The cation diffusion facilitator protein MamM's cytoplasmic domain exhibits metal-type dependent binding modes and discriminates against Mn2+

Cation diffusion facilitator (CDF) proteins are a conserved family of divalent transition metal cation transporters. CDF proteins are usually composed of two domains: the transmembrane domain, in which the metal cations are transported through, and a regulatory cytoplasmic C-terminal domain (CTD). Each CDF protein transports either one specific metal or multiple metals from the cytoplasm, and it is not known whether the CTD takes an active regulatory role in metal recognition and discrimination during cation transport. Here, the model CDF protein MamM, an iron transporter from magnetotactic bacteria, was used to probe the role of the CTD in metal recognition and selectivity. Using a combination of biophysical and structural approaches, the binding of different metals to MamM CTD was characterized. Results reveal that different metals bind distinctively to MamM CTD in terms of their binding sites, thermodynamics, and binding-dependent conformations, both in crystal form and in solution, which suggests a varying level of functional discrimination between CDF domains. Furthermore, these results provide the first direct evidence that CDF CTDs play a role in metal selectivity. We demonstrate that MamM''s CTD can discriminate against Mn²⁺, supporting its postulated role in preventing magnetite formation poisoning in magnetotactic bacteria via Mn²⁺ incorporation.     

Modes of Biomineralization of Magnetite by Microbes

Biomineralization processes have traditionally been grouped into two distinct modes; biologically induced mineralization (BIM) and biologically controlled mineralization (BCM). In BIM, microbes cause mineral formation by sorbing solutes onto their cell surfaces or extruded organic polymers and/or releasing reactive metabolites which alter the saturation state of the solution proximal to the cell or polymer surface. Such mineral products appear to have no specific recognized functions. On the other hand, in BCM microbes exert a great degree of chemical and genetic control over the nucleation and growth of mineral particles, presumably because the biominerals produced serve some physiological function. Interestingly, there are examples where the same biomineral is produced by both modes in the same sedimentary environment. For example, the magnetic mineral magnetite (Fe ₃ O ₄ ) is generated extracellularly in the bulk pore waters of sediments by various Fe(III)-reducing bacteria under anaerobic conditions, while some other anaerobic and microaerophilic bacteria and possibly protists form magnetite intracellularly within preformed vesicles. Differences in precipitation mechanisms might be caused by enzymatic activity at specific sites on the surface of the cell. Whereas one type of microbe might facilitate the transport of dissolved Fe(III) into the cell, another type will express its reductive enzymes and cause the reduction of Fe(III) external to the cell. Still other microbes might induce magnetite formation indirectly through the oxidation of Fe(II), followed by the reaction of dissolved Fe(II) with hydrolyzed Fe(III). The biomineralization of magnetite has significant effect on environmental iron cycling, the magnetization of sediments and thus the geologic record, and on the use of biomarkers as microbial fossils.     

Analysis of Magnetotactic Behavior by Swimming Assay

Melanoidin, which was obtained by the Maillard reaction between D-glucose and glycine, was found to exert a potent inhibitory effect on the activity of trypsin with BANA as a synthetic substrate. The concentration of melanoidin required to reduce the activity of trypsin by 50% was less than 1 μg/ml, similar to that of soybean trypsin inhibitor and leupeptin. On the other hand, chymotrypsin was not affected by melanoidin. The specific interaction between melanoidin and trypsin is discussed.     

趋磁细菌及其应用于生物导航的研究进展

趋磁性细菌是一种由于体内含有对磁场具有敏感性的磁小体,而能够沿着磁力线运动的特殊细菌,本文综述了趋磁细菌的分布、分类、特性、磁小体研究以及趋磁细菌在生物导航方面的研究进展。     

A biogeographic distribution of magnetotactic bacteria influenced by salinity

Magnetotactic bacteria (MTB), which synthesize intracellular ferromagnetic magnetite and/or greigite magnetosomes, have significant roles in global iron cycling in aquatic systems, as well as sedimentary magnetism. The occurrence of MTB has been reported in aquatic environments from freshwater to marine ecosystems; however, the distribution of MTB across heterogeneous habitats remains unclear. Here we examined the MTB communities from diverse habitats across northern and southern China, using comprehensive transmission electron microscopy and comparison of 16S rRNA gene analyses. A total of 334 16S rRNA gene sequences were analyzed, representing the most comprehensive analysis on the diversity and distribution of MTB to date. The majority (95%) of sequences belong to the Alphaproteobacteria, whereas a population of giant magnetotactic rod is affiliated with the Nitrospirae phylum. By a statistical comparison of these sequence data and publicly available MTB sequences, we infer for the first time that the composition of MTB communities represents a biogeographic distribution across globally heterogeneous environments, which is influenced by salinity.     

X-ray Absorption Spectroscopy and Magnetism of Synthetic Greigite and Greigite Magnetosomes in Magnetotactic Bacteria

Scanning transmission X-ray microscopy at the Fe 2p (L_2,3), O1s, C1s, and S2p edges was used to study greigite magnetosomes and other cellular content of a magnetotactic bacterium known as a multicellular magnetotactic prokaryote (MMP). X-ray absorption spectrum (XAS) and X-ray magnetic circular dichroism (XMCD) spectra of greigite (Fe₃S₄) nanoparticles, synthesized via a hydrothermal method, were measured. Although XAS of the synthetic greigite nanoparticles and biotic magnetosome crystals in MMPs are slightly different due to partial oxidation of the MMP greigite, the XMCD spectra of the two materials are in good agreement. The Fe 2p XAS and XMCD spectra of Fe₃S₄ are quite different from those of its oxygen analog, magnetite (Fe₃O₄), suggesting Fe₃S₄ has a different electronic and magnetic structure than Fe₃O₄ despite having the same crystal structure. Sulfate and sulfide species were also identified in MMPs, both of which are likely involved in sulfur metabolism.     

Interruption of the denitrification pathway influences cell growth and magnetosome formation in Magnetospirillum magneticum AMB-1

Aims: Intracellular magnetosome synthesis in magnetotactic bacteria has been proposed to be a process involving functions of a variety of proteins. To learn more about the genetic control that is involved in magnetosome formation, nonmagnetic mutants are screened and characterized. Methods and Results: Conjugation‐mediated transposon mutagenesis was applied to screen for nonmagnetic mutants of Magnetospirillum magneticum AMB‐1 that were unable to respond to the magnetic field. A mutant strain with disruption of a gene locus encoding nitric oxide reductase was obtained. Growth and magnetosome formation under different conditions were further characterized. Conclusions: Interruption of denitrification by inactivating nitric oxide reductase was responsible for the compromised growth and magnetosome formation in the mutant with shorter intracellular chains of magnetite crystals than those of wild‐type cells under anaerobic conditions. Nevertheless, the mutant displayed apparently normal growth in aerobic culture. Significance and Impact of the Study: Efficient denitrification in the absence of oxygen is not only necessary for maintaining cell growth but may also be required to derive sufficient energy to mediate the formation of magnetosome vesicles necessary for the initiation or activation of magnetite formation.     

趋磁细菌生态学研究进展

趋磁细菌是一类革兰氏阴性的原核生物,广泛分布于淡水和海水环境中的有氧-无氧过渡区.趋磁细菌的分布与其环境中的氧、硫化物及铁等的浓度相关,不同种类分布在不同的物化梯度范围内.趋磁细菌的生长、磁小体的合成及磁小体的成分对环境有一定程度的指示作用.它们在生物地球化学循环中起着重要的作用.主要针对以上研究内容进行回顾,同时结合本实验室的一些研究结果做初步的分析,并对趋磁细菌生态学研究进行展望.     

磁杆菌HMB—1的磁小体特性及其合成条件的研究

从西安段家坡黄土剖面６１个土样中,分离纯化出一株磁杆菌ＨＭＢ１,并对其磁性特点及磁小体的合成条件进行了研究;同时用Ｘ射线能谱分析测定了磁小体的组成成分,主要为铁和钴。综合实验结果得出ＨＭＢ１生长及磁小体形成的最适培养基     

趋磁细菌及磁小体在肿瘤治疗中的应用

提高抗肿瘤药物的靶向性是肿瘤治疗、降低药物副作用的重要手段。在肿瘤组织内部由于癌细胞的快速增殖致使其形成低氧区,低氧区会对多种肿瘤治疗方案产生耐受。趋磁细菌 (Magnetotactic bacteria, MTB) 是一类能在细胞内产生外包生物膜、纳米尺寸、单磁畴磁铁矿 (Fe3O4) 或硫铁矿 (Fe3S4) 晶体颗粒-磁小体的微生物的统称。在磁场的作用下,趋磁细菌可凭借鞭毛运动至厌氧区。趋磁细菌在动物体内毒性较低且生物相容性良好,其磁小体与人工合成的磁性纳米材料相比优势显著。文中在介绍趋磁细菌及其磁小体生物学特点、理化性能的基础上,综述了趋磁细菌作为载体偶联药物进入肿瘤内部,并通过感受低氧信号定位于肿瘤低氧区,以及趋磁细菌竞争肿瘤细胞铁源的研究进展,总结了磁小体运载化疗药物、抗体、DNA疫苗靶向结合肿瘤的研究进展,分析了趋磁细菌及磁小体肿瘤治疗中面临的问题,并对趋磁细菌和磁小体在肿瘤治疗中的应用进行了展望。