General stress response of Bacillus subtilis and other bacteria

One of the strongest and most noticeable responses of a Bacillus subtilis cell to a range of stress and starvation conditions is the dramatic induction of a large number of general stress proteins. The alternative sigma factor σB is responsible for the induction of the genes encoding these general stress proteins that occurs following heat, ethanol, salt or acid stress, or during energy depletion. σB was detected more than 20 years ago by Richard Losick and William Haldenwang as the first alternative sigma factor of bacteria, but interest in σB declined after it was realized that σB is not involved in sporulation. It later turned out that σB,whose activity itself is tightly controlled, is absolutely required for the induction of this regulon, not only in B. subtilis, but also in other Gram-positive bacteria. These findings may have been responsible for the recent revival of interest in σB. This chapter summarizes the current information on this σB response including the latest results on the signal transduction pathways, the structure of the regulon and its physiological role. More than 150 general stress proteins/genes belong to this σB regulon, which is believed to provide the non-growing cell with a non-specific, multiple and preventive stress resistance. σB-dependent stress proteins are involved in non-specific protection against oxidative stress and also protect cells against heat, acid, alkaline or osmotic stress. A cell in the transition from a growing to a non-growing state induced by energy depletion will be equipped with a comprehensive stress resistance machine to protect it against future stress. The protection against oxidative stress may be an essential part of this response. In addition, preloading of cells with σB-dependent stress proteins, induced by mild heat or salt stress, will protect cells against a severe, potentially lethal, future stress. Both the specific protection against an acute emerging stress, as well as the non-specific, prospective protection against future stress, are adaptive functions crucial for surviving stress and starvation in nature. We suggest that the σB response is one essential component of a survival strategy that ensures survival in a quiescent, vegetative state as an alternative to sporulation. The role of σB in related Gram-positive bacteria (including cyanobacteria) with special emphasis on pathogenic bacteria is discussed.