Lambda repressor-operator complex. Molecular model of the lambda repressor protein (purple and blue) binding to a region of DNA (deoxyribonucleic acid, red and green) known as the lambda operator. The DNA and protein are both from the lambda phage, a bacterial virus (bacteriophage) that infects the Escherichia coli bacterium. The repressor protein acts to turn off transcription, and plays a role in regulating the life cycle of the lambda phage.