Figure 4.1 (b). The Lac operon in bacteria when the system is turned on. When lactose molecules (green triangles) are present, they bind with the regulatory proteins. This changes the the physical shape of lactose-regulatory protein molecule and prevents it from binding with the DNA. (Note how the red-green complexes no longer have the appropriate shape to fit into the promoter region. The transcription enzymes are now free to bind to the promoter DNA sequence initiating the transctiption of messenger RNA. The mRNA transcripts are translated into enzymes that metabolize the lactose. Once all the lactose has been metabolized, the regulatory proteins bind to the DNA, prevent further transcription, and the system is shut off.