Genes and Proteins Involved in Induction

Expression of the quinolone resistance gene is increased by quinolones, but unlike induction of some other genes, the bacterial SOS system is not involved and no box is found upstream. Nonetheless, at least 205 bp of upstream sequence is required for induction to take place. An upstream sequence bound to beads trapped potential binding proteins from cell extracts that were identified by mass spectrometry as Dps, Fis, Ihf, Lrp, CysB, and YjhU. To further elucidate their role, a reporter plasmid linking the upstream sequence to was introduced into cells of the Keio collection with single-gene deletions and screened for expression. Mutants in and had decreased induction, while induction in a mutant was increased and , , , , and other mutants showed no change. The essential upstream sequence contains potential binding sites for Ihf and DnaA. A deletion could not be tested because it provides essential functions in cell replication; however, increased expression decreased induction while decreased expression enhanced it, implying a role for DnaA as a repressor. In a mobility shift assay, purified IhfA, IhfB, and DnaA proteins (but not CysB) were shown to bind to the upstream segment. Induction decreased in a quinolone-resistant mutant, indicating that GyrA also has a role. Thus, quinolones acting through proteins DnaA, GyrA, IhfA, and IhfB regulate expression of .