The lacI Gene: Its Role in lac Operon Control and Its Use as a Genetic System

Abstract

INTRODUCTION
One of the lasting contributions of the work of Jacob and Monod is that we can now envision the previously abstract notion of genetic control in terms of concrete, physical models. This is principally due to the conceptual breakthrough made during their studies of phage λ immunity and the enzymes concerned with lactose metabolism in Escherichia coli (Jacob and Monod 1961), namely, the idea that genes can be under the control of other “regulatory genes,” and that regulation is carried out by an intermediate which is often the direct product of the regulatory gene itself.

A dominant theme in biology is that nature favors selective rather than instructive modes of development and control. This is evident not only for evolution and embryogenesis, but also for antibody synthesis and now gene control. For instance, rather than instructing a cell to acquire the capacity to synthesize a specific antibody molecule, a particular antigen is recognized, thereby selecting for the proliferation of cells in which this capacity preexisted. Jacob and Monod showed that, in a similar fashion, lactose does not dictate to cells how to synthesize an enzyme with the appropriate stereospecific properties; instead, recognition of lactose at the molecular level permits selection of enhanced expression of several of the many genes already present which are involved in the metabolism of different sugars.1

In the specific case of the lac operon, the product of the I gene, the repressor, was postulated to bind to a small region of the DNA, termed the...