Regulation of gene expression refers to controlling the timing of appearance and amount of protein that is synthesised. Gene expression is regulated differently in eukaryotes and prokaryotes.
In prokaryotes, the predominant site of gene regulation is the initiation of transcription. In the transcription unit, the ability of RNA polymerase to recognise and bind to a promoter, that is, the start sites, is regulated by its interaction with accessory or regulatory proteins from regulatory genes. The accessibility of promoter regions on the DNA in many cases is regulated by the interaction of proteins with sequences called operators. The operator region lies adjacent to the promoter. In most cases, a repressor protein binds to the operator sequence due to which the operator is inactivated and hence transcription does not takes place. But when repressor protein is removed, it activates the operator and hence transcription occurs.
An operon consists of a structural gene which codes for several proteins and is regulated by common promoter and regulatory genes. Each operon has its specific operator and specific repressor. For example, the lac operator is present only in the lac operon and it interacts specifically with lac repressor. The lac operon consists of one regulatory gene, or i gene, and three structural genes: lac z, lac y and lac a. The i gene codes for the repressor. The structural genes code for enzymes required for the metabolism of lactose. The lac z gene codes for beta-galactosidase, which hydrolyses lactose into galactose and glucose. The lac y gene codes for permease, which increases the permeability of the cell to lactose while the lac a gene codes for transacetylase, whose function is unknown. The switching on and off of the operon is controlled by lactose and is hence the inducer. Since lactose needs a small amount of permease enzyme to enter the cell, a very low level expression of the lac operon is continuously required. The lac operon functions in the following manner: The repressor protein is continuously synthesised from the gene. The repressor, in its active state, binds to the operator sequence, thus preventing RNA polymerase from binding to the promoter site and transcribing the operon. In the presence of the inducer lactose or allolactose, the repressor is inactivated and transcription proceeds. This regulation of the lac operon by the repressor is called negative regulation. In this way, the lac operon demonstrates how gene expression is regulated in an organism.