An influential model of prokaryotic gene expression by François Jacob and Jacques Monod (1910-1976) first published in 1960, and in which the distinctions were made between structural, operator and regulator genes. The operator genes determine when protein synthesis in the adjacent structural genes will occur and achieve this by interacting with the cell environment by means of regulator gene products called repressor molecules. The regulator genes, therefore, form the ‘boss’ of a larger system in that they serve to repress operator genes and thereby structural genes. The model was criticized for only allowing repressor or negative control. Subsequent models of gene expression included the mechanism of positive control by means of activator molecules. The problem with all these models is that the distinction between regulator and structural genes is not so clear as when it was first suggested by Jacob and Monod because examples of what were considered to be regulator genes turned out to be structural genes. Nevertheless, the regulator gene hypothesis remains a useful way of accounting for rapid evolutionary change by a mutation in a single regulator gene resulting in abrupt and major shifts in the timing of ontogeny for features held in common by ancestors and descendants, an issue of interest to evolutionary developmental biology.
See Evolutionary developmental biology, Gene, Morphogenetic field, Mutation (biology), Ontogeny, Prokaryote, Punctuated equilibrium and phyletic gradualism, Regulator (or regulatory) genes, Structural genes