Cloning and sequencing of the Vibrio parahaemolyticus fur gene

Strategies of Vibrio parahaemolyticus to acquire nutritional iron during host colonization

Iron is an essential element for the growth and development of virtually all living organisms. As iron acquisition is critical for the pathogenesis, a host defense strategy during infection is to sequester iron to restrict the growth of invading pathogens. To counteract this strategy, bacteria such as Vibrio parahaemolyticus have adapted to such an environment by developing mechanisms to obtain iron from human hosts. This review focuses on the multiple strategies employed by V. parahaemolyticus to obtain nutritional iron from host sources. In these strategies are included the use of siderophores and xenosiderophores, proteases and iron-protein receptor. The host sources used by V. parahaemolyticus are the iron-containing proteins transferrin, hemoglobin, and hemin. The implications of iron acquisition systems in the virulence of V. parahaemolyticus are also discussed.

Identification of an iron acquisition machinery in Flavobacterium columnare

Flavobacterium columnare, a fastidious Gram-negative pathogen and the causative agent of columnaris disease, is one of the most harmful pathogens in the freshwater fish-farming industry. Nevertheless the virulence mechanisms of F. columnare are not well understood. Bacterial iron uptake from the host during infection is an important mechanism of virulence. Here we identified and analyzed part of the iron uptake machinery of F. columnare. Under iron-limited conditions during in vitro growth, synthesis of an outer membrane protein of ~86 kDa was upregulated. This protein was identified as a TonB-dependent ferrichrome-iron receptor precursor (FhuA). Synthesis of siderophores in F. columnare was corroborated by chrome azurol S assays. A putative ferric uptake regulator (Fur) protein was also identified in the F. columnare genome. Structural analysis of the F. columnare Fur protein revealed that it was similar to Fur proteins involved in iron uptake regulation of other bacteria. Furthermore, Salmonella enterica serovar Typhimurium (S. Typhimurium) Δfur mutants were partially complemented by the F. columnare fur gene. We conclude that a siderophore-mediated iron uptake system exists in F. columnare, and fur from F. columnare could partially complement S. Typhimurium Δfur mutant.

Identification and characterization of genes required for biosynthesis and transport of the siderophore vibrioferrin in Vibrio parahaemolyticus

In response to low iron availability, Vibrio parahaemolyticus synthesizes and secretes a polyhydroxycarboxylate-type siderophore vibrioferrin which is composed of 1 mol each of 2-ketoglutaric acid, L-alanine, ethanolamine, and citric acid. We have previously reported the cloning and characterization of the pvuA gene, which encodes the 78-kDa outer membrane receptor protein for ferric vibrioferrin. In this study, nine genes involved in the biosynthesis and transport of vibrioferrin have been identified in the genomic regions surrounding the pvuA gene. The genes were sequenced, and gene disruptants were constructed by insertion mutation for phenotype analysis. Five of the genes, named pvsABCDE, constitute an operon that is expressed under iron-limiting conditions. Homology searches of their predicted protein products suggested that the four genes pvsABDE are implicated in the biosynthesis of the siderophore. Another gene in the same operon, pvsC, encodes a putative exporter that is homologous to members of the major facilitator superfamily of multidrug efflux pumps. The remaining four genes, named pvuBCDE, encode proteins strongly homologous to Escherichia coli FecBCDE, respectively, which are components of the ATP-binding cassette transporter system for ferric dicitrate. Reverse transcriptase PCR analysis revealed that these transport genes are transcribed as a single mRNA with the upstream genes, psuA and pvuA. Phenotypic comparison between the wild-type strain and its targeted gene disruptants supported the biological functions for the respective operons that were expected on the basis of the homology search.

Cloning, characterisation and phylogenetic analysis of the fur gene in Vibrio salmonicida and Vibrio logei

The gene encoding the ferric uptake regulator protein (fur gene) of Vibrio salmonicida 87/09/1193 was located following hybridisation of an EcoRI digest of chromosomal V. salmonicida DNA with a 316 base pairs (bp) probe internal to the fur gene of Vibrio anguillarum. A 2088 bp fragment including an open reading frame of 441 bp, encoding a protein of 147 amino acids, and homologous with fur, was identified, cloned and sequenced. A plasmid bound V. salmonicida fur gene was found capable of complementing the fur mutation of Escherichia coli H1681. Although no 'iron-box' was identified upstream of the start-codon, beta-galactosidase activity in E. coli H1681 was regulated by iron availability in the media, indicating that in V. salmonicida fur, as in other fur genes, iron functions as a co-repressor. Southern blot hybridizations demonstrated that fur is conserved amongst V. salmonicida strains and several other closely related Vibrio strains in which fur remains as yet, uncharacterized. The fur gene of Vibrio logei NCIMB 2252 was subsequently amplified using polymerase chain reaction primers external to the V. salmonicida fur gene. Comparison of phylogenetic analyses using fur and 16S DNA coding for rRNA sequences, confirmed the usefulness of fur as an evolutionary marker within the genus Vibrio.

Identification and characterization of pvuA, a gene encoding the ferric vibrioferrin receptor protein in Vibrio parahaemolyticus

We previously reported that Vibrio parahaemolyticus expresses two outer membrane proteins of 78 and 83 kDa concomitant with production of siderophore vibrioferrin in response to iron starvation stress and that these proteins are the ferric vibrioferrin receptor and heme receptor, respectively (S. Yamamoto, T. Akiyama, N. Okujo, S. Matsuura, and S. Shinoda, Microbiol. Immunol. 39:759-766, 1995; S. Yamamoto, Y. Hara, K. Tomochika, and S. Shinoda, FEMS Microbiol. Lett. 128:195-200, 1995). In this study, the Fur titration assay (FURTA) system was applied to isolate DNA fragments containing a potential Fur box from a genomic DNA library of V. parahaemolyticus WP1. Sequencing a 3.2-kb DNA insert in one FURTA-positive clone revealed that an amino acid sequence deduced from a partial gene, which was preceded by a full-length gene (psuA) encoding a receptor for a siderophore of unknown origin, was consistent with the N-terminal amino acid sequence of the 78-kDa ferric vibrioferrin receptor. Then, the full-length gene (pvuA) encoding the ferric vibrioferrin receptor was cloned and characterized. The deduced protein encoded by pvuA displayed the highest similarity (31% identity; 48% similarity) to RumA, a ferric rhizoferrin receptor of Morganella morganii. Primer extension and Northern blot analyses indicated that psuA and pvuA constitute an operon which is transcribed from a Fur-repressed promoter upstream of psuA. The product of the pvuA gene and its function were confirmed by generating a pvuA-disrupted mutant, coupled with genetic complementation studies. A mutant with disruption in the upstream psuA gene also displayed a phenotype impaired in the utilization of ferric vibrioferrin.

A serine protease-encoding gene (aprII) of Alteromonas sp. Strain O-7 is regulated by the iron uptake regulator (Fur) protein

The ferric uptake regulator (Fur) box-like sequence was located upstream of the serine protease-encoding gene (aprII) from a marine bacterium, Alteromonas sp. strain O-7. To clarify whether the production of AprII (the gene product of aprII) is regulated by the environmental iron concentrations, this strain was cultured under iron-depleted or iron-rich conditions and the level of AprII in the culture supernatant was analyzed by Western blotting. The production of AprII was significantly repressed under iron-rich conditions. Northern hybridization analysis demonstrated that AprII biosynthesis was regulated by iron through the control of transcription. These results indicate that aprII is a new member of the iron regulon and plays an important role in the iron acquisition system of the strain. Furthermore, the gene encoding Fur was cloned and sequenced. The deduced amino acid sequence of the cloned Fur showed high sequence similarity with that from gram-negative bacteria.

Characterization of Vibrio parahaemolyticus manganese-resistant mutants in reference to the function of the ferric uptake regulatory protein

In many bacteria, the ferric uptake regulatory protein (Fur) has a central role in the negative regulation of genes affected by iron limitation. In this study, Vibrio parahaemolyticus strains carrying mutations in the fur gene encoding Fur were isolated by the manganese selection method to assess the function of Fur in connection with alternations in the coordinate expression of the siderophore vibrioferrin (VF) and iron-repressible outer membrane proteins (IROMPs). Ten out of 25 manganese-resistant mutants constitutively produced VF and expressed at least two IROMPs irrespective of the iron concentration in the medium. PCR-direct DNA sequencing of the fur genes in these mutants identified four different point mutations causing amino acid changes. Moreover, a fur overexpressing plasmid was constructed to prepare antiserum against V. parahaemolyticus Fur. Western blotting with this antiserum revealed that the intracellular abundance of the wild-type Fur was not significantly affected by the iron concentrations in the growth medium, and that the Fur proteins of the mutant strains occurred at substantially smaller amounts and/or migrated more rapidly in sodium dodecyl sulfate-polyacrylamide gel electrophoresis than the wild-type Fur. These data afford an additional insight into the structure-function relationship of Fur and imply its involvement in the iron acquisition systems of V. parahaemolyticus, although it is yet unknown whether its action on the target genes is direct or indirect.

Iron and oxidative stress in bacteria

The appearance of oxygen on earth led to two major problems: the production of potentially deleterious reactive oxygen species and a drastic decrease in iron availability. In addition, iron, in its reduced form, potentiates oxygen toxicity by converting, via the Fenton reaction, the less reactive hydrogen peroxide to the more reactive oxygen species, hydroxyl radical and ferryl iron. Conversely superoxide, by releasing iron from iron-containing molecules, favors the Fenton reaction. It has been assumed that the strict regulation of iron assimilation prevents an excess of free intracellular iron that could lead to oxidative stress. Studies in bacteria supporting that view are reviewed. While genetic studies correlate oxidative stress with increase of intracellular free iron, there are only few and sometimes contradictory studies on direct measurements of free intracellular metal. Despite this weakness, the strict regulation of iron metabolism, and its coupling with regulation of defenses against oxidative stress, as well as the role played by iron in regulatory protein in sensing redox change, appear as essential factors for life in the presence of oxygen.