An illustration with three images showing gene expression in prokaryotes: First image showing a prokaryotic gene is marked with the following parts: Lac Repressor gene - Codes for repressor protein Promoter - RNA polymerase binds here Operator - Repressor binds here Lac genes Second image is marked with the following texts: When lactose is not present, the repressor protein binds to the operating region. This blocks RNA polymerase from transcribing the lac genes. RNA polymerase Third image is marked with the following texts: When lactose is present, it binds to the repressor. This causes the release of the repressor which then moves away from the operating region. Transcription can now take place. mRNA Lactose