[PubMed] [CrossRef] [Google Scholar] 9. the bactericidal activity of normal human serum, but only TonB3 appears to be essential for virulence in insect and mouse models of infection. Our findings highlight a central role of the TonB3 system for pathogenicity. Hence, TonB3 represents a promising target for novel antibacterial therapies and for the generation of attenuated vaccine strains. has emerged as one of the most dreaded opportunistic pathogens in hospitals, being responsible for local and systemic Sitravatinib infections, especially in immunocompromised and severely ill patients (1). While the genetic and functional basis of multidrug resistance in clinical isolates is matter of intensive research, the LAMP3 mechanisms of pathogenicity are still poorly understood. Iron (Fe) is an essential nutrient for all living organisms, since it is required as a cofactor for several enzymes, such as those implicated in electron transport and in amino acid and DNA biosynthesis (2, 3). In aerobic environments, iron exists in the oxidized ferric form [Fe(III)], which aggregates in insoluble oxy-hydroxy polymers. Conversely, in anaerobic and/or reducing environments, the prevalent iron species is the more soluble ferrous form [Fe(II)]. It has been postulated that the ability to acquire iron from the environment contributes to pathobiology and virulence (4,C6). Upon entry into the human host, is faced with the low level of free iron imposed by the hypoferremic response and by the presence of high-affinity iron-binding proteins (e.g., transferrin and lactoferrin) (7). To counteract iron starvation, has developed several iron acquisition strategies, such as the production of different siderophores which are variably present in different strains and likely account for Fe(III) scavenging from different sources (8). Production of siderophores is stimulated under iron-limiting conditions and repressed when sufficient iron is present. The Fur (ferric uptake regulator) repressor protein acts as the master regulator of iron homeostasis; in bacteria containing sufficient iron levels, the Fur-Fe(II) complex blocks transcription arising from Fur-controlled promoters, which conversely are transcribed during iron starvation due to detachment of apo-Fur from iron-repressible promoters (9). In Gram-negative bacteria, Feo is the main system for Fe(II) uptake (10), and it consists of three proteins encoded by the operon: FeoA, a small cytosolic protein with still-unknown functions; FeoB, a large protein involved in active translocation of Fe(II) across the cytoplasmic membrane with a cytosolic N-terminal G-protein domain and a C-terminal integral inner membrane domain; and FeoC, a small cytosolic protein likely acting as transcriptional repressor (11). Bacterial systems involved in Fe(III) acquisition (via either siderophores or heme) require the TonB energy transducing machinery, consisting of the TonB-ExbB-ExbD protein complex (12). This complex transduces the proton Sitravatinib motive force (PMF) of the cytoplasmic membrane into energy required for high-affinity active transport of Fe(III)-loaded carriers across outer membrane transporter proteins into the periplasmic space (13). Structurally, TonB consists Sitravatinib of a short hydrophobic N-terminal transmembrane domain associated with ExbB and ExbD proteins, a proline-rich linker domain and a C-terminal domain interacting with a variety of the outer membrane transporters (12, 14). Up to 21 putative TonB-dependent outer membrane transporter genes have been identified or predicted in genomes, most often associated with putative or confirmed ferri-siderophore and heme uptake genes (8). TonB-dependent transporter proteins are all characterized by a short conserved signature at the N terminus called TonB box. Once TonB proficiently interacts with the TonB box of an outer membrane transporter, translocation of the transporter-bound ligand into the Sitravatinib periplasmic space occurs (14,C16). Although the TonB and Feo systems have extensively been studied in prototypic Gram-negative bacteria, including and (16,C21), knowledge about these systems in is still limited. Three genes coding.