Bacteria adapt to changes in their surroundings by using regulatory proteins to turn groups of genes on and off in response to various environmental signals.
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.
The operator is a short region of DNA that lies partially within the promoter and that interacts with a regulatory protein that controls the transcription of the operon.
The regulatory gene lacI produces an mRNA that produces a Lac repressor protein, which can bind to the operator of the lac operon.
In some texts, the lacI regulatory gene is called the lacI regulator gene. Regulatory genes are not necessarily close to the operons they affect.
The general term for the product of a regulatory gene is a regulatory protein. The Lac regulatory protein is called a repressor because it keeps RNA polymerase from transcribing the structural genes. Thus the Lac repressor inhibits transcription of the lac operon.
In the absence of lactose, the Lac repressor binds to the operator and keeps RNA polymerase from transcribing the lac genes.
It would be energetically wasteful for E. coli if the lac genes were expressed when lactose was not present.
The effect of the Lac repressor on the lac genes is referred to as negative regulation.
When lactose is present, the lac genes are expressed because allolactose binds to the Lac repressor protein and keeps it from binding to the lac operator.
Allolactose is an isomer of lactose. Small amounts of allolactose are formed when lactose enters E. coli. Allolactose binds to an allosteric site on the repressor protein causing a conformational change. As a result of this change, the repressor can no longer bind to the operator region and falls off. RNA polymerase can then bind to the promoter and transcribe the lac genes.
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