Signal integration in gene expression. An activator and a repressor integrate two different environmental signals (the presence of glucose and of lactose) to control gene expression in the lactose-metabolic pathway of bacteria. (a) When lactose is absent, the Lac repressor binds the promoter and blocks RNA polymerase; there is no gene expression. (b) In the presence of lactose, the repressor binds a small signal molecule and separates from the DNA. The lac genes are now transcribed at a low, basal level. The presence of glucose keeps the activator in a nonfunctional state. (c) In the absence of glucose and presence of lactose, the activator binds a different small signal molecule, which causes it to bind DNA and recruit RNA polymerase for high-level gene expression.