2.7 The Control Regions of Genes
It is essential that the cell be able to regulate the relative amounts of proteins and enzymes that are synthesized according to the needs of cell in its environment. The process is very well studied in bacteria and an example of one such regulatory region and how it works was worked out by Jacob and Monod and is shown in the figure. E. coli makes the proteins and enzymes that are needed to metabolize lactose only if lactose is present in the environment, and actually prefers to use glucose if it is available. Whether or not these genes are transcribed is under control of a repressor protein and several other factors, including cyclic AMP. The gene for the repressor protein lies upstream from the lactose control region and, in the absence of lactose, the repressor (as a tetramer) binds to a gene segment called the operator and prevents RNA polymerase from binding at the adjacent promoter site. If lactose is present, it binds to repressor and lowers the binding affinity of the repressor for the operator site. An enhanced reading of the lactose genes occurs upon binding of CAP protein (catabolite activator protein) and cyclic AMP leading to an active transcription of the genes and enabling the cell to metabolize lactose. The level of cyclic AMP in the cell is regulated by glucose. Thus, when glucose concentration is high, cyclic AMP concentration declines, and the lactose genes will not be read. We will use parts of this same system later in cloning experiments. The lacZ gene is the structural gene for b-galactosidase, the enzyme that hydrolyzes lactose to glucose plus galactose. It can accumulate to equal as much as 10% of total soluble, cellular protein following induction.
Figure 13. Control of transcription of genes involved in lactose metabolism