Effects of Hypomagnetic Field on Magnetosome Formation of Magnetospirillum Magneticum AMB-1

Magnetotactic bacteria synthesize intracellular magnetic particles, magnetosomes, which arrange in chain(s) and confer on cell a magnetic dipolar moment. To explore the function of geomagnetic field to magnetotactic bacteria, the effects of hypomagnetic field on magnetosome formation in Magnetospirillum magneticum AMB-1 were studied. Cells were cultivated in a specially designed device where geomagnetic field was reduced by about 100-fold to less than 500nT. AMB-1 cultures were incubated in hypomagnetic field or geomagnetic field. Results showed that hypomagnetic field had no significant effects on the average number of magnetic particles per bacterium and bacterial iron depletion. However, the growth (OD) of cell at stationary-phase was lower and cellular magnetism (R _mag) at exponential growth phase was higher than that of bacteria cultivated in geomagnetic field. Statistic results on transmission electron microscopy (TEM) micrographs showed that the average size of magnetic particles in AMB-1 cells in hypomagnetic field group was larger than that of in geomagnetic field group and more ratio of larger-size magnetic particles (>50 nm) was observed when cultivated 16 h under hypomagnetic field. Furthermore, the influences of hypomagnetic field on gene expression were studied in AMB-1 cells. Quantitative RT-PCR results showed that hypomagnetic field up-regulated mms13, down-regulated mms6 and had no effect on magA. Together, the results showed that hypomagnetic field could affect the growth of AMB-1 at the stationary-phase, the crystallization process of magnetosomes, and mms13, mms6 expressions. In addition, our results suggested that the geomagnetic field plays an important role in the biomineralization of magnetosomes.     

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.     

Cloning and functional analysis of the sequences flanking mini-Tn5 in the magnetosomes deleted mutant NM4 of <Emphasis Type="Italic">Magnetospirillum gryphiswaldense</Emphasis> MSR-1

A magnetosome deleted mutant NM4 of Magnetospirillum gryphiswaldense MSR-1 was generated by mini-Tn5 transposon mutagenesis, and a 5045-bp fragment flanking mini-Tn5 in NM4 was cloned by Anchored PCR. Sequencing analysis showed that this fragment involved six putative open reading frames (ORFs); the mini-Tn5 was inserted into ORF4. Functional complementary test indicated that the 5045-bp fragment was required for biosynthesis of magnetosomes in M. gryphiswaldense MSR-1. The protein encoded by ORF4 had 25% of identity with the chemotaxis protein CheYIII of Caulobacter crescentus CB15, and the protein encoded by ORF4 contained a conserved signal receiver domain that can receive the signal from the sensor partner of the bacterial two-component systems. It was suggested that the protein encoded by ORF4 may take part in the signal transduction relating to biosynthesis of magnetosomes.     

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。     

Genetics and cell biology of magnetosome formation in magnetotactic bacteria

The ability of magnetotactic bacteria (MTB) to orient in magnetic fields is based on the synthesis of magnetosomes, which are unique prokaryotic organelles comprising membrane-enveloped, nano-sized crystals of a magnetic iron mineral that are aligned in well-ordered intracellular chains. Magnetosome crystals have species-specific morphologies, sizes, and arrangements. The magnetosome membrane, which originates from the cytoplasmic membrane by invagination, represents a distinct subcellular compartment and has a unique biochemical composition. The roughly 20 magnetosome-specific proteins have functions in vesicle formation, magnetosomal iron transport, and the control of crystallization and intracellular arrangement of magnetite particles. The assembly of magnetosome chains is under genetic control and involves the action of an acidic protein that links magnetosomes to a novel cytoskeletal structure, presumably formed by a specific actin-like protein. A total of 28 conserved genes present in various magnetic bacteria were identified to be specifically associated with the magnetotactic phenotype, most of which are located in the genomic magnetosome island. The unique properties of magnetosomes attracted broad interdisciplinary interest, and MTB have recently emerged as a model to study prokaryotic organelle formation and evolution.     

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.     

固体平板磁泳分离细菌新方法的研究

氧化亚铁硫杆菌（Acidithiobacillus ferrooxidans）能够在胞内形成电子致密的磁性颗粒,它的这种特性使利用氧化亚铁硫杆菌合成生物纳米磁性材料成为了可能。本课题组为了筛选出合成磁性颗粒能力强的菌株,对原有的液体磁泳进行了改进,采用了新的固体平板磁泳方法来筛选纯化目的菌株。经过磁泳分离后,细菌中含磁性颗粒的细胞比例由原始菌群的30％上升到90％,胞内含有的磁颗粒数目也由1~2颗增加至2~5颗,筛选得到的细菌在人工磁场下会进行趋磁运动。实验结果表明,氧化亚铁硫杆菌具有较弱的趋磁性,在人工磁场下会进行趋磁运动,但仅在地磁场作用下不能定向运动,利用固体平板磁泳筛选纯化含有磁性颗粒的氧化亚铁硫杆菌的方法是切实可行的,磁泳分离技术的进一步完善和改进为传统的微生物菌种分离提供了新的途径,为研究纯氧化亚铁硫杆菌菌株胞内磁性颗粒的形成条件及机理提供了前提条件,也为今后从浸矿细菌中分离筛选更多的含有磁性颗粒的菌株打下基础。     

Disruption of a <Emphasis Type="Italic">fur</Emphasis>-like gene inhibits magnetosome formation in <Emphasis Type="Italic">Magnetospirillum gryphiswaldense</Emphasis> MSR-1

In this study, a genomic library of Magnetospirillum gryphiswaldense MSR-1 strain was constructed and a fur-like gene (encoding Fur protein, ferric uptake regulator) was isolated and sequenced. This gene consisted of 420 bp and encoded 139 amino acid residues. To investigate the function of this gene in MSR-1, a fur mutant was generated by double crossover with a kanamycin cassette inserted into its coding region. Iron uptake and magnetosome formation were dramatically inhibited by disruption of fur. Iron content analysis of the fur mutant indicated that it contained approximately 0.037% by dry weight, which was at least 10-fold less than that observed in the wild type. Electron microscopy revealed the absence of a magnetosome in the fur mutant, although it was able to tolerate 1 mM H₂O₂ at 10-fold higher level than wild-type. These data suggest that Fur protein may possess a novel function in magnetic bacteria. Published in Russian in Biokhimiya, 2007, Vol. 72, No. 11, pp. 1532–1539.     

矿物作用下A.ferrooxidans中磁小体相关基因的差异表达

目的:研究氧化亚铁硫杆菌(Acidithiobacillus ferrooxidans,A.f)中与磁小体形成相关的mpsA、magA、thy和mamB基因分别在黄铁矿、黄铜矿、磁黄铁矿和闪锌矿的作用下的表达差异,寻找有利于磁小体形成的最佳培养矿物能源。方法:测量以不同硫化矿为能源时的菌体生长特性,用实时定量PCR方法研究与磁小体形成相关基因的表达差异。结果:在以磁黄铁矿为能源时,菌的生长量及多数基因的表达量优于其它三种矿,四个基因相对表达量分别为1.15、2.35、1.32、2.68。结论:磁黄铁矿是A.f中磁小体形成的最佳矿物能源。