STUDY HINTS

The problem of how genes are turned on and off at the proper time is a fascinating one. Geneticists still have a lot to learn in this area. Particularly in eukaryotic cells, the interaction between the nucleus and cytoplasm and the coordinated activation of functionally related genes at different times or in different tissues make development a complicated process, even in the simplest organisms. The operon model in prokaryotes is probably not directly analogous to the control systems of higher organisms, but it is an excellent place to begin getting the feel of the logic of regulatory systems.

There are two main types of operons: inducible and repressible. Inducible operons are normally turned off, since a repressor protein is bound at the operator site (O) and thus blocks RNA polymerase, which binds at the promoter site (P). Active transcription is blocked. Inducible operons can be activated by some substrate (the inducer) that binds with, and deforms, the repressor protein. Typically the inducer is some substance that is acted upon by the enzymes coded in the operon, so that the operon is turned on only when its products are needed by the cell. The repressor gene (i) may be quite distant from the operon, but the promoter and operator must be adjacent to the structural genes (SG1, SG2, etc.) that they control. The key is that the represser gene's diffusible product is synthesized in an active form.