the expression of genes can be transcriptionally regulated such regulation is a critical feature of both eukaryotic and procaryotic organisms one of the classical systems used to investigate transcriptional regulation was originally investigated by Jacob Mano and Pari their studies involve the transcriptional regulation of the genes used to ferment lactose in the bacterium a sherikia coli three genes clustered together on the chromosome are required for the bacterium to utilize lactose these were determined to be the Lac Z Gene that codes for the enzyme beta galactosidase the Lac y Gene that codes for a lactose permease and the Lac a gene that codes for the enzyme galactoside transacetylase the Lac P region is the promoter that is needed to transcribe the Lac Z Lac Y and Lac a genes as a single polycistronic mRNA the Lac o region is an operator site that is involved in the transcriptional regulation of the Lac operon the unit consisting of the Lac promoter Lac operator and the Lac Z Lac Y and Lac a genes is called the Lac operon located near but not in tandem withd the Lac region is the Lac ey regulatory Gene that codes for an mRNA that is translated to produce a protein referred to as the Lac repressor in the absence of lack lactose inside the cell the Lac repressor protein is active in its active State the Lac repressor recognizes and binds to the Lac operator site when the Lac repressor is bound to the lack operator site RNA polymerase is prohibited from recognizing the Lac promoter and the Lac Z Lac Y and Lac a genes are not transcribed when lactose enters the cell a small amount of it is converted to allolactose Via beta galactosidase allolactose binds to the Lac repressor this causes a confirmational change in the Lac repressor protein that prevents it from binding to the lack operator site inactive Lac repressor is unable to bind to the Lac operator site without an active Lac repressor bound to the Lac operator site RNA polymerase is able to bind the promoter and a polycistronic messenger RNA is transcribed that includes the LAX Z Lac Y and Lac a genes this polycistronic messenger RNA is then translated to produce the beta galactosidase lactose permease and galactoside transacetylase proteins therefore the induction of the Lac operon enables the bacterium to efficiently transport lactose into the cell and metabolize it