Structural genes: There are many structural genes associated in an operon system. These direct the synthesis of the mRNA and govern the sequence of amino acids in a protein molecule. Each structural gene might produce a particular kind of protein or all structural genes might regulate the production of a single protein. The activities of the structural gene (synthesis) are controlled by the promoter and operator of the operon system. The most well studied structural genes (z.y and a) are those of the Lac operon system in Escherichia coli.
Operator gene: The operator gene is situated adjacent to the first structural gene. It switches on or switches off the functioning of the structural gene (protein synthesis). In case a structural gene has to be suppressed, a repressor attaches itself to the operator to form an Operator – repressor complex. In the case of protein synthesis, the operator – repressor complex prevents the transcription by blocking the movement of RNA polymerase.
Promoter gene: The promoter gene is continuous with the operator gene and is believed to lie left to it. It is suggested that RNA polymerase binds to the promoter site during transcription. Three regions have been recognized in the promoter site. These are a) recognition site, initial binding site and the mRNA initiation site (operator site).
(i) Recognition site: Also called the cga site (catabolic gene activator site), it consists of certain palindromic sequences of DNA. These symmetrical sections of DNA are recognised by proteins having symmetrically placed sub units. This site, also called the CRP site (cyclic AMP receptor protein site) binds a CR protein to the promoter gene and thus facilitates the binding of the enzyme RNA polymerase. It has been found that in E.coli CRP combines with cAMP (cyclic adenosine monophosphate) forming a CRP – cAMP complex which binds to the promoter enhancing the binding of RNA polymerase and activates transcription. This regulation is called positive control.
(ii) Initial binding site: This consists of seven bases (DNA) to which the RNA polymerase binds.
(iii) RNA initiation site: The site where transcription begins is called initiation site. This is the region overlapping with the operator region.
Regulator gene: The regulator gene directs the activity of the operator gene by producing inhibitor proteins called repressors. This repressor protein binds to the operator gene and blocks the path of RNA polymerase, thus preventing transcription. If an inducer is present in the system, it binds to the repressor which undergoes conformational change and becomes inactive. As the inactive repressor cannot bind to the promoter, the strucutral genes get activated and protein synthesis continues.