Most of the information about stress response in prokaryotes comes from studies in bacteria. Bacteria developed a set of measures to maintain homeostasis in unstable environments. In response to a particular stress, bacteria switch their transcription to an alternative mode that better serves survival in the changed environment. They do it by using alternative sigma subunits of RNA polymerase (RNAP). Some sigma factors induce a transcription switch to a particular program that adjusts bacteria to specific stress, such as heat. Other sigma factors are responsible for the general stress response that protects the bacteria from various stresses. In addition to alternative transcriptional modes, bacteria use other mechanisms to cope with stress including DNA damage stress response and programmed cell death in response to severe stress.