Toxins MazF and MqsR cleave <i>Escherichia coli</i> ribosomal RNA precursors at multiple sites
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The endoribonuclease toxins of the <i>E. coli</i> toxin-antitoxin systems arrest bacterial growth and protein synthesis by targeting cellular mRNAs. As an exception, <i>E. coli</i> MazF was reported to cleave also 16S rRNA at a single site and separate an anti-Shine-Dalgarno sequence-containing RNA fragment from the ribosome. We noticed extensive rRNA fragmentation in response to induction of the toxins MazF and MqsR, which suggested that these toxins can cleave rRNA at multiple sites. We adapted differential RNA-sequencing to map the toxin-cleaved 5′- and 3′-ends. Our results show that the MazF and MqsR cleavage sites are located within structured rRNA regions and, therefore, are not accessible in assembled ribosomes. Most of the rRNA fragments are located in the aberrant ribosomal subunits that accumulate in response to toxin induction and contain unprocessed rRNA precursors. We did not detect MazF- or MqsR-cleaved rRNA in the stationary phase bacteria and in assembled ribosomes. Thus, we conclude that MazF and MqsR cleave rRNA precursors before the ribosomes are assembled and potentially facilitate the decay of surplus rRNA transcripts during stress.
大肠杆菌（E. coli）毒素-抗毒素系统中的核糖核酸内切酶毒素，可通过靶向细胞内mRNA抑制细菌生长与蛋白质合成。作为特例，已有研究报道大肠杆菌MazF还可在单个位点切割16S rRNA，并将含有抗Shine-Dalgarno（SD）序列的RNA片段从核糖体上分离下来。我们观察到，在诱导表达毒素MazF与MqsR后，会出现广泛的rRNA片段化现象，这提示上述两类毒素可在多个位点切割rRNA。我们采用差分RNA测序（differential RNA-sequencing）技术，绘制了毒素切割产生的5'端与3'端图谱。研究结果显示，MazF与MqsR的切割位点位于具有高级结构的rRNA区域内，因此在组装完成的核糖体中无法被接触到。绝大多数rRNA片段存在于因毒素诱导而积累的异常核糖体亚基中，这类亚基包含未加工的rRNA前体。我们并未在稳定期细菌以及组装完成的核糖体中检测到被MazF或MqsR切割的rRNA。综上，我们认为MazF与MqsR可在核糖体组装完成前切割rRNA前体，并可能在应激过程中促进过剩rRNA转录本的降解。
