The gram-positive bacterium Bacillus subtilis secretes high levels of proteins into its environment. Most of these secretory proteins are exported from the cytoplasm in an unfolded state and have to fold efficiently after membrane translocation. As previously shown for ␣-amylases of Bacillus species, inefficient posttranslocational protein folding is potentially detrimental and stressful. In B. subtilis, this so-called secretion stress is sensed and combated by the CssRS two-component system. Two known members of the CssRS regulon are the htrA and htrB genes, encoding potential extracytoplasmic chaperone proteases for protein quality control. In the present study, we investigated whether high-level production of a secretory protein with two disulfide bonds, PhoA of Escherichia coli, induces secretion stress in B. subtilis. Our results show that E. coli PhoA production triggers a relatively moderate CssRS-dependent secretion stress response in B. subtilis. The intensity of this response is significantly increased in the absence of BdbC, which is a major determinant for posttranslocational folding of disulfide bond-containing proteins in B. subtilis. Our findings show that BdbC is required to limit the PhoA-induced secretion stress. This conclusion focuses interest on the BdbC-dependent folding pathway for biotechnological production of proteins with disulfide bonds in B. subtilis and related bacilli.The gram-positive eubacterium Bacillus subtilis is well known for its high capacity to secrete proteins, both in its natural habitat (the soil) and in biotechnological applications. Recent studies have suggested that most secretory proteins of this organism are translocated across the cytoplasmic membrane in an unfolded state via the Sec pathway (14,38,39,42). When the posttranslocational folding of secretory proteins, such as the ␣-amylase AmyQ from Bacillus amyloliquefaciens, is not efficient enough, extracytoplasmic accumulation of malfolded proteins can harm the cell (12, 33). B. subtilis uses the CssR-CssS two-component system to sense this stress situation and combat its effects (12). In general, two-component systems mediate the relay between specific stimuli and adequate transcriptional behavior (28, 36). The B. subtilis CssRS system consists of a typical sensor kinase, CssS, and its cognate cytoplasmic response regulator, CssR. Most likely, the membrane protein CssS senses stimuli at the membrane-cell wall interface (12). This information is then transmitted to CssR, using a phosphorylation-dephosphorylation relay. Finally, the CssR protein modulates the transcription of certain genes in response to the initial stimuli. So far, the CssRS two-component system is known to respond not only to stress induced by overproduction of ␣-amylases from different Bacillus species but also to heat (6, 12). Collectively, the conditions that trigger a CssRS-dependent cellular response have been termed secretion stress. At least four genes have been identified as members of the CssRS regulon: htrA, htrB, cssR, and cssS, with ...