E. coli mazEF is a stress-induced toxin-antitoxin module. MazF is a stable endoribonuclease that cleaves at ACA sites in RNAs. Here we further studied the Stress-induced Translation Machinery (STM) generated by MazF under stress. STM is composed of MazF processed mRNAs and selective ribosomes that specifically translate the processed mRNAs. We found that MazF cleaves only ACA sites located in the open reading frames of processed mRNAs, while out-of-frame ACAs are resistant.