Secretion and Transcription: New Regulatory Mechanisms of Virulence Control By ESX-1

Pathogenic mycobacteria are a global health burden. Mycobacterium tuberculosis accounts the most pathogenic mycobacterial infections in the world, but nontuberculous mycobacteria (NTM) infections are on the rise. M. tb and M. marinum, an NTM, share conserved mechanisms of pathogenesis, including the ESX-1 secretion system. We use M. marinum as a model for M. tb to study ESX-1 secretion and pathogenesis. Here, we use molecular genetics, biochemistry, and proteo-genetic approaches to interrogate integral aspects of mycobacterial pathobiology with specific emphasis on regulation of the ESX-1 secretion system. Using these approaches, we identified a novel ESX-1 protein substrate, MMAR_2894, that is unique to M. marinum. MMAR_2894 is important for ESX-1 secretion in vitro but is dispensable in vivo, suggesting there may be compensatory mechanisms to rescue loss of MMAR_2894. We also defined roles of additional ESX-1 substrates inside the mycobacterial cell. The EspE and EspF ESX-1 substrates are required for the lytic activity of ESX-1 outside the cell and they are negative regulators of gene expression inside the mycobacterial cell. Finally, we identified a novel transcriptional regulator of the ESX-1 system, EspN, that is required for pathogenesis and whose activity is infection-dependent. Collectively, our findings have expanded our understanding of ESX-1 regulation, emphasizing that this regulation is both multifaceted and complex, involving coordination of ESX-1 substrates and transcription factors themselves.