Figure 3. Cell-to-cell signaling systems. Cell-to-cell signaling systems are composed of two genes. The I gene encodes an autoinducer synthase and the R gene encodes a transcriptional activator protein (R-protein). The autoinducer synthase is responsible for the synthesis of an autoinducer molecule (AI), which crosses the cell membrane. With increasing cell-density the intracellular concentration of AI reaches a threshold level, and the AI then binds to the transcriptional activator. The complex R-protein/AI activates the expression of specific target genes.