topA and gyrB gene expression mirror predicted effects of supercoiling. topA encodes topoisomerase I, andgyrB encodes the β subunit of gyrase. (A)Escherichia coli MG1655 was cultured to optical density (OD) 0.4 and sampled (no treatment), or induced with novobiocin, salt, or novobiocin + salt. Treated cultures were sampled 10 min postinduction. (B) E. coli was cultured in a separate experiment to OD 0.6 and treated with pefloxacin. (C) Predicted changes in DNA supercoiling from salt or gyrase inhibitors based on the literature.