The antagonistic transcription factors, EspM and EspN, regulate the ESX-1 secretion system inM. marinum

Bacterial pathogens use protein secretion systems to transport virulence factors and regulate gene expression. Among pathogenic mycobacteria, including Mycobacterium tuberculosis andMycobacterium marinum, ESX-1 (ESAT-6 system 1) secretion is crucial for host interaction. Secretion of protein substrates by the ESX-1 secretion system disrupts phagosomes, allowing mycobacteria cytoplasmic access during macrophage infections. Deletion or mutation of the ESX-1 system attenuates mycobacterial pathogens. Pathogenic mycobacteria respond to the presence or absence of the ESX-1 system in the cytoplasmic membrane by altering transcription. Under laboratory conditions, the EspM repressor and WhiB6 activator control transcription of specific ESX-1 responsive genes, including the ESX-1 substrate genes. However, deleting theespMorwhiB6genes does not phenocopy the deletion of the ESX-1 substrate genes during macrophage infection byM. marinum. In this study, we identified EspN, a critical transcription factor whose activity is masked by the EspM repressor under laboratory conditions. In the absence of EspM, EspN activates transcription ofwhiB6and ESX-1 genes both during laboratory growth and during macrophage infection. EspN is also independently required forM. marinumgrowth within and cytolysis of macrophages, similar to the ESX-1 genes, and for disease burden in a zebrafish larval model of infection. These findings suggest that EspN and EspM coordinate to counterbalance the regulation of the ESX-1 system and support mycobacterial pathogenesis.