Department of Microbiology and Molecular Genetics, Michigan State University, East Lansing, MI 48824, USA.
Proc Natl Acad Sci U S A. 2011 Sep 13;108(37):15248-52. doi: 10.1073/pnas.1108616108. Epub 2011 Sep 6.
The in situ stimulation of Fe(III) oxide reduction by Geobacter bacteria leads to the concomitant precipitation of hexavalent uranium [U(VI)] from groundwater. Despite its promise for the bioremediation of uranium contaminants, the biological mechanism behind this reaction remains elusive. Because Fe(III) oxide reduction requires the expression of Geobacter's conductive pili, we evaluated their contribution to uranium reduction in Geobacter sulfurreducens grown under pili-inducing or noninducing conditions. A pilin-deficient mutant and a genetically complemented strain with reduced outer membrane c-cytochrome content were used as controls. Pili expression significantly enhanced the rate and extent of uranium immobilization per cell and prevented periplasmic mineralization. As a result, pili expression also preserved the vital respiratory activities of the cell envelope and the cell's viability. Uranium preferentially precipitated along the pili and, to a lesser extent, on outer membrane redox-active foci. In contrast, the pilus-defective strains had different degrees of periplasmic mineralization matching well with their outer membrane c-cytochrome content. X-ray absorption spectroscopy analyses demonstrated the extracellular reduction of U(VI) by the pili to mononuclear tetravalent uranium U(IV) complexed by carbon-containing ligands, consistent with a biological reduction. In contrast, the U(IV) in the pilin-deficient mutant cells also required an additional phosphorous ligand, in agreement with the predominantly periplasmic mineralization of uranium observed in this strain. These findings demonstrate a previously unrecognized role for Geobacter conductive pili in the extracellular reduction of uranium, and highlight its essential function as a catalytic and protective cellular mechanism that is of interest for the bioremediation of uranium-contaminated groundwater.
原位刺激铁（III）氧化物还原可导致六价铀[U（VI）]从地下水中同时沉淀。尽管它有希望用于生物修复铀污染物，但这种反应背后的生物学机制仍难以捉摸。因为铁（III）氧化物还原需要表达 Geobacter 的导电菌毛，所以我们评估了它们在导电菌毛诱导或非诱导条件下生长的 Geobacter sulfurreducens 中对铀还原的贡献。使用菌毛缺陷突变体和外膜 c-细胞色素含量降低的遗传互补菌株作为对照。菌毛表达显着提高了每细胞铀固定的速率和程度，并防止了周质矿化。结果，菌毛表达还保留了细胞包膜的重要呼吸活性和细胞活力。铀优先沿菌毛沉淀，在较小程度上沿外膜氧化还原活性焦点沉淀。相比之下，菌毛缺陷菌株的周质矿化程度不同，与它们的外膜 c-细胞色素含量相匹配。X 射线吸收光谱分析表明，通过菌毛将 U（VI）体外还原为单核四价铀 U（IV），与含碳配体络合，与生物还原一致。相比之下，菌毛缺陷突变体细胞中的 U（IV）也需要额外的磷配体，与该菌株中观察到的铀主要周质矿化一致。这些发现证明了 Geobacter 导电菌毛在铀的细胞外还原中具有以前未被认识的作用，并强调了其作为一种催化和保护细胞机制的重要作用，这对于生物修复铀污染的地下水具有重要意义。
