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Abstract：
Microbial reduction of organic disulfides affects the macromolecular structure and chemical reactivity of natural organic matter. Currently, the enzymatic pathways that mediate disulfide bond reduction in soil and sedimentary organic matter are poorly understood. In this study, we examined the extracellular reduction of 5,5\'-dithiobis(2-nitrobenzoic acid) (DTNB) by Shewanella oneidensis strain MR-1. A transposon mutagenesis screen performed with S. oneidensis resulted in the isolation of a mutant that lost ~90% of its DTNB reduction activity. Genome sequencing of the mutant strain revealed that the transposon was inserted into the dsbD gene, which encodes for an oxidoreductase involved in cytochrome c maturation. Complementation of the mutant strain with the wild-type dsbD partially restored DTNB reduction activity. Because DsbD catalyzes a critical step in the assembly of multi-heme c-type cytochromes, we further investigated the role of extracellular electron transfer cytochromes in organic disulfide reduction. The results indicated that mutants lacking proteins in the Mtr system were severely impaired in their ability to reduce DTNB. These findings provide new insights into extracellular organic disulfide reduction and the enzymatic pathways of organic sulfur redox cycling.IMPORTANCEOrganic sulfur compounds in soils and sediments are held together by disulfide bonds. This study investigates how Shewanella oneidensis breaks apart extracellular organic sulfur compounds. The results show that an enzyme involved in the assembly of c-type cytochromes as well as proteins in the Mtr respiratory pathway is needed for S. oneidensis to transfer electrons from the cell surface to extracellular organic disulfides. These findings have important implications for understanding how organic sulfur decomposes in terrestrial ecosystems.
摘要：
微生物还原有机二硫化物会影响天然有机物的大分子结构和化学反应性。目前,对介导土壤和沉积有机质中二硫键还原的酶促途径知之甚少。在这项研究中,我们检查了Shewanellaoneidensis菌株MR-1对5,5'-二硫代双（2-硝基苯甲酸）（DTNB）的细胞外还原。用S.oneidensis进行的转座子诱变筛选导致损失〜90％的DTNB还原活性的突变体的分离。突变菌株的基因组测序显示转座子被插入dsbD基因中，它编码参与细胞色素c成熟的氧化还原酶。突变菌株与野生型dsbD的互补部分恢复了DTNB减少活性。因为DsbD催化多血红素c型细胞色素组装的关键步骤，我们进一步研究了细胞外电子转移细胞色素在有机二硫化物还原中的作用。结果表明，在Mtr系统中缺乏蛋白质的突变体在减少DTNB的能力方面受到严重损害。这些发现为细胞外有机二硫化物还原和有机硫氧化还原循环的酶途径提供了新的见解。重要元素土壤和沉积物中的有机硫化合物通过二硫键结合在一起。这项研究调查了希瓦氏菌是如何分解细胞外有机硫化合物的。结果表明，一种酶参与c型细胞色素以及Mtr呼吸途径中的蛋白质的组装。这些发现对于理解有机硫如何在陆地生态系统中分解具有重要意义。
