The regulation of the genes in prokaryotes and eukaryotes determines the cell's growth, differential development, and responses to changing conditions. In eukaryotes, for example, hormone-producing cells in the human pituitary gland normally only produce growth hormone during childhood and adolescence. The same cells remain in the pituitary in adulthood, but do not function. The activation and suppression of these and other genes is necessary for normal and healthy functioning of the individual. An example for prokaryotes is provided by the ability of a bacterium to use different sources of nitrogen--direct incorporation of atmospheric nitrogen, the incorporation and breakdown of ammonia, or the removal of nitrogen groups from amino acids. Different enzymes are required for these processes and, since the direct use of nitrogen gas is less energetically costly to the bacterium, there needs to b a mechanism for switching on and of the different nitrogen acquisition...