在清除放射性鈾污染的隊伍中,，有望增加一批新成員,。這些只有千分之一毫米長的清潔工揮舞著細(xì)長的“毛發(fā)”，能把溶解在水里的鈾清除掉,。美國研究人員新近發(fā)現(xiàn),，一類稱為地桿菌的細(xì)菌有潛力用于鈾污染的生物治理。
此前已有研究表明,，一些地桿菌能夠通過還原周圍環(huán)境里的金屬（也就是向金屬添加電子）來獲取能量,。溶解在水里的鈾經(jīng)過這樣的還原之后,，會變得難以溶解，從而縮小污染范圍,，并且容易被清除掉。
美國密歇根州立大學(xué)的研究人員杰瑪·雷格拉猜測,，這些細(xì)菌外面的細(xì)長絲狀物——菌毛可能是問題的關(guān)鍵,。這些由蛋白質(zhì)組成的菌毛能夠?qū)щ姡挥糜谘兄?ldquo;納米電線”,。
雷格拉與同事以硫還原地桿菌為對象,，培育出因缺乏某種基因而不能產(chǎn)生菌毛的菌株，與能正常產(chǎn)生菌毛的菌株進(jìn)行比較,。結(jié)果顯示,，菌毛能大大增強(qiáng)細(xì)菌清除鈾污染的能力。
研究發(fā)現(xiàn),，如果沒有菌毛,，鈾的還原反應(yīng)是在細(xì)菌內(nèi)部進(jìn)行的，會傷害到細(xì)菌自身,。而有菌毛時,，大部分反應(yīng)圍繞著菌毛完成，不僅擴(kuò)大了反應(yīng)過程中可用于電子傳輸?shù)目臻g,，還拉遠(yuǎn)了鈾與細(xì)菌的距離,，提高安全性。
研究人員用一種熒光染料測量了地桿菌細(xì)胞的呼吸酶在接觸鈾之后的活性,。結(jié)果顯示,，有菌毛的細(xì)菌呼吸酶活性更高，因而生存能力更強(qiáng),。有菌毛的菌株在接觸鈾之后還能恢復(fù)過來,，并且比沒有菌毛的菌株生長更快。
雷格拉說,，由于菌毛的成分是蛋白質(zhì),，可以比較容易地往上面添加不同的官能團(tuán)（決定有機(jī)化合物化學(xué)性質(zhì)的原子或原子團(tuán)），來調(diào)節(jié)菌毛功能,。
相關(guān)論9月文5日發(fā)表在美國新一期《國家科學(xué)院院刊》上,。雷格拉認(rèn)為，這種方法理論上也適用于其他一些金屬元素的放射性同位素,，包括锝,、钚和鈷等。因此,，該成果不僅可用于治理以往核試驗(yàn)造成的鈾污染,，還有可能幫助應(yīng)對日本核電站事故,。(生物谷 Bioon.com)
doi:10.1073/pnas.1108616108
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PMID:
Extracellular reduction of uranium via Geobacter conductive pili as a protective cellular mechanism
Cologgi, Dena L.; Lampa-Pastirk, Sanela; Speers, Allison M.; Kelly, Shelly D.; Reguera, Gemma
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.
