The lassystem, the first to be described, comprises the lasR gene coding for the LasRregulatory protein and the lasI gene coding for an enzyme, autoinducersynthase, LasI, which is necessary for the synthesis of a type of AHL: N-(3-oxododecanoyl) -L-homoserine lactone (3-oxo-C12-HSL). As in V. ficheri,3-oxo-C12-HSL have the property of easily crossing bacterial membranes and thusconstitute a real means of communication between bacteria. When the3-oxo-C12-HSL concentration reaches a critical, high bacterial concentrationthreshold, one AHL molecule binds to two LasR proteins to form an activatorcomplex for the transcription of several genes. This activation is triggeredsynchronously throughout the bacterial population at the junction between theexponential growth phase and the beginning of the stationary phase. The genesactivated by this system include:• lasB, lasA, aprA coding respectively for two elastases, and for analkaline protease each contributing to the destruction of the lung tissues,• toxA, coding for an ADP-ribosylating exotoxin,• xcpR and xcpP, coding for proteins of the type II secretion machineryrequired for export.these factors outside the bacterium,• and lasI,allowing a rapid increase in the synthesis of 3-oxo-C12-HSL and therefore anamplification of signal by self-induction.
The second system rhl operates according to thesame scheme and comprises the rhlR gene, coding for the regulatory protein RhlRand the rhlI gene, coding for an autoinducer synthase enzyme, RhlI required forthe synthesis of a second type of AHL: N-butyryl L-homoserine lactone (C4-HSL).The RhlR-C4-HS1 complex controls the expression of the rhlAB operon requiredfor the production of rhamnolipid, and the expression of a series of genesincluding lasB, lasA, aprA, and rhlI. several other QS-regulated genes wereidentified in three studies using a transcriptional analysis approach of the P.aeruginosa genome and DNA microarray technology.
The majority oftranscripts regulated by QS would translate proteins whose functions arehypothetical or even unknown. Ninety-seven genes are found in common in allthree studies. To confirm the validity of these results, it will be necessaryto identify and characterize these proteins Despitethe structural similarity between the activator/inductor couples (LasR/LasI andRhlR/RhlI) of the two systems, none of the components of one system can replacethose of the other. However, there are interactions between these two systems.The LasR-3-oxo-C12-HSL complex activates the transcription of rhlR and rhlI.
Inaddition, 3-oxo-C12-HSL may compete with C4-HSL for the RhlI binding site andthus may act as an antagonism of the rhl system. So, there is a hierarchybetween these two systems with the las system which positively regulates thesystem rhl