Grkovic Steve, Hardie Kate M, Brown Melissa H, Skurray Ronald A
School of Biological Sciences, University of Sydney, Sydney, New South Wales 2006, Australia.
Biochemistry. 2003 Dec 30;42(51):15226-36. doi: 10.1021/bi035447+.
The QacR multidrug-binding repressor protein regulates the expression of the Staphylococcus aureus qacA gene, a multidrug resistance (MDR) locus that is prevalent in clinical isolates of this important human pathogen. In this paper we demonstrate that the range of structurally diverse compounds capable of inducing qacA transcription is significantly more varied than previously appreciated, particularly in relation to bivalent cations. For all of the newly identified inducing compounds, induction of qacAexpression was correlated with a matching ability to dissociate QacR from operator DNA. Development of a ligand-binding assay based on intrinsic tryptophan fluorescence permitted dissociation constants to be determined for the majority of known QacR ligands, with values ranging from 0.2 to 82 microM. High-affinity binding of a compound to QacR in vitro was not found to correlate very strongly with either its in vivo inducing abilities or its structure. The latter observation indicated that the QacR ligand-binding pocket appears to have evolved to accommodate a wide range of toxic hydrophobic cations, rather than a specific class of compound. Importantly, the antimicrobial ligands of QacR included several plant alkaloids that share structural similarities with synthetic MDR substrates. This is consistent with the suggestion that the qacA-qacR MDR locus was recently derived from genes that protect against natural antimicrobial compounds.
QacR多药结合阻遏蛋白调控金黄色葡萄球菌qacA基因的表达，qacA基因是一个多药耐药（MDR）位点，在这种重要的人类病原体的临床分离株中普遍存在。在本文中，我们证明能够诱导qacA转录的结构多样的化合物范围比之前认识到的要广泛得多，特别是在二价阳离子方面。对于所有新鉴定出的诱导化合物，qacA表达的诱导与将QacR从操纵子DNA上解离的匹配能力相关。基于内在色氨酸荧光的配体结合测定法的开发使得能够确定大多数已知QacR配体的解离常数，其值范围为0.2至82微摩尔。未发现化合物在体外与QacR的高亲和力结合与其体内诱导能力或结构有很强的相关性。后一观察结果表明，QacR配体结合口袋似乎已经进化以容纳广泛的有毒疏水阳离子，而不是特定类别的化合物。重要的是，QacR的抗菌配体包括几种与合成MDR底物具有结构相似性的植物生物碱。这与qacA - qacR MDR位点最近源自抵御天然抗菌化合物的基因的观点一致。
