Fig. 6.

HHQ/PQS cell-to-cell communication model. 1, HHQ is synthesized and released by bacterial cells. 2, Extracellular HHQ is taken up by adjacent bacteria and converted into PQS, possibly in the periplasm. 3, PQS is released to act as a signaling molecule for other cells. 4, PQS activates target gene expression, such as the phz1 operon. Note that both HHQ availability and PqsH activity determine the final PQS concentration. In the experimental paradigm used to test the model, cell A and cell B were a lasR mutant and an mvfR mutant, respectively. In the case of a wild-type population, both the A and B cells are producing HHQ and PQS.