Branco Rita, Chung Ana-Paula, Morais Paula V
IMAR-Laboratory of Microbiology 3004-517 Coimbra, Portugal.
BMC Microbiol. 2008 Jun 13;8:95. doi: 10.1186/1471-2180-8-95.
Arsenic (As) is a natural metalloid, widely used in anthropogenic activities, that can exist in different oxidation states. Throughout the world, there are several environments contaminated with high amounts of arsenic where many organisms can survive. The most stable arsenical species are arsenate and arsenite that can be subject to chemically and microbiologically oxidation, reduction and methylation reactions. Organisms surviving in arsenic contaminated environments can have a diversity of mechanisms to resist to the harmful effects of arsenical compounds.
The highly metal resistant Ochrobactrum tritici SCII24 was able to grow in media with arsenite (50 mM), arsenate (up to 200 mM) and antimonite (10 mM). This strain contains two arsenic and antimony resistance operons (ars1 and ars2), which were cloned and sequenced. Sequence analysis indicated that ars1 operon contains five genes encoding the following proteins: ArsR, ArsD, ArsA, CBS-domain-containing protein and ArsB. The ars2 operon is composed of six genes that encode two other ArsR, two ArsC (belonging to different families of arsenate reductases), one ACR3 and one ArsH-like protein. The involvement of ars operons in arsenic resistance was confirmed by cloning both of them in an Escherichia coli ars-mutant. The ars1 operon conferred resistance to arsenite and antimonite on E. coli cells, whereas the ars2 operon was also responsible for resistance to arsenite and arsenate. Although arsH was not required for arsenate resistance, this gene seems to be important to confer high levels of arsenite resistance. None of ars1 genes were detected in the other type strains of genus Ochrobactrum, but sequences homologous with ars2 operon were identified in some strains.
A new strategy for bacterial arsenic resistance is described in this work. Two operons involved in arsenic resistance, one giving resistance to arsenite and antimonite and the other giving resistance to arsenate were found in the same bacterial strain.
砷是一种天然类金属，广泛应用于人类活动中，可呈现不同的氧化态。在全球范围内，有若干被大量砷污染的环境，许多生物仍能在其中存活。最稳定的砷化合物是砷酸盐和亚砷酸盐，它们可发生化学和微生物氧化、还原及甲基化反应。在砷污染环境中生存的生物具有多种机制来抵抗砷化合物的有害影响。
具有高度金属抗性的小麦苍白杆菌SCII24能够在含有亚砷酸盐（50 mM）、砷酸盐（高达200 mM）和亚锑酸盐（10 mM）的培养基中生长。该菌株含有两个砷和锑抗性操纵子（ars1和ars2），对其进行了克隆和测序。序列分析表明，ars1操纵子包含五个基因，编码以下蛋白质：ArsR、ArsD、ArsA、含CBS结构域的蛋白质和ArsB。ars2操纵子由六个基因组成，编码另外两个ArsR、两个ArsC（属于不同家族的砷酸盐还原酶）、一个ACR3和一个ArsH样蛋白质。通过将这两个操纵子克隆到大肠杆菌ars突变体中，证实了ars操纵子与砷抗性有关。ars1操纵子赋予大肠杆菌细胞对亚砷酸盐和亚锑酸盐的抗性，而ars2操纵子也负责对亚砷酸盐和砷酸盐的抗性。虽然arsH对于砷酸盐抗性不是必需的，但该基因似乎对于赋予高水平的亚砷酸盐抗性很重要。在苍白杆菌属的其他模式菌株中未检测到ars1基因，但在一些菌株中鉴定出了与ars2操纵子同源的序列。
本研究描述了一种细菌抗砷的新策略。在同一菌株中发现了两个与砷抗性有关的操纵子，一个赋予对亚砷酸盐和亚锑酸盐的抗性，另一个赋予对砷酸盐的抗性。
