphate (SDS) resistance of biofilm-grown Pseudomonas aeruginosa. P. aeruginosa QS requires two transcriptional activator proteins known as LasR and RhlR and their cognate autoinducers PAI-1 (N-(3-oxododecanoyl)-L-homoserine lactone) and PAI-2 (N-butyryl-L-homoserine lactone) respectively. This study provides evidence of QS control of genes essential for relieving oxidative stress. Mutants devoid of one or both autoinducers were more sensitive to hydrogen peroxide and phenazine methosulphate, and some PAI mutant strains also demonstrated decreased expression of two superoxide dismutases (SODs), Mn-SOD and Fe-SOD, and the major catalase, KatA. The expression of sodA (encoding Mn-SOD) was particularly dependent on PAI-1, whereas the influence of autoinducers on Fe-SOD and KatA levels was also apparent but not to the degree observed with Mn-SOD. beta-Galactosidase reporter fusion results were in agreement with these findings. Also, the addition of both PAIs to suspensions of the PAI-1/2-deficient double mutant partially restored KatA activity, while the addition of PAI-1 only was sufficient for full restoration of Mn-SOD activity. In biofilm studies, catalase activity in wild-type bacteria was significantly reduced relative to planktonic bacteria; catalase activity in the PAI mutants was reduced even further and consistent with relative differences observed between each strain grown planktonically. While wild-type and mutant biofilms contained less catalase activity, they were more resistant to hydrogen peroxide treatment than their respective planktonic counterparts. Also, while catalase was implicated as an important factor in biofilm resistance to hydrogen peroxide insult, other unknown factors seemed potentially important, as PAI mutant biofilm sensitivity appeared not to be incrementally correlated to catalase levels.

Pseudomonas aeruginosa biofilm sensitivity to biocides: use of hydrogen peroxide as model antimicrobial agent for examining resistance mechanisms

The biofilm mode of bacterial growth may be the preferred form of existence in nature. Because of the global impact of problematic biofilms, study of the mechanisms affording resistance to various biocides is of dire importance. Furthermore, understanding the physiological differences between biofilm and planktonic organisms ranks particularly high on the list of important and necessary research. Such contributions will only serve to broaden our knowledge base, especially regarding the development of better antimicrobials while also fine-tuning the use of current highly effective antimicrobials. Using H2O2 as a model oxidizing biocide, we demonstrate the marked resistance of biofilm bacteria relative to planktonic cells. Because many biocides are good oxidizing agents (e.g., H2O2, HOCl), understanding the mechanisms by which genes involved in combating oxidative stress are activated is important in determining the overall efficacy of such biocides. Future studies will focus on determining mechanisms of oxidative stress gene regulation in bacterial biofilms.

The response of Pseudomonas aeruginosa to iron: genetics, biochemistry and virulence

During the past decade significant progress has been made towards identifying some of the schemes that Pseudomonas aeruginosa uses to obtain iron and towards cataloguing and characterizing many of the genes and gene products that are likely to play a role in these processes. This review will largely recount what we have learned in the past few years about how P. aeruginosa regulates its acquisition, intake and, to some extent, trafficking of iron, and the role of iron acquisition systems in the virulence of this remarkable opportunistic pathogen. More specifically, the genetics, biochemistry and biology of an essential regulator (Ferric uptake regulator - Fur) and a Fur-regulated alternative sigma factor (PvdS), which are central to these processes, will be discussed. These regulatory proteins directly or indirectly regulate a substantial number of other genes encoding proteins with remarkably diverse functions. These genes include: (i) other regulatory genes, (ii) genes involved in basic metabolic processes (e.g. Krebs cycle), (iii) genes required to survive oxidative stress (e.g. superoxide dismutase), (iv) genes necessary for scavenging iron (e.g. siderophores and their cognate receptors) or genes that contribute to the virulence (e.g. exotoxin A) of this opportunistic pathogen. Despite this recent expansion of knowledge about the response of P. aeruginosa to iron, many significant biological issues surrounding iron acquisition still need to be addressed. Virtually nothing is known about which of the distinct iron acquisition mechanisms P. aeruginosa brings to bear on these questions outside the laboratory, whether it be in soil, in a pipeline, on plants or in the lungs of cystic fibrosis patients.

Characterization of a ferric uptake regulator (fur) gene from Xanthomonas campestris pv. phaseoli with unusual primary structure, genome organization, and expression patterns

A 1.5kb DNA fragment from Xanthomonas campestris pv. phaseoli containing fur was characterized. fur is a single copy gene that is transcribed as a monocistronic mRNA. The predicted amino acid sequence of Xp Fur showed extensive identity to other Fur proteins. However, Xp Fur has many distinct features, particularly a lack of cysteine residues in the conserved metal-binding motifs and unusual modifications in the carboxy-terminus region. The nucleotide sequences of fur genes from four other Xanthomonas spp. were determined. Deduced amino acid sequences all showed the distinct features of Xp Fur. Functionally, Xp Fur partially repressed a Fur-regulated promoter in E. coli. Expression analysis of fur showed increased fur mRNA levels in response to a low iron growth condition. The fur transcription start site was identified by primer extension.

Transcriptional regulation by iron of genes encoding iron- and manganese-superoxide dismutases from Pseudomonas putida

Genes from Pseudomonas putida (Pp), sodA, encoding manganese-superoxide dismutase (MnSOD) and, sodB, iron-superoxide dismutase (FeSOD) were cloned by hybridization with digoxigenin (dig)-labeled PCR products generated from Pp genomic DNA. The sodB gene had a 594 bp open reading frame (ORF), corresponding to 198 amino acids (aa), and a transcript of 880 bases. The sodA gene contained a 609 bp ORF encoding 203 aa and was transcribed as part of a polycistronic operon, consisting of orfY-fumC-orfX-sodA. Pp sodA or sodB genes both restored aerobic growth, growth on paraquat, and growth on minimal medium to an Escherichia coli (Ec) mutant deficient in SOD activity. Paraquat treatment did not enhance mRNA transcription of the sod genes or increase SOD activity in Pp. The Pp sodB gene was highly expressed throughout logarithmic-(log) growth phase and stationary-phase cells grown in medium supplemented with FeCl3, but was down-regulated in iron-deficient conditions, such as in stationary-phase or generated by 2,2'-dipyridyl (DP) treatment. This is the first evidence that iron regulates expression of the sodB gene at the transcriptional level. In contrast, iron-deficient conditions, or addition of MnCl2 to the growth medium, induced transcripts (2.4 kb and 1.2 kb) from the sodA operon. Our results reveal an intricate role of iron in the transcriptional regulation of both Pp sodA and sodB genes.

Bacterioferritin A modulates catalase A (KatA) activity and resistance to hydrogen peroxide in Pseudomonas aeruginosa

We have cloned a 3.6-kb genomic DNA fragment from Pseudomonas aeruginosa harboring the rpoA, rplQ, katA, and bfrA genes. These loci are predicted to encode, respectively, (i) the alpha subunit of RNA polymerase; (ii) the L17 ribosomal protein; (iii) the major catalase, KatA; and (iv) one of two iron storage proteins called bacterioferritin A (BfrA; cytochrome b1 or b557). Our goal was to determine the contributions of KatA and BfrA to the resistance of P. aeruginosa to hydrogen peroxide (H2O2). When provided on a multicopy plasmid, the P. aeruginosa katA gene complemented a catalase-deficient strain of Escherichia coli. The katA gene was found to contain two translational start codons encoding a heteromultimer of approximately 160 to 170 kDa and having an apparent Km for H2O2 of 44.7 mM. Isogenic katA and bfrA mutants were hypersusceptible to H2O2, while a katA bfrA double mutant demonstrated the greatest sensitivity. The katA and katA bfrA mutants possessed no detectable catalase activity. Interestingly, a bfrA mutant expressed only approximately 47% the KatA activity of wild-type organisms, despite possessing wild-type katA transcription and translation. Plasmids harboring bfrA genes encoding BfrA altered at critical amino acids essential for ferroxidase activity could not restore wild-type catalase activity in the bfrA mutant. RNase protection assays revealed that katA and bfrA are on different transcripts, the levels of which are increased by both iron and H2O2. Mass spectrometry analysis of whole cells revealed no significant difference in total cellular iron levels in the bfrA, katA, and katA bfrA mutants relative to wild-type bacteria. Our results suggest that P. aeruginosa BfrA may be required as one source of iron for the heme prosthetic group of KatA and thus for protection against H2O2.

Iron-responsive gene regulation in a campylobacter jejuni fur mutant

The expression of iron-regulated systems in gram-negative bacteria is generally controlled by the Fur protein, which represses the transcription of iron-regulated promoters by using Fe2+ as a cofactor. Mutational analysis of the Campylobacter jejuni fur gene was carried out by generation of a set of mutant copies of fur which had a kanamycin or chloramphenicol resistance gene introduced into the regions encoding the N and C termini of the Fur protein. The mutated genes were recombined into the C. jejuni NCTC 11168 chromosome, and putative mutants were confirmed by Southern hybridization. C. jejuni mutants were obtained only when the resistance genes were transcribed in the same orientation as the fur gene. The C. jejuni fur mutant grew slower than the parental strain. Comparison of protein profiles of fractionated C. jejuni cells grown in low- or high-iron medium indicated derepressed expression of three iron-regulated outer membrane proteins with molecular masses of 70, 75, and 80 kDa. Characterization by N-terminal amino acid sequencing showed the 75-kDa protein to be identical to CfrA, a Campylobacter coli siderophore receptor homologue, whereas the 70-kDa protein was identified as a new siderophore receptor homologue. Periplasmic fractions contained four derepressed proteins with molecular masses of 19, 29, 32, and 36 kDa. The 19-kDa protein has been previously identified, but its function is unknown. The cytoplasmic fraction contained two iron-repressed and two iron-induced proteins with molecular masses of 26, 55, 31, and 40 kDa, respectively. The two iron-repressed proteins have been previously identified as the oxidative stress defense proteins catalase (KatA) and alkyl hydroperoxide reductase (AhpC). AhpC and KatA were still iron regulated in the fur mutant, suggesting the presence of Fur-independent iron regulation. Further analysis of the C. jejuni iron and Fur regulons by using two-dimensional gel electrophoresis demonstrated the total number of iron- and Fur-regulated proteins to be lower than for other bacterial pathogens.

Influence of the TonB energy-coupling protein on efflux-mediated multidrug resistance in Pseudomonas aeruginosa

TonB couples the energized state of the cytoplasmic membrane to the operation of outer membrane receptors responsible for Fe(III) siderophore uptake across the outer membrane of gram-negative bacteria. A tonB mutant of Pseudomonas aeruginosa deficient in iron siderophore uptake was shown in the present study to be hypersusceptible to a wide variety of antibiotics, reminiscent of the phenotype of mutants defective in the mexAB-oprM antibiotic efflux operon. This was not related to influences of a tonB mutation on the iron status of the cell, and indeed, intrinsic antibiotic susceptibility and mexAB-oprM expression were unaffected by iron levels in the growth medium. The presence of tonB on a multicopy plasmid increased the level of resistance of a MexAB-OprM+ strain but not that of a MexAB-OprM- strain to a variety of antimicrobial agents. mexAB-oprM expression was not, however, altered in a tonB deletion mutant, indicating that any influence of TonB on MexAB-OprM-mediated multidrug resistance was at the level of pump activity. Consistent with this, drug accumulation assays revealed that the tonB deletion mutant exhibited decreased levels of drug efflux. Still, the multidrug resistance of a nalB strain was not wholly abrogated by a tonB mutation, indicating that it is likely not an essential component of the efflux apparatus. Similarly, elimination of tonB from an nfxB strain only partially compromised MexCD-OprJ-mediated multidrug resistance. Intriguingly, the drug susceptibility of a mexAB-oprM deletion strain was increased following deletion of tonB, suggesting that TonB may also influence antibiotic resistance mediated by determinants other than MexAB-OprM (and MexCD-OprJ). Thus, TonB plays an important role in both intrinsic and acquired antibiotic resistance in P. aeruginosa.

The catecholate siderophores of Azotobacter vinelandii: their affinity for iron and role in oxygen stress management

In iron-limited medium, Azotobacter vinelandii strain UW produces three catecholate siderophores: the tricatecholate protochelin, the dicatecholate azotochelin and the monocatecholate aminochelin. Each siderophore was found to bind Fe3+ preferentially to Fe2+, in a ligand:Fe ratio of 1:1, 3:2 and 3:1, respectively. Protochelin had the highest affinity for Fe3+, with a calculated proton-independent solubility coefficient of 10439, comparable to ferrioxamine B. Iron-limited wild-type strain UW grown under N2-fixing or nitrogen-sufficient conditions hyper-produced catecholate siderophores in response to oxidative stress caused by high aeration. In addition, superoxide dismutase activity was greatly diminished in iron-limited cells, whereas catalase activity was maintained. The ferredoxin I (Fdl)-negative A. vinelandii strain LM100 also hyper-produced catecholates, especially protochelin, under oxidative stress conditions, but had decreased activities of both superoxide dismutase and catalase, and was about 10 times more sensitive to paraquat than strain UW. Protochelin and azotochelin held Fe3+ firmly enough to prevent its reduction by.O- 2 and did not promote the generation of hydroxyl radical by the Fenton reaction. Ferric-aminochelin was unable to resist reduction by O- 2 and was a Fenton catalyst. These data suggest that under iron-limited conditions, A. vinelandii suffers oxidative stress caused by.O- 2. The catecholate siderophores azotochelin, and especially protochelin, are hyper-produced to offer chemical protection from oxidative damage catalysed by.O- 2 and Fe3+. The results are also consistent with Fdl being required for oxidative stress management in A. vinelandii.

Bacillus subtilis contains multiple Fur homologues: identification of the iron uptake (Fur) and peroxide regulon (PerR) repressors

Fur (ferric uptake regulator) proteins control iron uptake in many Gram-negative bacteria. Although Fur homologues have been identified in Gram-positive bacteria, their roles in gene regulation are unknown. Genome sequencing has revealed three fur homologues in Bacillus subtilis: yqkL, yqfV and ygaG. We demonstrate that yqkL encodes an iron uptake repressor: both siderophore biosynthesis and transcription of ferri-siderophore uptake genes is constitutive in the yqkL mutant. Thus, yqkL encodes a repressor that is functionally as well as structurally related to Fur. B. subtilis peroxide stress genes are induced by either H2O2 or by metal ion limitation. Previous genetic studies defined a regulatory locus, perR, postulated to encode the peroxide regulon repressor. We demonstrate that a ygaG mutant has the perR phenotype: It is highly resistant to peroxides and overexpresses catalase, alkyl hydroperoxide reductase and the DNA binding protein MrgA. Nine spontaneous perR mutations, isolated by virtue of their ability to derepress mrgA transcription in the presence of managanous ion, all contain sequence changes in the ygaG locus and can be complemented by the cloned ygaG gene. Thus, ygaG encodes the peroxide regulon repressor and is allelic with perR.

Cloning and characterization of the Pseudomonas aeruginosa zwf gene encoding glucose-6-phosphate dehydrogenase, an enzyme important in resistance to methyl viologen (paraquat)

In this study, we cloned the Pseudomonas aeruginosa zwf gene, encoding glucose-6-phosphate dehydrogenase (G6PDH), an enzyme that catalyzes the NAD+- or NADP+-dependent conversion of glucose-6-phosphate to 6-phosphogluconate. The predicted zwf gene product is 490 residues, which could form a tetramer with a molecular mass of approximately 220 kDa. G6PDH activity and zwf transcription were maximal in early logarithmic phase when inducing substrates such as glycerol, glucose, or gluconate were abundant. In contrast, both G6PDH activity and zwf transcription plummeted dramatically when bacteria approached stationary phase, when inducing substrate was limiting, or when the organisms were grown in a citrate-, succinate-, or acetate-containing basal salts medium. G6PDH was purified to homogeneity, and its molecular mass was estimated to be approximately 220 kDa by size exclusion chromatography. Estimated Km values of purified G6PDH acting on glucose-6-phosphate, NADP+, and NAD+ were 530, 57, and 333 microM, respectively. The specific activities with NAD+ and NADP+ were calculated to be 176 and 69 micromol/min/mg. An isogenic zwf mutant was unable to grow on minimal medium supplemented with mannitol. The mutant also demonstrated increased sensitivity to the redox-active superoxide-generating agent methyl viologen (paraquat). Since one by-product of G6PDH activity is NADPH, the latter data suggest that this cofactor is essential for the activity of enzymes critical in defense against paraquat toxicity.

A rubrerythrin operon and nigerythrin gene in Desulfovibrio vulgaris (Hildenborough)

Rubrerythrin is a nonheme iron protein of unknown function isolated from Desulfovibrio vulgaris (Hildenborough). We have sequenced a 3.3-kbp Sal1 fragment of D. vulgaris chromosomal DNA containing the rubrerythrin gene, rbr, identified additional open reading frames (ORFs) adjacent to rbr, and shown that these ORFs are part of a transcriptional unit containing rbr. One ORF, designated fur, lies just upstream of rbr and encodes a 128-amino-acid-residue protein which shows homology to Fur (ferric uptake regulatory) proteins from other purple bacteria. The other ORF, designated rdl, lies just downstream of rbr and encodes a 74-residue protein with significant sequence homology to rubredoxins but with a different number and spacing of cysteine residues. Overexpression of rdl in Escherichia coli yielded a protein, Rdl, which has spectroscopic properties and iron content consistent with one Fe3+(SCys)4 site per polypeptide but is clearly distinct from both rubrerythrin and a related protein, nigerythrin. Northern analysis indicated that fur, rbr, and rdl were each present on a transcript of 1.3 kb; i.e., these three genes are cotranscribed. Because D. vulgaris nigerythrin appears to be closely related to rubrerythrin, and its function is also unknown, we cloned and sequenced the gene encoding nigerythrin, ngr. The amino acid sequence of nigerythrin is 33% identical to that of rubrerythrin, and all residues which furnish iron ligands to both the FeS4 and diiron-oxo sites in rubrerythrin are conserved in nigerythrin. Despite the close resemblance of these two proteins, ngr was found to be no closer than 7 kb to rbr on the D. vulgaris chromosome, and Northern analysis showed that, in contrast to rbr, ngr is not cotranscribed with other genes. Possible redox-linked functions for rubrerythrin and nigerythrin in iron homeostasis are proposed.

An operon containing fumC and sodA encoding fumarase C and manganese superoxide dismutase is controlled by the ferric uptake regulator in Pseudomonas aeruginosa: fur mutants produce elevated alginate levels

The activities of fumarase- and manganese-cofactored superoxide dismutase (SOD), encoded by the fumC and sodA genes in Pseudomonas aeruginosa, are elevated in mucoid, alginate-producing bacteria and in response to iron deprivation (D. J. Hassett, M. L. Howell, P. A. Sokol, M. L. Vasil, and G. E. Dean, J. Bacteriol. 179:1442-1451, 1997). In this study, a 393-bp open reading frame, fagA (Fur-associated gene), was identified immediately upstream of fumC, in an operon with orfX and sodA. Two iron boxes or Fur (ferric uptake regulatory protein) binding sites were discovered just upstream of fagA. Purified P. aeruginosa Fur caused a gel mobility shift of a PCR product containing these iron box regions. DNA footprinting analysis revealed a 37-bp region that included the Fur binding sites and was protected by Fur. Primer extension analysis and RNase protection assays revealed that the operon is composed of at least three major iron-regulated transcripts. Four mucoid fur mutants produced 1.7- to 2.6-fold-greater fumarase activity and 1.7- to 2.3-greater amounts of alginate than wild-type organisms. A strain devoid of the alternative sigma factor AlgT(U) produced elevated levels of one major transcript and fumarase C and manganase-cofactored SOD activity, suggesting that AlgT(U) may either play a role in regulating this transcript or function in some facet of iron metabolism. These data suggest that the P. aeruginosa fagA, fumC, orfX, and sodA genes reside together on a small operon that is regulated by Fur and is transcribed in response to iron limitation in mucoid, alginate-producing bacteria.

Fumarase C activity is elevated in response to iron deprivation and in mucoid, alginate-producing Pseudomonas aeruginosa: cloning and characterization of fumC and purification of native fumC

We report the discovery of fumC, encoding a fumarase, upstream of the sodA gene, encoding manganese superoxide dismutase, in Pseudomonas aeruginosa. The fumC open reading frame, which terminates 485 bp upstream of sodA, contains 1,374 bp that encode 458 amino acids. A second 444-bp open reading frame located between fumC and sodA, called orfX, showed no homology with any genes or proteins in database searches. A fumarase activity stain revealed that P. aeruginosa possesses at least two and possibly three fumarases. Total fumarase activity was at least approximately 1.6-fold greater in mucoid, alginate-producing bacteria than in nonmucoid bacteria and decreased 84 to 95% during the first 5 h of aerobic growth, followed by a rapid rise to maximum activity in stationary phase. Bacteria exposed to the iron chelator 2,2'-dipyridyl, but not ferric chloride, demonstrated an increase in fumarase activity. Mucoid bacteria produced approximately twofold-higher levels of the siderophores pyoverdin and pyochelin than nonmucoid bacteria. Northern blot analysis revealed a transcript that included fumC, orfX, and sodA, the amount of which was increased in response to iron deprivation. A P. aeruginosa fumC mutant produced only approximately 40% the alginate of wild-type bacteria. Interestingly, a sodA mutant possessed an alginate-stable phenotype, a trait that is typically unstable in vitro. These data suggest that mucoid bacteria either are in an iron-starved state relative to nonmucoid bacteria or simply require more iron for the process of alginate biosynthesis. In addition, the iron-regulated, tricarboxylic acid cycle enzyme fumarase C is essential for optimal alginate production by P. aeruginosa.