Cryo-EM reveals unique structural features of the FhuCDB Escherichia coli ferrichrome importer

New insights into iron uptake in Streptococcus mutans: evidence for a role of siderophore-like molecules

Streptococcus mutans, a gram-positive coccus commonly found in the human oral cavity, is the primary causative agent of dental caries as well as infective endocarditis. Bacteria produce potent iron chelators called siderophores to absorb iron. Because, there are few studies on siderophore-mediated iron transport in S. mutans, the current study investigates the presence of such a mechanism in S. mutans GS-5. Deferration of culture medium and different concentrations of 2, 2'-Bipyridyl has been used to simulate iron-restricted conditions. Iron restriction alters the colony morphology and slows bacterial growth. Cross-feeding conditioned medium into an iron-restricted medium promotes bacterial growth, indicating the presence of siderophore-like molecules. This was further confirmed by Chrome Azurol S (CAS) assay and Modified CAS-agar assay. Cśaky's and Arnow's assays detected the presence of hydroxamate and catecholate-type molecules in optimal and iron-restricted conditions, respectively. Further, the siderophore-like molecules were extracted and purified with thin layer chromatography (TLC). TLC elutes were also found to be positive for iron-chelation in CAS-agar assay and aided growth of S. mutans under iron-restricted conditions. LC-MS analysis of culture supernatants under iron-restricted conditions identified iron-binding small molecules, including a catechol structural motif. Computational analysis utilizing KEGG and BLASTp suggested homologues of siderophore biosynthesis and transport proteins, including genes associated with mutanobactin production. These findings indicate a possible siderophore-mediated iron uptake mechanism in S. mutans GS-5, warranting further molecular studies and advanced spectroscopic characterization of this unidentified siderophore. Once confirmed, this mechanism can be used as a potential drug target to control streptococcal infection.

Successful strategies for expression and purification of ABC transporters

ATP-binding cassette (ABC) transporters are proteins responsible for active transport of various compounds, from small ions to macromolecules, across membranes. Proteins from this superfamily also pump drugs out of the cell resulting in multidrug resistance. Based on the cellular functions of ABC-transporters they are commonly associated with diseases like cancer and cystic fibrosis. To understand the molecular mechanism of this critical family of integral membrane proteins, structural characterization is a powerful tool which in turn requires successful recombinant production of stable and functional protein in good yields. In this review we have used high resolution structures of ABC transporters as a measure of successful protein production and summarized strategies for prokaryotic and eukaryotic proteins, respectively. In general, Escherichia coli is the most frequently used host for production of prokaryotic ABC transporters while human embryonic kidney 293 (HEK293) cells are the preferred host system for eukaryotic proteins. Independent of origin, at least two-steps of purification were required after solubilization in the most used detergent DDM. The purification tag was frequently cleaved off before structural characterization using cryogenic electron microscopy, or crystallization and X-ray analysis for prokaryotic proteins.

Location, Location, Location: Establishing Design Principles for New Antibacterials from Ferric Siderophore Transport Systems

This review strives to assemble a set of molecular design principles that enables the delivery of antibiotic warheads to Gram-negative bacterial targets (ESKAPE pathogens) using iron-chelating siderophores, known as the Trojan Horse strategy for antibiotic development. Principles are derived along two main lines. First, archetypical siderophores and their conjugates are used as case studies for native iron transport. They enable the consideration of the correspondence of iron transport and antibacterial target location. The second line of study charts the rationale behind the clinical antibiotic cefiderocol. It illustrates the potential versatility for the design of new Trojan Horse-based antibiotics. Themes such as matching the warhead to a location where the siderophore delivers its cargo (i.e., periplasm vs. cytoplasm), whether or not a cleavable linker is required, and the relevance of cheaters to the effectiveness and selectivity of new conjugates will be explored. The effort to articulate rules has identified gaps in the current understanding of iron transport pathways and suggests directions for new investigations.

Actinobacterial chalkophores: the biosynthesis of hazimycins

Copper is a transition metal element with significant effects on the morphological development and secondary metabolism of actinobacteria. In some microorganisms, copper-binding natural products are employed to modulate copper homeostasis, although their significance in actinobacteria remains largely unknown. Here, we identified the biosynthetic genes of the diisocyanide natural product hazimycin in Kitasatospora purpeofusca HV058, through gene knock-out and heterologous expression. Biochemical analyses revealed that hazimycin A specifically binds to copper, which diminishes its antimicrobial activity. The presence of a set of putative importer/exporter genes surrounding the biosynthetic genes suggested that hazimycin is a chalkophore that modulates the intracellular copper level. A bioinformatic survey of homologous gene cassettes, as well as the identification of two previously unknown hazimycin-producing Streptomyces strains, indicated that the isocyanide-based mechanism of copper homeostasis is prevalent in actinobacteria.