Figure 5. Biosensor system designed for detection of AHL and AHL mimics. In this intracellular screen, the biosensor that detects active clones is inside the same cell as the metagenomic DNA. In the case of the intercellular screen, the biosensor that detects active clones is in a different cell than the metagenomic DNA. The biosensor detects small diffusible signal molecules that induce quorum sensing. When the signal molecules reach a sufficient concentration, they bind the LuxR transcriptional activator that activates the luxR promoter and induces the expression of target genes, in this case GFP. Luciferase and β-galactosidase genes have been also used as reporter genes.