How are the vast majority of the genes kept inactive, and how can a gene that is switched off be switched on? In short, how are genes controlled?
The answer involves a process called gene regulation. This process is more efficient than feedback inhibition because the enzymes involved in any unnecessary metabolic step are simply not present (thus saving the cell energy). Click here for another outline of gene regulation
Most genes contain 2 regions: a coding region and a regulatory region. It is the function of the regulatory region to act as an on/off switch for the coding region.
Within the regulatory region is a nucleotide sequence termed the promoter. RNA polymerase binds most efficiently on the promoter sequence and transcribes downstream in the 5' to 3' direction.
In Prokaryotic gene regulation the enzymes for a particular metabolic pathway are grouped together (polycistronic) and can be controlled by a single regulatory region. In addition to a promoter the regulatory region has a nucleotide sequence called the operator which is the binding site for repressor molecules. This combination of promoter, operator, and several related structural genes is called an operon (see below)
The main parts of a bacterial operon are (a) the grouping of one or more structural genes coding for enzymes or other proteins (1, 2, ...n) under the control of a single regulatory region and (b) a regulatory region that consists of a promoter (P) and an operator (O).
The operator is the site where a repressor molecule (usually a metabolite) may bind preventing RNA polymerase from beginning transcription.