Perez J Christian, Groisman Eduardo A
Program in Molecular Genetics, Howard Hughes Medical Institute, Washington University School of Medicine, Campus Box 8230, 660 S. Euclid Ave., St Louis, MO 63110, USA.
Mol Microbiol. 2007 Jan;63(1):283-93. doi: 10.1111/j.1365-2958.2006.05512.x.
Acid pH often triggers changes in gene expression. However, little is known about the identity of the gene products that sense fluctuations in extracytoplasmic pH. The Gram-negative pathogen Salmonella enterica serovar Typhimurium experiences a number of acidic environments both inside and outside animal hosts. Growth in mild acid (pH 5.8) promotes transcription of genes activated by the response regulator PmrA, but the signalling pathway(s) that mediates this response has thus far remained unexplored. Here we report that this activation requires both PmrA's cognate sensor kinase PmrB, which had been previously shown to respond to Fe(3+) and Al(3+), and PmrA's post-translational activator PmrD. Substitution of a conserved histidine or of either one of four conserved glutamic acid residues in the periplasmic domain of PmrB severely decreased or abolished the mild acid-promoted transcription of PmrA-activated genes. The PmrA/PmrB system controls lipopolysaccharide modifications mediating resistance to the antibiotic polymyxin B. Wild-type Salmonella grown at pH 5.8 were > 100 000-fold more resistant to polymyxin B than organisms grown at pH 7.7. Our results suggest that protonation of the PmrB periplasmic histidine and/or of the glutamic acid residues activate the PmrA protein, and that mild acid promotes cellular changes resulting in polymyxin B resistance.
酸性pH值常常会引发基因表达的变化。然而，对于感知胞外pH值波动的基因产物的身份却知之甚少。革兰氏阴性病原体鼠伤寒沙门氏菌在动物宿主体内和体外都会经历多种酸性环境。在轻度酸性环境（pH 5.8）中生长会促进由应答调节因子PmrA激活的基因的转录，但介导这种应答的信号通路迄今为止仍未得到探索。在此我们报告，这种激活既需要PmrA的同源传感器激酶PmrB（此前已证明其对Fe(3+)和Al(3+)有反应），也需要PmrA的翻译后激活因子PmrD。替换PmrB周质结构域中一个保守的组氨酸或四个保守谷氨酸残基中的任何一个，都会严重降低或消除轻度酸性环境促进的PmrA激活基因的转录。PmrA/PmrB系统控制着介导对抗生素多粘菌素B耐药性的脂多糖修饰。在pH 5.8条件下生长的野生型沙门氏菌对多粘菌素B的耐药性比在pH 7.7条件下生长的菌株高100000倍以上。我们的结果表明，PmrB周质组氨酸和/或谷氨酸残基的质子化会激活PmrA蛋白，并且轻度酸性环境会促进导致多粘菌素B耐药性的细胞变化。
