ESAT-6 Secretion-Independent Impact of ESX-1 Genes espF and espG1 on Virulence of Mycobacterium tuberculosis

Methods. To elucidate the role of EspF/G(1), whose orthologs in Mycobacterium marinum and Mycobacterium smegmatis are reportedly involved in EsxA/B secretion, we constructed 3 M. tuberculosis knockout strains deleted for espF, espG(1) or the segment corresponding to the combined RD1(bcg)-RD1(mic) region of bacille Calmette-Guerin (BCG) and Mycobacterium microti, which also contains espF and espG(1). Results. Analysis of these strains revealed that, unlike observations with the model organisms M. smegmatis or M. marinum, disruption of espF and espG(1) in M. tuberculosis did not impact the secretion and T cell recognition of EsxA/B but still caused severe attenuation. Conclusions. The separation of the 2 ESX-1-connected phenotypes (ie, EsxA/B secretion and virulence) indicates that EsxA/B secretion is not the only readout for a functional ESX-1 system and suggests that other processes involving EspF/G(1) also play important roles in ESX-1-mediated pathogenicity.