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An operon is a cluster of bacterial genes along with an adjacent promoter that controls the transcription of those genes.When the genes in an operon are transcribed, a single mRNA is produced for all the genes in that operon. This mRNA is said to be polycistronic because it carries the information for more than one type of protein.

Prokaryotic as well as eukaryotic organisms possess different mechanisms to control the regulation of their genes. Cells need to be efficient and avoid wasting energy in the production of unnecessary proteins. Most of these mechanims take place at the transcriptional level. Gene regulation can be negative or positive. In negative regulation, a repressor molecule binds to the operator of an operon and terminates transcription. In positive regulation, an activator interacts with the RNA polymerase in the promoter region to initiate transcription.

The lactose operon is an example of negative regulation. When Escherichia coli cells are growing in a medium containing lactose as the only carbon source, some of the lactose is converted to allolactose. Allolactose acts as an inducer and turns on the lac operon. In the presence of glucose and the absence of lactose, allolactose is not produced and the lac operon is turned off. Since glucose is a readily available energy source, cells won't use lactose until they consume all the glucose. When both glucose and lactose are present in the medium, the operon is also off. In this case, the operon is regulated by catabolite repression.