Figure 3a. Quorum-sensing systems in Gram-positive micro-organisms. The oligopeptide signal precursor () is processed and exported by ABC-transporters. When a threshold level is reached, the extracellular peptides () bind to a histidine kinase receptor (HK) in the cell membrane, resulting in autophosphorylation of the histidine phosphotransferase domain. The phosphate (P) is transferred to the cognate response regulator (RR), which can then bind to target DNA and alter gene expression.