Daimon Yasushi, Narita Shin-ichiro, Akiyama Yoshinori
Institute for Virus Research, Kyoto University, Kyoto, Japan.
Institute for Virus Research, Kyoto University, Kyoto, Japan Faculty of Nutritional Sciences, University of Morioka, Takizawa, Iwate, Japan
J Bacteriol. 2015 Jul;197(14):2316-24. doi: 10.1128/JB.00079-15. Epub 2015 Apr 27.
σ(E), an alternative σ factor that governs a major signaling pathway in envelope stress responses in Gram-negative bacteria, is essential for growth of Escherichia coli not only under stressful conditions, such as elevated temperature, but also under normal laboratory conditions. A mutational inactivation of the hicB gene has been reported to suppress the lethality caused by the loss of σ(E). hicB encodes the antitoxin of the HicA-HicB toxin-antitoxin (TA) system; overexpression of the HicA toxin, which exhibits mRNA interferase activity, causes cleavage of mRNAs and an arrest of cell growth, while simultaneous expression of HicB neutralizes the toxic effects of overproduced HicA. To date, however, how the loss of HicB rescues the cell lethality in the absence of σ(E) and, more specifically, whether HicA is involved in this process remain unknown. Here we showed that simultaneous disruption of hicA abolished suppression of the σ(E) essentiality in the absence of hicB, while ectopic expression of wild-type HicA, but not that of its mutant forms without mRNA interferase activity, restored the suppression. Furthermore, HicA and two other mRNA interferase toxins, HigB and YafQ, suppressed the σ(E) essentiality even in the presence of chromosomally encoded cognate antitoxins when these toxins were overexpressed individually. Interestingly, when the growth media were supplemented with low levels of antibiotics that are known to activate toxins, E. coli cells with no suppressor mutations grew independently of σ(E). Taken together, our results indicate that the activation of TA system toxins can suppress the σ(E) essentiality and affect the extracytoplasmic stress responses.
σ(E) is an alternative σ factor involved in extracytoplasmic stress responses. Unlike other alternative σ factors, σ(E) is indispensable for the survival of E. coli even under unstressed conditions, although the exact reason for its essentiality remains unknown. Toxin-antitoxin (TA) systems are widely distributed in prokaryotes and are composed of two adjacent genes, encoding a toxin that exerts harmful effects on the toxin-producing bacterium itself and an antitoxin that neutralizes the cognate toxin. Curiously, it is known that inactivation of an antitoxin rescues the σ(E) essentiality, suggesting a connection between TA systems and σ(E) function. We demonstrate here that toxin activation is necessary for this rescue and suggest the possible involvement of TA systems in extracytoplasmic stress responses.
σ(E)是一种替代σ因子，它在革兰氏阴性菌的包膜应激反应中控制着一条主要的信号通路，不仅在诸如高温等应激条件下，而且在正常实验室条件下，对大肠杆菌的生长都是必不可少的。据报道，hicB基因的突变失活可抑制因σ(E)缺失而导致的致死性。hicB编码HicA-HicB毒素-抗毒素（TA）系统的抗毒素；具有mRNA干扰酶活性的HicA毒素的过表达会导致mRNA的切割和细胞生长的停滞，而HicB的同时表达可中和过量产生的HicA的毒性作用。然而，迄今为止，HicB的缺失如何在没有σ(E)的情况下挽救细胞致死性，更具体地说，HicA是否参与这一过程仍不清楚。在这里，我们表明，在没有hicB的情况下，hicA的同时破坏消除了对σ(E)必需性的抑制，而野生型HicA的异位表达（而非没有mRNA干扰酶活性的突变形式的表达）恢复了这种抑制。此外，当单独过表达时，HicA以及另外两种mRNA干扰酶毒素HigB和YafQ即使在存在染色体编码的同源抗毒素的情况下也能抑制σ(E)的必需性。有趣的是，当生长培养基中添加已知可激活毒素的低水平抗生素时，没有抑制突变的大肠杆菌细胞能够独立于σ(E)生长。综上所述，我们的结果表明TA系统毒素的激活可以抑制σ(E)的必需性并影响胞外应激反应。
σ(E)是一种参与胞外应激反应的替代σ因子。与其他替代σ因子不同，σ(E)即使在无应激条件下对大肠杆菌的存活也是必不可少的，尽管其必需性的确切原因仍不清楚。毒素-抗毒素（TA）系统广泛分布于原核生物中，由两个相邻基因组成，分别编码对产生毒素的细菌本身产生有害影响的毒素和中和同源毒素的抗毒素。奇怪的是，已知抗毒素的失活可挽救σ(E)的必需性，这表明TA系统与σ(E)功能之间存在联系。我们在此证明毒素激活对于这种挽救是必要的，并提示TA系统可能参与胞外应激反应。
