ABSTRACT
All organisms, from bacteria to plants and animals, respond to hyperosmotic stress by accumulating intracellular solutes. These solutes increase the osmotic strength of the cytoplasm to counterbalance the high extracellular osmolarity. Amino acids constitute a major class of osmolytes, but only certain amino acids, such as taurine, ectoine, glutamate, and proline, are involved in cell volume regulation. Positive regulation is usually mediated by an activator protein binding to the deoxyribonucleic acid just upstream of the -35 region. Genes expressed in response to a common environmental stimulus constitute a stimulon. If the mechanism of gene activation involves a common regulatory protein, the network is termed a regulon. The heat shock regulon is a good example of this type of global regulation. An increase in environmental osmotic pressure triggers a number of physiological and biochemical changes in E. coli. When bacterial cells run out of nutrients, cell division stops and their metabolism adapts to the starvation conditions.
