Transcription of mqsRA and mazEF operons in response to amino acid starvation. Mupirocin (MUP) was added to cultures of BW25113 (wt) and BW25113 ∆relBEF to inhibit isoleucine tRNA synthetase and induce stringent response. RNA was extracted at timepoints −1 (before addition of MUP), 15, 60, and 120 min; 10-μg aliquots were subjected to northern blotting and hybridized with probes mqsR (A) and mazF (B). The full-length mqsRA and mazEF transcripts are marked by arrowheads (◄). A longer mqsRA transcript can be seen above the marked band and has been described previously .