Gene regulation is the process in which the expression of genes is activated or inhibited. This is an essential process within all organisms as they have finite resources and thus must use their resources economically by synthesising proteins only when they are needed. Moreover, not all proteins are required by all cells, and therefore gene regulation helps to inhibit their expression.
Before we begin, here are some terms which you should familiarise yourself with.
Bacteria such as E. coli need amino acids such as Tryptophan to survive, Tryptophan is an amino acid that E. coli can ingest from the environment. E. coli can also synthesize tryptophan using enzymes that are encoded by five genes. Five structural genes are next to each other in what is called the tryptophan (trp) operon.
If tryptophan is not plentiful / is absent in E. coli’s environment, there is not enough tryptophan to bind to the repressor protein, therefore the repressor protein goes back to its original shape and can no longer bind to the operator region of the operon. RNA Polymerase would be able to bind to the promoter region of the trp operon and transcribe the whole strand.
When lactose is absent, the repressor protein is active and binds to the operator, thereby physically blocking the RNA polymerase from binding to the promotor region and therefore transcription of the gene is inhibited.
When lactose is present, it can bind to the repressor protein inducing a conformation change and thereby inactivating it so that it can no longer bind to the operator allowing for RNA polymerase to bind to the promotor region and catalyse transcription of the gene.