Background:The whiB7 gene is a multidrug resistance determinant in mycobacteria. Results: WhiB7 autoregulates its own promoter in response to both antibiotics (in a structure-and target-independent manner) and to redox changes. Conclusion: WhiB7 links cell metabolism, redox homeostasis, and antibiotic resistance. Significance: Understanding antibiotic-induced metabolic stress responses in mycobacteria may lead to therapies for mycobacterial diseases, including tuberculosis.

Tuberculosis therapeutic options are limited by the high intrinsic antibiotic resistance of Mycobacterium tuberculosis. The putative transcriptional regulator WhiB7 is crucial for the activation of systems that provide resistance to diverse antibiotic classes. Here, we used in vitro run-off, two-hybrid assays, as well as mutagenic, complementation and protein pull-down experiments, to characterize WhiB7 as an auto-regulatory, redox-sensitive transcriptional activator in Mycobacterium smegmatis. We provide the first direct biochemical proof that a WhiB protein promotes transcription and also demonstrate that this activity is sensitive to oxidation (diamide). Its partner protein for transcriptional activation was identified as SigA, the primary sigma factor subunit of RNA polymerase. Residues required for the interaction mapped to region 4 of SigA (including R515H) or adjacent domains of WhiB7 (including E63D). WhiB7’s ability to provide a specific spectrum of antibiotic-resistance was dependent on these residues as well as its C-terminal AT-hook module that binds to an AT-rich motif immediately upstream of the −35 hexamer recognized by SigA. These experimentally established constrains, combined with protein structure predictions, were used to generate a working model of the WhiB7–SigA-promoter complex. Inhibitors preventing WhiB7 interactions could allow the use of previously ineffective antibiotics for treatment of mycobacterial diseases.

Current tuberculosis treatment regimens are notoriously limited, lengthy and becoming increasingly ineffective due to the emergence of drug-resistant mutant strains of Mycobacterium tuberculosis. The intrinsic resistance of M. tuberculosis to the majority of available drugs relies both on the impermeability of its cell envelope, and its ability to activate specific genes and physiological states. WhiB7 is a transcriptional regulatory protein underlying this adaptive process. Transcription of the whiB7 gene is upregulated in response to a variety of antibiotics having different structures and targets, as well as in response to metabolic signals. The whiB7 regulon activates various systems of intrinsic drug resistance involving antibiotic export, antibiotic inactivation (by chemical modifications of the drug or its target) and significant changes to thiol redox balance. Drugs have been identified that inactivate resistance determinants in the whiB7 regulon, thereby potentiating the activities of diverse antibiotics against M. tuberculosis.

Background: WhiB7 is essential for antibiotic resistance in M. tuberculosis. Results: WhiB7 requires conserved residues, including a redox-sensitive center and DNA-binding motif, to coordinate transcription of species-specific drug resistance genes in diverse Actinobacteria. Conclusion: WhiB7 activates species-specific drug resistance genes in Actinobacteria. Significance: Understanding WhiB7 activity may allow the development of drugs that sensitize bacteria to antibiotics.

Combinations of antibiotics, each individually effective against Mycobacterium abscessus, are routinely coadministered based on the concept that this minimizes the spread of antibiotic resistance. However, our in vitro data contradict this assumption and instead document antagonistic interactions between two antibiotics (clarithromycin and amikacin) used to treat M. abscessus infections. Clinically relevant concentrations of clarithromycin induced increased resistance to both amikacin and itself. The induction of resistance was dependent on whiB7, a transcriptional activator of intrinsic antibiotic resistance that is induced by exposure to many different antibiotics. In M. abscessus, the deletion of whiB7 (MAB_3508c) resulted in increased sensitivity to a broad range of antibiotics. WhiB7 was required for transcriptional activation of genes that confer resistance to three commonly used anti-M. abscessus drugs: clarithromycin, amikacin, and tigecycline. The whiB7-dependent gene that conferred macrolide resistance was identified as erm(41) (MAB_2297), which encodes a ribosomal methyltransferase. The whiB7-dependent gene contributing to amikacin resistance was eis2 (MAB_4532c), which encodes a Gcn5-related N-acetyltransferase (GNAT). Transcription of whiB7 and the resistance genes in its regulon was inducible by subinhibitory concentrations of clarithromycin but not by amikacin. Thus, exposure to clarithromycin, or likely any whiB7-inducing antibiotic, may antagonize the activities of amikacin and other drugs. This has important implications for the management of M. abscessus infections, both in cystic fibrosis (CF) and non-CF patients.