The Operon Revisited

Simple observation of living organisms allows one to conclude that the entire genetic potential of any given cell is never expressed at any given moment. In bacteria this phenomenon is termed genetic adaptation and it occurs in two apparently contradictory ways. The synthesis of enzymes responsible for a catabolic process generally appears to be turned on (induced) by the substrate (1) while formation of anabolic enzymes appears to be turned off (repressed) by the end product of each given pathway (2). These observations have led to the question-what mechanisms are responsible for genetic adaptation ? In 1961 Francois Jacob and Jacques Monod addressed themselves to this problem and, in an attempt to describe the mechanisms of genetic adaptation (and hopefully how gene regulation occurs in all organisms), they presented the (3, 4). This model in fact described two phenomena: (a) structural genes (those coding for proteins) express themselves, and (b) how expression is regulated. Some parts of the operon model have been so consistent with experimental results that they have become articles of faith for most biologists. These will be described only briefly. Other aspects of the model have either had to be modified with time or have been found to apply to only a few systems. These eases will be discussed in greater detail. This review will be restricted almost entirely to Escherichia coli and Salmonella typhimurium. The reader is strongly advised to search out other recent reviews (5-17).