Nitroaromatic compounds pose severe threats to public health and environmental safety. Nitro group removal via ammonia release is an important strategy for bacterial detoxification of nitroaromatic compounds, such as the conversion of 4-nitrobenzoate (4-NBA) to protocatechuate by the bacterial pnb operon. In contrast to the LysR-family transcriptional regulator PnbR in proteobacteria, the actinomycete-derived pnb locus (4-NBA degradation structural genes) formed an operon with the TetR-family transcriptional regulator gene pnbX, implying that it has a distinct regulatory mechanism. Here, pnbBA from the actinomycete Nocardioides sp. strain LMS-CY was biochemically confirmed to express 4-NBA degradation enzymes, and pnbX was essential for inducible degradation of 4-NBA. Purified PnbX-6His could bind the promoter probe of the pnb locus in vitro, and 4-NBA prevented this binding. 4-NBA could bind PnbX at a 1:1 molar ratio with K_D = 26.7 ± 4.2 nM. Low-nanomolar levels of 4-NBA induced the transcription of the pnb operon in strain LMS-CY. PnbX bound a palindromic sequence motif (5'-TTACGTTACA-N₈-TGTAACGTAA-3′) that encompasses the pnb promoter. This study identified a TetR-family repressor for the actinomycete-derived pnb operon that recognizes 10⁻⁸ M 4-NBA as its ligand, implying that nitro group removal of nitroaromatic compounds may be especially important for actinomycetes.

中文翻译：

---

低纳摩尔 4-硝基苯甲酸酯响应性阻遏物 PnbX 负调控放线菌衍生的 4-硝基苯甲酸酯降解 pnb 基因座

硝基芳香族化合物对公众健康和环境安全构成严重威胁。通过氨释放去除硝基是硝基芳香化合物细菌解毒的重要策略，例如细菌pnb操纵子将 4-硝基苯甲酸酯 (4-NBA) 转化为原儿茶酸。与变形菌中的 LysR 家族转录调节因子 PnbR 相比，放线菌衍生的pnb基因座（4-NBA 降解结构基因）与 TetR 家族转录调节基因pnbX形成操纵子，这意味着它具有独特的调节机制。在这里，来自放线菌Nocardioides的pnbBAsp。菌株 LMS-CY 经生化证实可表达 4-NBA 降解酶，而pnbX对 4-NBA 的可诱导降解至关重要。纯化的PnbX-6His可以在体外结合pnb基因座的启动子探针，而4-NBA阻止了这种结合。4-NBA 可以以 1:1 的摩尔比结合 PnbX，K _D  = 26.7 ± 4.2 nM。低纳摩尔水平的 4-NBA 诱导菌株 LMS-CY 中pnb操纵子的转录。PnbX 结合了一个包含pnb启动子的回文序列基序（5'-TTACGTTACA-N ₈ -TGTAACGTAA-3'） 。本研究为放线菌衍生的pnb操纵子确定了一个 TetR 家族阻遏物，可识别 10^-8  M 4-NBA 作为其配体，这意味着硝基芳族化合物的硝基去除可能对放线菌尤为重要。