BackgroundBacteria produce small molecule iron chelators, known as siderophores, to facilitate the acquisition of iron from the environment. The synthesis of more than one siderophore and the production of multiple siderophore uptake systems by a single bacterial species are common place. The selective advantages conferred by the multiplicity of siderophore synthesis remains poorly understood. However, there is growing evidence suggesting that siderophores may have other physiological roles besides their involvement in iron acquisition.Methods and Principal FindingsHere we provide the first report that pyochelin displays antibiotic activity against some bacterial strains. Observation of differential sensitivity to pyochelin against a panel of bacteria provided the first indications that catecholate siderophores, produced by some bacteria, may have roles other than iron acquisition. A pattern emerged where only those strains able to make catecholate-type siderophores were resistant to pyochelin. We were able to associate pyochelin resistance to catecholate production by showing that pyochelin-resistant Escherichia coli became sensitive when biosynthesis of its catecholate siderophore enterobactin was impaired. As expected, supplementation with enterobactin conferred pyochelin resistance to the entE mutant. We observed that pyochelin-induced growth inhibition was independent of iron availability and was prevented by addition of the reducing agent ascorbic acid or by anaerobic incubation. Addition of pyochelin to E. coli increased the levels of reactive oxygen species (ROS) while addition of ascorbic acid or enterobactin reduced them. In contrast, addition of the carboxylate-type siderophore, citrate, did not prevent pyochelin-induced ROS increases and their associated toxicity.ConclusionsWe have shown that the catecholate siderophore enterobactin protects E. coli against the toxic effects of pyochelin by reducing ROS. Thus, it appears that catecholate siderophores can behave as protectors of oxidative stress. These results support the idea that siderophores can have physiological roles aside from those in iron acquisition.

Microorganisms produce siderophores to facilitate iron uptake and even though this trait has been extensively studied, there is growing evidence suggesting that siderophores may have other physiological roles aside from iron acquisition. In support of this notion, we previously linked the archetypal siderophore enterobactin with oxidative stress alleviation. To further characterize this association, we studied the sensitivity of Escherichia coli strains lacking different components of the enterobactin system to the classical oxidative stressors hydrogen peroxide and paraquat. We observed that strains impaired in enterobactin production, uptake and hydrolysis were more susceptible to the oxidative damage caused by both compounds than the wild-type strain. In addition, meanwhile iron supplementation had little impact on the sensitivity, the reducing agent ascorbic acid alleviated the oxidative stress and therefore significantly decreased the sensitivity to the stressors. This indicated that the enterobactin-mediated protection is independent of its ability to scavenge iron. Furthermore, enterobactin supplementation conferred resistance to the entE mutant but did not have any protective effect on the fepG and fes mutants. Thus, we inferred that only after enterobactin is hydrolysed by Fes in the cell cytoplasm and iron is released, the free hydroxyl groups are available for radical stabilization. This hypothesis was validated testing the ability of enterobactin to scavenge radicals in vitro. Given the strong connection between enterobactin and oxidative stress, we studied the transcription of the entE gene and the concomitant production of the siderophore in response to such kind of stress. Interestingly, we observed that meanwhile iron represses the expression and production of the siderophore, hydrogen peroxide and paraquat favour these events even if iron is present. Our results support the involvement of enterobactin as part of the oxidative stress response and highlight the existence of a novel regulation mechanism for enterobactin biosynthesis.

Tyl activation of gene expression observed in haploid cell types of Saccharomyces cerevisiae requires the STE7 and STE12 gene products. An activator sequence within Tyl that is responsive to these two regulators has been defined. Complex formation between a factor in whole-cell extracts and the DNA regulatory element showed the same dependence on the STE7 and STE12 gene products as did reporter gene expression. Base pair substitutions within the binding site abolished the ability to form the factor-DNA complex and to activate gene expression. The correlation between complex formation and reporter gene expression indicates that factor binding to the cis-acting element is essential for gene activation. Because the predicted protein for the STE7 gene product is homologous to protein kinases, we suggest that protein phosphorylation may directly or indirectly regulate formation of this DNA-protein complex.Eucaryotic cells differentiate into distinct cell types that express characteristic subsets of genes and are able to perform specialized functions. Regulation at the transcriptional level is one of the primary steps for control of cell-type-specific gene expression. Existing evidence indicates that transcriptional control involves interactions between specific DNA recognition elements, site-specific DNA-binding proteins, and the transcriptional machinery (for reviews, see references 8, 28, and 37). In general, eucaryotic promoters appear to be a mosaic of distinct DNA recognition elements that function in different combinations to achieve specific patterns of gene expression reflecting inducible, positive, or negative controls. Proteins that bind to specific DNA sequences have been identified in many systems, and some have been purified. However, the mechanisms that regulate formation of these DNA-protein complexes and the mechanisms by which the complexes affect gene expression are not understood.Saccharomyces cerevisiae is a model for the study of events controlling cell type specialization. Although this yeast is a unicellular organism, it exhibits three cell types. The a and a haploid cell types are specialized to mate with each other, and the a/a diploid cell type is specialized for meiosis and sporulation (for reviews, see references 22 and 31). The a or a alleles present at the mating type locus (MAT) are the primary determinants of cell type. The a allele encodes two site-specific DNA-binding proteins, al and a2, that regulate the steady-state abundance of mRNA from cell-type-specific genes (2,20,25). The a allele encodes the al regulatory protein. Expression of both al and a2 in the diploid cell type causes repression of haploid specific genes.The specialized function of mating between haploid cell types requires intercellular recognition followed by cell and nuclear fusion to form a diploid cell. Expression of the genes required for these functions depends on several regulatory determinants in addition to those encoded at MAT. Some of the regulators have been identified by sterile mutations that prevent...