Abstract
Regulation by a repressor protein is the mechanism selected when, in the organism''s natural environment, there is low demand for expression of the regulated structural genes. Regulation by an activator protein is selected when there is high demand for expression of the regulated structural genes. These general conclusions are useful in relating physiological function to underlying molecular determinants in a wide variety of systems [in microbes and eukaryotes] that includes repressible biosynthetic pathways, inducible biosynthetic enzymes, inducible drug resistance, prophage induction and inducible catabolic pathways, for which a special case of this prediction previously was reported.