As mentioned, Gram-negative bacteria primarily use acylated homoserine lactones (AHLs) as autoinducer molecules.  The minimum quorum sensing circuit in Gram-negative bacteria consists of a protein that synthesizes an AHL and a second, different protein that detects it and causes a change in gene expression.  First identified in V. fischeri, these two such proteins are LuxI and LuxR, respectively.  Other Gram-negative bacteria use LuxI-like and LuxR-like proteins (homologs), suggesting a high degree of evolutionary conservation.  However, among Gram-negatives, the LuxI/LuxI-type circuit has been modified in different species.  Described in more detail below, these modifications reflect bacterial adaptations to grow and respond to particular niche environments.