The effects of the ampR mutation on QS las and rhl gene transcription. The alteration in the transcription of the QS systems in Alg− PAO1 (hatched bars), Alg− PAOampR (grey bars), Alg+ PDO300 (white bars) and Alg+ PDOampR (black bars) was monitored using four transcriptional fusions, PlasI-lacZ, PlasR-lacZ, PrhlI-lacZ and PrhlR-lacZ. The promoterless lacZ vector has a low basal level of activity of <20 Miller units.