FIGURE 3.

Quorum sensing system and biosurfactant biosynthesis regulation in Pseudomonas aeruginosa: increasing cell density activates the three major quorum sensing mechanisms, the las, rhl, and PQS. The las system has transcriptional activators, LasR and LasI (produces autoinducer 3-oxo-C₁₂HSL). The 3-oxo-C₁₂HSL activates LasR by complex formation leading to the induction of various genes. The 3-oxo-C₁₂HSL–LasR complex also activates PQS system, where PQS binds to its receptor, PqsR, thus regulating the expression of various virulence factors and biofilm formation. The rhl system has rhlA and rhlB that produces rhamnosyltransferase, arranged with transcriptional activators, rhlR and rhlI (produce autoinducer C₄–HSL). The autoinducer C₄–HSL acts and forms complex with RhlR, which induces gene expression of various factors. The C₄–HSL–RhlR complex also activates the rhlA promoter, leading to the transcription of the rhlAB genes to produce rhamnolipids. Thick blue lines depict the sequential reactions for the formation of mono- and di-rhamnolipids, also showing the biosynthesis routes to produce their dTDP–rhamnose and fatty acid precursors. Green dashed lines depict the transport of various gene factors outside the cell membrane.