School of Biological Sciences, Georgia Institute of Technology, Atlanta, Georgia, USA.
Energy and Environment Directorate, Pacific Northwest National Laboratory, Richland, Washington, USA.
Appl Environ Microbiol. 2019 Jan 23;85(3). doi: 10.1128/AEM.02115-18. Print 2019 Feb 1.
The metal-reducing gammaproteobacterium reduces iodate (IO) as an anaerobic terminal electron acceptor. Microbial IO electron transport pathways are postulated to terminate with nitrate (NO) reductase, which reduces IO as an alternative electron acceptor. Recent studies with , however, have demonstrated that NO reductase is not involved in IO reduction. The main objective of the present study was to determine the metal reduction and protein secretion genes required for IO reduction by with lactate, formate, or H as the electron donor. With all electron donors, the type I and type V protein secretion mutants retained wild-type IO reduction activity, while the type II protein secretion mutant lacking the outer membrane secretin GspD was impaired in IO reduction. Deletion mutants lacking the cyclic AMP receptor protein (CRP), cytochrome maturation permease CcmB, and inner membrane-tethered -type cytochrome CymA were impaired in IO reduction with all electron donors, while deletion mutants lacking -type cytochrome MtrA and outer membrane β-barrel protein MtrB of the outer membrane MtrAB module were impaired in IO reduction with only lactate as an electron donor. With all electron donors, mutants lacking the -type cytochromes OmcA and MtrC of the metal-reducing extracellular electron conduit MtrCAB retained wild-type IO reduction activity. These findings indicate that IO reduction by involves electron donor-dependent metal reduction and protein secretion pathway components, including the outer membrane MtrAB module and type II protein secretion of an unidentified IO reductase to the outer membrane. Microbial iodate (IO) reduction is a major component in the biogeochemical cycling of iodine and the bioremediation of iodine-contaminated environments; however, the molecular mechanism of microbial IO reduction is poorly understood. Results of the present study indicate that outer membrane (type II) protein secretion and metal reduction genes encoding the outer membrane MtrAB module of the extracellular electron conduit MtrCAB are required for IO reduction by On the other hand, the metal-reducing -type cytochrome MtrC of the extracellular electron conduit is not required for IO reduction by These findings indicate that the IO electron transport pathway terminates with an as yet unidentified IO reductase that associates with the outer membrane MtrAB module to deliver electrons extracellularly to IO
金属还原γ-变形菌 将碘酸盐 (IO) 还原为厌氧末端电子受体。微生物 IO 电子传递途径被假定以硝酸盐 (NO) 还原酶为终点，后者将 IO 还原为替代电子受体。然而，最近对 的研究表明，NO 还原酶不参与 IO 还原。本研究的主要目的是确定利用乳酸盐、甲酸盐或 H 作为电子供体还原 IO 时所需的金属还原和蛋白分泌基因。对于所有电子供体，I 型和 V 型蛋白分泌突变体保留了野生型 IO 还原活性，而缺乏外膜分泌蛋白 GspD 的 II 型蛋白分泌突变体在 IO 还原中受损。缺乏环腺苷酸受体蛋白 (CRP)、细胞色素成熟渗透酶 CcmB 和内膜连接的 -型细胞色素 CymA 的缺失突变体在所有电子供体中还原 IO 时受损，而缺乏外膜 MtrAB 模块的外膜 β-桶蛋白 MtrB 和 -型细胞色素 MtrA 的缺失突变体在仅以乳酸盐作为电子供体时还原 IO 时受损。对于所有电子供体，缺乏金属还原细胞外电子导管 MtrCAB 的 -型细胞色素 OmcA 和 MtrC 的突变体保留了野生型 IO 还原活性。这些发现表明， 对 IO 的还原涉及电子供体依赖性的金属还原和蛋白分泌途径成分，包括外膜 MtrAB 模块和对 外膜的未鉴定 IO 还原酶的 II 型蛋白分泌。微生物碘酸盐 (IO) 还原是碘的生物地球化学循环和碘污染环境生物修复的主要组成部分；然而，微生物 IO 还原的分子机制知之甚少。本研究结果表明，细胞外电子导管的外膜 (II 型) 蛋白分泌和编码外膜 MtrAB 模块的金属还原基因是 还原 IO 所必需的。另一方面，细胞外电子导管的金属还原 -型细胞色素 MtrC 并不需要 还原 IO。这些发现表明，IO 电子传递途径的末端是一个尚未鉴定的 IO 还原酶，它与外膜 MtrAB 模块结合，将电子体外传递到 IO 上。
