Atypical Mycobacterium abscessus BlaRI Ortholog Mediates Regulation of Energy Metabolism but Not β-Lactam Resistance

Mycobacterium abscessus (Mab) is highly drug resistant, and understanding regulation of antibiotic resistance is critical to future antibiotic development. Regulatory mechanisms controlling Mab's β-lactamase (Bla Mab) that mediates β-lactam resistance remain unknown. S. aureus encodes a prototypical protease-mediated two-component system BlaRI regulating the β-lactamase BlaZ. BlaR binds extracellular β-lactams, activating an intracellular peptidase domain which cleaves BlaI to derepress blaZ. Mycobacterium tuberculosis (Mtb) encodes homologs of BlaRI (which we will denote as BlaIR to reflect the inverted gene order in mycobacteria) that regulate not only the Mtb β-lactamase, blaC, but also additional genes related to respiration. We identified orthologs of blaIR Mtb in Mab and hypothesized that they regulate bla Mab. Surprisingly, neither deletion of blaIR Mab nor overexpression of only blaI Mab altered bla Mab expression or β-lactam susceptibility. However, BlaI Mab did bind to conserved motifs upstream of several Mab genes involved in respiration, yielding a putative regulon that partially overlapped with BlaI Mtb. Prompted by evidence that respiration inhibitors including clofazimine induce the BlaI regulon in Mtb, we found that clofazimine triggers induction of blaIR Mab and its downstream regulon. Highlighting an important role for BlaIR Mab in adapting to disruptions in energy metabolism, constitutive repression of the BlaI Mab regulon rendered Mab highly susceptible to clofazimine. In addition to our unexpected findings that BlaIR Mab does not regulate β-lactam resistance, this study highlights the novel role of mycobacterial BlaRI-type regulators in regulating electron transport and respiration.