WhiB6 regulation of ESX-1 gene expression is controlled by a negative feedback loop inMycobacterium marinum

Abstract

ESX (ESAT-6 system) export systems play diverse roles across mycobacterial species. Interestingly, genetic disruption of ESX systems in different species does not result in an accumulation of protein substrates in the mycobacterial cell. However, the mechanisms underlying this observation are elusive. We hypothesized that the levels of ESX substrates were regulated by a feedback-control mechanism, linking the levels of substrates to the secretory status of ESX systems. To test this hypothesis, we used a combination of genetic, transcriptomic, and proteomic approaches to define export-dependent mechanisms regulating the levels of ESX-1 substrates inMycobacterium marinum. WhiB6 is a transcription factor that regulates expression of genes encoding ESX-1 substrates. We found that, in the absence of the genes encoding conserved membrane components of the ESX-1 system, the expression of thewhiB6gene and genes encoding ESX-1 substrates were reduced. Accordingly, the levels of ESX-1 substrates were decreased, and WhiB6 was not detected inM. marinumstrains lacking genes encoding ESX-1 components. We demonstrated that, in the absence of EccCb1, a conserved ESX-1 component, substrate gene expression was restored by constitutive, but not native, expression of thewhiB6gene. Finally, we found that the loss of WhiB6 resulted in a virulentM. marinumstrain with reduced ESX-1 secretion. Together, our findings demonstrate that the levels of ESX-1 substrates inM. marinumare fine-tuned by negative feedback control, linking the expression of thewhiB6gene to the presence, not the functionality, of the ESX-1 membrane complex.