| Iron is an essential ion for the growth of most bacteria. However, the concentration of soluble iron in the environment is as low as is not sufficient for bacterial survival.Most mammal hosts have evolved a defensive mechanism against pathogenic bacteria by synthesizing iron-chelating proteins such as lactoferrin and transferrin, which push the free iron concentration to an extremely low level that does not support bacterial growth. To obtain sufficient iron, many pathogen have developed the ability to synthe iron chelators known as siderophores tochelate iron with high efficiency. Siderophores can be divided into three groups:catecholates, hydroxamates, and carboxylates, among which catecholate siderophores are the most powerful iron chelators.Mycobacterium tuberculosis(MTB) is an important human pathogen that causes tuberculosis. On one hand, pathogens have to compete for iron in the host so that they can multiply and establish a successful infection but, on the other hand, they must regulate iron metabolism to prevent excess iron that can initiate a cascade of lethal reactions. There are4potential iron-dependent regulators, furA and furB, encode proteins of the Fur (ferric uptake regulator) family, while IdeR and SirR are members of the DtxR (diphtheria toxin repressor) family. IdeR plays the central role as an iron-dependent regulator. It is the only one of these mycobacterial proteins that has been well characterized as to structure and function. However, many iron-regulated genes in the pathogen escape modulation by IdeR. These are speculated to be under the control of SirR.In our study,we obtained the crystal structure of SirR at2.4A resolution.The structure revealed that there are three manganese binding sites in SirR, two of which are conserved in its homologues. Meanwhile, manganese regulates the dimerization of SirR:the analysis through gel filtration indicates that native SirR is dimer while upon bound with Mn2+, SirR becames monomer.This discovery is novel in metal-dependent transcriptional regulators. Usually, most metal-dependent transcriptional regulators can bind and be activated by Fe2+, Co2+, Ni2+, Mn2+, Cd2+, and Zn2+, upon bound with them, the dimer regulators can interact with DNA. As for SirR,characterist is opposite, we conclude that SirR regulates the genes responsible for the excretion or utilization of manganese.Vibrio cholerae, the causative agent of the severe diarrheal disease cholera, acquires iron through secretion of a catecholate siderphore called vibriobactin. Furthermore, V. cholerae can also use the siderophore enterobactin, which is produced by other microorganisms, such as Escherichia coli。 Periplasmic binding proteins(PBPs) have long been observed as essential elements for the siderophore-mediated iron transport system of gram-negative bacteria and as a necessity for the growth, survival, and pathogenicity of various microorganisms VctP, one of the two indispensable periplasmic siderophore-binding proteins from V. cholerae, plays an important role in the enterobactin and vibriobactin transportation. Our study reports the crystal structure of VctP from V. cholerae N16961at1.7A resolution. Structural comparisons of VctP with its homologues and docking results indicate that enterobactin and vibriobactin share the same binding pocket of VctP.Notably, in the model of ferric-enterobactin bound VctP, a basic triad consisting ofArg137, Arg226and Arg270is used to balance the three negative charges of ferricenterobactin through specific hybrid electrostatic/cation-π interactions,while in the model of ferric-vibriobactin bound VctP, a basic dyad consisting of Arg137and Arg270is used to balance the two negative charges of ferricvibriobactin. |
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