ABSTRACT
Since their earliest appearance on earth, living organisms have been exposed to environments which altered gradually and sometimes rapidly. Long-term adaptation to environmental change has been possible through processes of mutation and natural selection. Another more immediate means of protection against environmental adversity involves the induction of stress gene expression as a pleiotropic response to stress. The products of stress genes play key roles in cellular homeostasis through protective and adaptive functions, enabling organisms to survive different types of environmental onslaught. These stress responses are orchestrated at the gene level and characteristically can be induced both quickly and repeatedly. They are thus particularly valuable to organisms living under rapidly altering environments. Moreover, a large proportion of these responses are seen in all organisms from bacteria to humans and are likely to have originated soon after primitive cells appeared on earth. With the dramatic advances in molecular biological techniques in recent years, it has been possible
to conduct detailed investigations of the underlying inductive mechanisms. Although the stress of DNA damage elicits SOS responses in bacteria and the activation of cell division control genes by p53 in eukaryotes (see Chapter 8), there are many environmental threats that have been shown to bring about the activation of other specific stress genes. These include temperature shifts, oxidative stress, ultraviolet radiation, water and salt stress, toxic metals, anaerobiosis and pathogenicity.
