The Mycobacterium tuberculosisphosphate-sensing Pst/SenX3-RegX3 system regulates ESX-5 secretion to evade host immunity
The Mycobacterium tuberculosis Type VII secretion system ESX-5, which has been implicated in virulence, is activated at the transcriptional level by the phosphate starvation responsive Pst/SenX3-RegX3 signal transduction system. Deletion of pstA1, which encodes a Pst phosphate transporter component, causes constitutive activation of the response regulator RegX3, hyper-secretion of ESX-5 substrates and attenuation in the mouse infection model. We hypothesized that constitutive activation of ESX-5 secretion causes attenuation of the {Delta}pstA1 mutant. To test this, we uncoupled ESX-5 from regulation by RegX3. Using electrophoretic mobility shift assays, we defined a RegX3 binding site in the esx-5 locus. Deletion or mutation of the RegX3 binding site reversed hyper-secretion of the ESX-5 substrate EsxN by the {Delta}pstA1 mutant and abrogated induction of EsxN secretion in response to phosphate limitation by wild-type M. tuberculosis. Deletion of the esx-5 RegX3 binding site ({Delta}BS) suppressed attenuation of the {Delta}pstA1 mutant in Irgm1-/- mice, suggesting that constitutive ESX-5 secretion limits M. tuberculosis evasion of host immune responses that are independent of Irgm1. However, the {Delta}pstA1{Delta}BS mutant remained attenuated in both NOS2-/- and C57BL/6 mice, suggesting that factors other than ESX-5 secretion also contribute to attenuation of the {Delta}pstA1 mutant. In addition, a {Delta}pstA1{Delta}esxN mutant lacking the hyper-secreted ESX-5 substrate EsxN remained attenuated in Irgm1-/- mice, suggesting that ESX-5 substrates other than EsxN cause increased susceptibility to host immunity. Our data indicate that while M. tuberculosis requires ESX-5 for virulence, it tightly controls secretion of ESX-5 substrates to avoid elimination by host immune responses.