Modeling rhl quorum-sensing regulation on rhamnolipid production by Pseudomonas aeruginosa.

The effect of autoinducer PAI2 on rhamnolipid (RL) production by Pseudomonas aeruginosa was evaluated using an rhlI null mutant of PAO1 added with PAI2 at various concentrations. A model has also been developed to describe the production kinetics regulated by the rhl quorum-sensing system in three steps: First, PAI2 combines with RhlR protein. Second, the activated complex RhlR:PAI2 triggers the transcription (and expression) of the rhlAB operon that encodes for rhamnosyltransferase. Finally, the enzyme catalyzes the RL synthesis. The model describes fairly well the experimental results/profiles from three different studies (this and two others reported in the literature). The overall picture predicted by the model is as follows: The induced enzyme synthesis proceeds at the highest rate following PAI2 addition. The rate decreases with time as the autoinducer is degraded. The enzyme concentration nonetheless continues to increase until reaching the plateau at the exhaustion of autoinducer. Higher added PAI2 concentrations thus give not only higher initial enzyme synthesis rates but also longer induced synthesis. As the enzyme concentration increases with time, the RL production rate also increases, resulting in an accelerated rise in RL concentrations initially. The increase in RL concentrations becomes linear at the exhaustion of PAI2. The best-fit model parameters obtained also provided important insights. To complex half of the intracellular RhlR proteins would require 1.61 microM PAI2, about half of the PAI2 concentration obtained in the stationary-phase culture of wild-type PAO1. On the other hand, to activate the rhamnosyltransferase synthesis at half of its maximum rate would require the binding of 39% of RhlR with PAI2. The maximum RL production rate of the culture was found to be 0.042 g/L.h, and the fully induced culture would require at least 1.61 h to synthesize the enzyme to the necessary level for producing RL at half of the maximum rate.