Almost one-third of all proteins require metal ions as an essential component in key biological processes and approximately half of all enzymes are associated with one or more metal ions. The naturally occurring selenium is very toxic at higher levels, but few bacteria can reduce it into the less toxic insoluble elemental selenium. Selenium is required for the synthesis of selenocysteine, an essential residue involved in the active sites of various enzymes. The purple non-sulphur bacteria, Rhodobacter sphaeroidesis demonstrated for its selenite reduction capacity. The exact mechanism of selenite toxicity is unknown but it reacts with glutathione to form selenodiglutathione, producing the highly toxic compounds namely, H₂O₂and O₂⁻. A R. sphaeroidesstrain with mutated takP gene, a member of the TRAP (tripartite ATP-independent periplasmic) family of transporter, was reported to be showing more resistance towards selenite in the growth medium but the reason for the resistance is unknown. TRAP transporters are the best-studied family of substrate-binding protein and in our previous study it was confirmed that the gene takP in R. sphaeroides is down-regulated by a small non-coding RNA SorY, providing more resistance to the bacterium against the oxidative stress. By comparative growth analysis and sensitivity assays in the presence of 2 mM selenite, it was observed that the SorY knockout strain is more sensitive to selenite while overexpression of the sRNA conferred more resistance to the bacterium like the takP mutant strain. TakP is involved in the import of malate into the cell, which under oxidative stress needs to be down-regulated to limit malate flux into the cell. Limited malate flux leads to metabolic rearrangements in the cell to avoid excessive generation of prooxidant NADH and facilitate constant generation of antioxidant NADPH. In the presence and absence of selenite, a drastic increase in the NADPH and decrease in the NADH levels are reported respectively. Accumulation of metallic selenium in the cytoplasm was detected via atomic absorption spectrophotometer and our analysis clearly demonstrated the presence of more selenium in the electron micrographs of the SorY knockout strain compared to the takP mutant grown under dark semi-aerobic growth conditions in the presence of selenite. Hence based on our analysis, it is confirmed that lack of TakP transporter led to reduced selenite influx into the cytoplasm, relieving cells with limited generation of ROS, eventually exhibiting more resistance against selenite-induced oxidative stress.

几乎三分之一的蛋白质需要金属离子作为关键生物过程的重要组成部分，并且大约一半的酶与一种或多种金属离子相关。天然存在的硒在较高水平时毒性很大，但很少有细菌可以将其还原为毒性较小的不溶性元素硒。硒是合成硒代半胱氨酸所必需的，硒代半胱氨酸是各种酶活性位点中必不可少的残留物。紫色的非硫细菌，球状红杆菌因其亚硒酸盐还原能力而被证明。亚硒酸盐毒性的确切机制尚不清楚，但它与谷胱甘肽反应形成硒二谷胱甘肽，产生剧毒化合物，即 H ₂ O ₂和 O ₂ ^-. 甲类球红细菌具有突变株takP基因，TRAP的成员（三方ATP无关的周质）家族转运，据报道，被示出朝向亚硒酸钠在生长培养基中，但用于电阻的原因是未知的更多的阻力。TRAP 转运蛋白是研究最深入的底物结合蛋白家族，在我们之前的研究中证实，球状红球藻中的基因takP被一个小的非编码 RNA SorY 下调，为细菌提供更多的抗氧化应激抵抗力。通过在 2 mM 亚硒酸盐存在下的比较生长分析和敏感性测定，观察​​到 SorY 敲除菌株对亚硒酸盐更敏感，而 sRNA 的过表达赋予了对takP等细菌的更多抗性突变株。TakP 参与将苹果酸输入细胞，在氧化应激下需要下调苹果酸以限制苹果酸流入细胞。有限的苹果酸流量导致细胞中的代谢重排，以避免过度生成促氧化剂 NADH 并促进抗氧化剂 NADPH 的持续生成。在亚硒酸盐存在和不存在的情况下，分别报告了 NADPH 的急剧增加和 NADH 水平的降低。通过原子吸收分光光度计检测在细胞质金属硒的积累和我们的分析清楚地表明更多硒在索里敲除菌株的电子显微照片的存在相比takP在亚硒酸盐存在下在黑暗半好氧生长条件下生长的突变体。因此，根据我们的分析，证实缺乏 TakP 转运蛋白导致亚硒酸盐流入细​​胞质减少，释放 ROS 产生有限的细胞，最终对亚硒酸盐诱导的氧化应激表现出更强的抵抗力。