CBS and infection: In light of previous studies showing that Mtb infection of macrophages induces upregulation of host H2S-producing enzymes CBS [7] and CSE [6], which leads to excessive levels of H2S to exacerbate disease, it was important to determine whether Mtb senses and responds to physiological levels of H2S. Here, we report a mechanism whereby DosS(Fe3+) directly binds H2S leading to increased DosS autokinase activity and DosR phosphorylation, and that during infection Mtb senses endogenous levels of H2S to induce key DosR regulon genes.