Only the proteins that the bacteria Escherichia coli (E. coli) needs are synthesized at any given time. For example, when lactose is available in the environment, E. coli will express the lac genes that allow it to break down lactose to obtain energy. When lactose is not present, these genes are not expressed. Which prevents the lac genes in the DNA of E. coli from being expressed most of the time

The lac repressor

Explanation:

In prokaryotic cells, an operon is a set of genes under the control of a single promoter which are transcribed together as a single messenger RNA (mRNA). In Escherichia coli, the lac operon is an operon containing genes involved in lactose metabolism, which is expressed when lactose is present and glucose is absent. This lac repressor is a DNA-binding protein that binds to a specific DNA sequence called the operator, thereby inhibiting transcription of the lac operon when lactose is present. When lactose is present, this disaccharide is converted into allolactose, which acts as an inducer by binding to the repressor, thereby the repressor dissociates from the operator and it allows the expression of the lac genes.