Role of metal-dependent regulation of ESX-3 secretion in intracellular survival of Mycobacterium tuberculosis

Abstract

More people die every year from Mycobacterium tuberculosis infection than from infection by any other bacterial pathogen.Type VII secretion systems (T7SS) are used by both environmental and pathogenic mycobacteria to secrete proteins across theircomplex cell envelope. In the nonpathogen Mycobacterium smegmatis, the ESX-1 T7SS plays a role in conjugation, and theESX-3 T7SS is involved in metal homeostasis. In M. tuberculosis, these secretion systems have taken on roles in virulence, andthey also are targets of the host immune response. ESX-3 secretes a heterodimer composed of EsxG (TB9.8) and EsxH (TB10.4),which impairs phagosome maturation in macrophages and is essential for virulence in mice. Given the importance of EsxG andEsxH during infection, we examined their regulation. With M. tuberculosis, the secretion of EsxG and EsxH was regulated in responseto iron and zinc, in accordance with the previously described transcriptional response of the esx-3 locus to these metals.While iron regulated the esx-3 expression in both M. tuberculosis and M. smegmatis, there is a significant difference in the dynamicsof this regulation. In M. smegmatis, the esx-3 locus behaved like other iron-regulated genes such as mbtB. In M. tuberculosis,both iron and zinc modestly repressed esx-3 expression. Diminished secretion of EsxG and EsxH in response to these metalsaltered the interaction of M. tuberculosis with macrophages, leading to impaired intracellular M. tuberculosis survival. Ourfindings detail the regulatory differences of esx-3 in M. tuberculosis and M. smegmatis and demonstrate the importance of metal-dependent regulation of ESX-3 for virulence in M. tuberculosis.