Acyl chain length of N-(3-oxoacyl)-L-homoserine lactones has an effect on the regulation of lasB expression in the mexB deletion strain. (a) Individual cultures of KG7403 (ΔlasI ΔrhlI PlasB-gfp) and KG703 (ΔlasI ΔrhlI ΔmexB PlasB-gfp) were grown in LB medium containing 5 μM 3-oxo-Cn-HSL, respectively. Transcription of lasB was determined by measurement of the fluorescence intensity (arbitrary units) depending on the amount of green-fluorescence protein (GFP) derived from PlasB-gfp (emission at 490 nm; excitation at 510 nm). (b) Individual culture supernatants of KG7403 (ΔlasI ΔrhlI PlasB-gfp) and KG7503 (ΔlasI ΔrhlI ΔmexB PlasB-gfp) grown in LB medium containing 5 μM 3-oxo-Cn-HSL, respectively, were assayed for LasB elastase activity. LasB activity was measured as the rate of hydrolysis of FRET-AGLA by the LasB protein. Hydrolysis rates were determined by measurement of fluorescence intensity depending on the N-methylanthranilyl derivative derived from an elastase substrate; emission at 355 nm and excitation at 460 nm. Open bars, KG7403; closed bars, KG7503. The data represent mean values of three independent experiments. Error bars represent the standard errors of the means.