Expression of the Resistance-Nodulation-Cell Division Pump AdeIJK in Acinetobacter baumannii Is Regulated by AdeN, a TetR-Type Regulator

Abstract

Resistance-nodulation-division efflux system AdeIJK contributes to intrinsic resistance to various drug classes in Acinetobacter baumannii . By whole-genome sequencing, we have identified in clinical isolate BM4587 the adeN gene, located 813 kbp upstream from adeIJK , which encodes a TetR transcriptional regulator. In one-step mutant BM4666 overexpressing adeIJK , the deletion of cytosine 582 (C ₅₈₂ ) in the 3′ portion of this gene was responsible for a frameshift mutation resulting in the deletion of the seven C-terminal residues. trans -Complementation of this BM4587 derivative with a plasmid expressing adeN restored antibiotic susceptibility to the host associated with the loss of adeJ overexpression. The inactivation of adeN in BM4587 led to a diminished susceptibility to antibiotics that are substrates for AdeIJK and to a 5-fold increase in adeJ expression. Taken together, these results indicate that AdeN represses AdeIJK expression. Quantitative reverse transcription-PCR (qRT-PCR) demonstrated that AdeN is constitutively expressed in BM4587 and does not regulate its own expression. Deletion of cytosine 582 and a 394-bp deletion of the 3′ part of adeN were found in independent one-step adeIJK -overexpressing mutants selected from clinical isolates BM4667 and BM4651, respectively. The corresponding alterations were located in the α9 helix, which is known to be involved in dimerization, a process essential for the activity of TetR regulators. The adeN gene was detected in all of the 30 A. baumannii strains tested and in Acinetobacter calcoaceticus , Acinetobacter nosocomialis , and Acinetobacter pittii .