Islam Sabina, Benedik Michael J, Wood Thomas K
Department of Chemical Engineering, the Pennsylvania State University, University Park, PA 16802-4400, USA.
Department of Biology, Texas A&M University, College Station, TX 77843-3258, USA.
Toxins (Basel). 2015 Jan 29;7(2):299-321. doi: 10.3390/toxins7020299.
Toxin/antitoxin (TA) systems are nearly universal in prokaryotes; toxins are paired with antitoxins which inactivate them until the toxins are utilized. Here we explore whether toxins may function alone; i.e., whether a toxin which lacks a corresponding antitoxin (orphan toxin) is physiologically relevant. By focusing on a homologous protein of the membrane-damaging toxin GhoT of the Escherichia coli GhoT/GhoS type V TA system, we found that YdcX (renamed OrtT for orphan toxin related to tetrahydrofolate) is toxic but is not part of TA pair. OrtT is not inactivated by neighboring YdcY (which is demonstrated to be a protein), nor is it inactivated by antitoxin GhoS. Also, OrtT is not inactivated by small RNA upstream or downstream of ortT. Moreover, screening a genomic library did not identify an antitoxin partner for OrtT. OrtT is a protein and its toxicity stems from membrane damage as evidenced by transmission electron microscopy and cell lysis. Furthermore, OrtT reduces cell growth and metabolism in the presence of both antimicrobials trimethoprim and sulfamethoxazole; these antimicrobials induce the stringent response by inhibiting tetrahydrofolate synthesis. Therefore, we demonstrate that OrtT acts as an independent toxin to reduce growth during stress related to amino acid and DNA synthesis.
毒素/抗毒素（TA）系统在原核生物中几乎普遍存在；毒素与抗毒素配对，抗毒素会使毒素失活，直到毒素被利用。在这里，我们探讨毒素是否可以单独发挥作用；即，缺乏相应抗毒素的毒素（孤儿毒素）在生理上是否相关。通过关注大肠杆菌GhoT/GhoS V型TA系统中膜损伤毒素GhoT的同源蛋白，我们发现YdcX（因与四氢叶酸相关的孤儿毒素而重新命名为OrtT）具有毒性，但不是TA对的一部分。OrtT不会被相邻的YdcY（已证明是一种蛋白质）失活，也不会被抗毒素GhoS失活。此外，ortT上下游的小RNA也不会使OrtT失活。而且，筛选基因组文库未发现OrtT的抗毒素伴侣。OrtT是一种蛋白质，其毒性源于膜损伤，这在透射电子显微镜和细胞裂解实验中得到了证实。此外，在同时存在抗菌药物甲氧苄啶和磺胺甲恶唑的情况下，OrtT会降低细胞生长和代谢；这些抗菌药物通过抑制四氢叶酸合成诱导严谨反应。因此，我们证明OrtT作为一种独立的毒素，在与氨基酸和DNA合成相关的应激过程中会降低生长。
