Three structural regions of this gene are all regulated by the same promotor region. The 3 enzymes produced by these structural regions are necessary to break down the milk sugar lactose as a source of food. When lactose is absent or has been digested away, there is none present to bind to the repressor protein and the repressor protein binds to the operator. So, for this to work the repressor must always be expressed. Five structural genes code for the enzymes needed for the production of tryptophan when tryptophan is present, it binds to the repressor protein so that the repressor CAN bind to the operator.
Transcription is blocked.
This prevents overproduction of tryptophan. Transcription proceeds, genes are transcribed, enzymes are synthesized, and the tryptophan is manufactured. The control of gene expression is much more complex in eukaryotes than in prokaryotes. It involves a large number of distinct but interacting mechanisms. Repressor and activator proteins are required, and these can involve regions quite far away from the gene.