The CssRS two-component regulatory system controls a general secretion stress response in Bacillus subtilis

Bacillus species are valuable producers of industrial enzymes and biopharmaceuticals, because they can secrete large quantities of high-quality proteins directly into the growth medium. This requires the concerted action of quality control factors, such as folding catalysts and 'cleaning proteases'. The expression of two important cleaning proteases, HtrA and HtrB, of Bacillus subtilis is controlled by the CssRS two-component regulatory system. The induced CssRS-dependent expression of htrA and htrB has been defined as a protein secretion stress response, because it can be triggered by high-level production of secreted alpha-amylases. It was not known whether translocation of these alpha-amylases across the membrane is required to trigger a secretion stress response or whether other secretory proteins can also activate this response. These studies show for the first time that the CssRS-dependent response is a general secretion stress response which can be triggered by both homologous and heterologous secretory proteins. As demonstrated by high-level production of a nontranslocated variant of the alpha-amylase, AmyQ, membrane translocation of secretory proteins is required to elicit this general protein secretion stress response. Studies with two other secretory reporter proteins, lipase A of B. subtilis and human interleukin-3, show that the intensity of the protein secretion stress response only partly reflects the production levels of the respective proteins. Importantly, degradation of human interleukin-3 by extracellular proteases has a major impact on the production level, but only a minor effect on the intensity of the secretion stress response.