Heat-shock and general stress response in Bacillus subtilis.

The induction of stress proteins is an important component of the adaptional network of a non-growing cell of Bacillus subtilis. A diverse range of stresses such as heat shock, salt stress, ethanol, starvation for oxygen or nutrients etc. induce the same set of proteins, called general stress proteins. Although the adaptive functions of these proteins are largely unknown, they are proposed to provide general and rather non-specific protection of the cell under these adverse conditions. In addition to these non-specific general stress proteins, all extracellular signals induce a set of specific stress proteins that may confer specific protection against a particular stress factor. In B. subtilis at least three different classes of heat-inducible genes can be defined by their common regulatory characteristics: Class I genes, as exemplified by the dnaK and groE operons, are most efficiently induced by heat stress. Their expression involves a sigma A-dependent promoter, an inverted repeat (called the CIRCE element) highly conserved among eubacteria, and probably a repressor interacting with the CIRCE element. The majority of general stress genes (class II, more than 40) are induced at sigma B-dependent promoters by different growth-inhibiting conditions. The activation of sigma B by stress or starvation is the crucial event in the induction of this large stress regulon. Only a few genes, including Ion, clpC, clpP, and ftsH, can respond to different stress factors independently of sigma B or CIRCE (class III). Stress induction of these genes occurs at promoters presumably recognized by sigma A and probably involves additional regulatory elements which remain to be defined.