tRNA synthetase mutants of Escherichia coli K-12 are resistant to the gyrase inhibitor novobiocin

In previous studies we demonstrated that mutations in the genes cysB, cysE, and cls (nov) affect resistance of Escherichia coli to novobiocin (J. Rakonjac, M. Milic, and D. J. Savic, Mol. Gen. Genet. 228:307-311, 1991; R. Ivanisevic, M. Milic, D. Ajdic, J. Rakonjac, and D. J. Savic, J. Bacteriol. 177:1766-1771, 1995). In this work we expand this list with mutations in rpoN (the gene for RNA polymerase subunit sigma54) and the tRNA synthetase genes alaS, argS, ileS, and leuS. Similarly to resistance to the penicillin antibiotic mecillinam, resistance to novobiocin of tRNA synthetase mutants appears to depend upon the RelA-mediated stringent response. However, at this point the overlapping pathways of mecillinam and novobiocin resistance diverge. Under conditions of stringent response induction, either by the presence of tRNA synthetase mutations or by constitutive production of RelA protein, inactivation of the cls gene diminishes resistance to novobiocin but not to mecillinam.