Nygaard Per, Saxild Hans H
Institute of Molecular Biology, Department of Biological Chemistry, University of Copenhagen, Copenhagen, Denmark.
J Bacteriol. 2005 Jan;187(2):791-4. doi: 10.1128/JB.187.2.791-794.2005.
In Bacillus subtilis, the expression of genes encoding enzymes and other proteins involved in purine de novo synthesis and salvage is affected by purine bases and phosphoribosylpyrophosphate (PRPP). The transcription of the genes belonging to the PurR regulon is negatively regulated by the PurR protein and PRPP. The expression of the genes belonging to the G-box (XptR) regulon, including the pbuE gene, is negatively regulated by a riboswitch-controlled transcription termination mechanism. The G-box regulon effector molecules are hypoxanthine and guanine. pbuE encodes a purine base efflux pump and is now recognized as belonging to a third purine regulon. The expression of the pbuE gene is positively regulated by a riboswitch that recognizes adenine. Here we show that the expression of pbuE'-lacZ transcriptional fusions are induced by adenine to the highest extent in mutants which do not express a functional PbuE pump. In a mutant defective in the metabolism of adenine, the ade apt mutant, we found a high intracellular level of adenine and constitutive high levels of PbuE. A growth test using a purine auxotroph provided further evidence for the role of PbuE in lowering the intracellular concentration of purine bases, including adenine. Purine analogs also affect the expression of pbuE, which might be of importance for the protection against toxic analogs. In a mutant that overexpresses PbuE, the expression of genes belonging to the PurR regulon was increased. Our findings provide further evidence for important functions of the PbuE protein, such as acting as a pump that lowers the purine base pool and affects the expression of the G-box and PurR regulons, including pbuE itself, and as a pump involved in protection against toxic purine base analogs.
在枯草芽孢杆菌中，参与嘌呤从头合成和补救途径的酶及其他蛋白质编码基因的表达受嘌呤碱基和磷酸核糖焦磷酸（PRPP）的影响。属于PurR调控子的基因转录受PurR蛋白和PRPP的负调控。属于G盒（XptR）调控子的基因，包括pbuE基因，其表达受核糖开关控制的转录终止机制负调控。G盒调控子的效应分子是次黄嘌呤和鸟嘌呤。pbuE编码一种嘌呤碱基外排泵，现在被认为属于第三个嘌呤调控子。pbuE基因的表达受识别腺嘌呤的核糖开关正调控。在此我们表明，在不表达功能性PbuE泵的突变体中，pbuE'-lacZ转录融合体的表达受腺嘌呤诱导达到最高水平。在腺嘌呤代谢缺陷的突变体ade apt突变体中，我们发现细胞内腺嘌呤水平很高且PbuE持续高水平表达。使用嘌呤营养缺陷型进行的生长试验进一步证明了PbuE在降低包括腺嘌呤在内的嘌呤碱基细胞内浓度方面的作用。嘌呤类似物也会影响pbuE的表达，这可能对抵御有毒类似物很重要。在过表达PbuE的突变体中，属于PurR调控子的基因表达增加。我们的研究结果进一步证明了PbuE蛋白的重要功能，例如作为一种降低嘌呤碱基库并影响G盒和PurR调控子（包括pbuE自身）表达的泵，以及作为一种参与抵御有毒嘌呤碱基类似物的泵。
