Oron-Gottesman Adi, Sauert Martina, Moll Isabella, Engelberg-Kulka Hanna
Department of Microbiology and Molecular Genetics, IMRIC, The Hebrew University-Hadassah Medical School, Jerusalem, Israel.
Department of Microbiology, Max F. Perutz Laboratories, Center for Molecular Biology, Immunobiology and Genetics, University of Vienna, Vienna, Austria.
mBio. 2016 Nov 15;7(6):e01855-16. doi: 10.1128/mBio.01855-16.
Escherichia coli mazEF is an extensively studied stress-induced toxin-antitoxin (TA) system. The toxin MazF is an endoribonuclease that cleaves RNAs at ACA sites. Thereby, under stress, the induced MazF generates a stress-induced translation machinery (STM), composed of MazF-processed mRNAs and selective ribosomes that specifically translate the processed mRNAs. Here, we further characterized the STM system, finding that MazF cleaves only ACA sites located in the open reading frames of processed mRNAs, while out-of-frame ACAs are resistant. This in-frame ACA cleavage of MazF seems to depend on MazF binding to an extracellular-death-factor (EDF)-like element in ribosomal protein bS1 (bacterial S1), apparently causing MazF to be part of STM ribosomes. Furthermore, due to the in-frame MazF cleavage of ACAs under stress, a bias occurs in the reading of the genetic code causing the amino acid threonine to be encoded only by its synonym codon ACC, ACU, or ACG, instead of by ACA.
The genetic code is a universal characteristic of all living organisms. It defines the set of rules by which nucleotide triplets specify which amino acid will be incorporated into a protein. Our results represent the first existing report on a stress-induced bias in the reading of the genetic code. We found that in E. coli, under stress, the amino acid threonine is encoded only by its synonym codon ACC, ACU, or ACG, instead of by ACA. This is because under stress, MazF generates a stress-induced translation machinery (STM) in which MazF cleaves in-frame ACA sites of the processed mRNAs.
大肠杆菌mazEF是一个经过广泛研究的应激诱导毒素-抗毒素（TA）系统。毒素MazF是一种内切核糖核酸酶，可在ACA位点切割RNA。因此，在应激状态下，诱导产生的MazF会生成一种应激诱导翻译机制（STM），该机制由MazF加工的mRNA和特异性翻译这些加工后mRNA的选择性核糖体组成。在此，我们进一步对STM系统进行了表征，发现MazF仅切割加工后mRNA开放阅读框中的ACA位点，而框外ACA具有抗性。MazF的这种框内ACA切割似乎取决于MazF与核糖体蛋白bS1（细菌S1）中类似细胞外死亡因子（EDF）的元件结合，这显然使MazF成为STM核糖体的一部分。此外，由于应激状态下MazF对ACA的框内切割，在遗传密码的解读中出现了偏差，导致苏氨酸仅由其同义密码子ACC、ACU或ACG编码，而不是ACA。
遗传密码是所有生物体的一个普遍特征。它定义了核苷酸三联体指定将哪种氨基酸掺入蛋白质的一组规则。我们的结果代表了关于应激诱导遗传密码解读偏差的首份现有报告。我们发现，在大肠杆菌中应激状态下，苏氨酸仅由其同义密码子ACC、ACU或ACG编码，而不是ACA。这是因为在应激状态下，MazF会生成一种应激诱导翻译机制（STM），其中MazF会切割加工后mRNA的框内ACA位点。
