Interestingly, in P. putida WCS358, ppoR expression shows substantial increase in the IBE5 ppuI AHL synthase mutant, indicating a QS system mediated repression of ppoR expression (Figure mTOR inhibitor 4e). The ppoR promoter levels in this genetic background were not restored to WCS358 wild-type levels by adding exogenously the four AHLs (3-oxo-C6-, 3-oxo-C8-, 3-oxo-C10- and 3-oxo-C12-HSL) produced by WCS358 (data not shown). The reason for this is not known and we cannot exclude that QS is particularly sensitive to growth phase and AHL concentration, thus exogenous addition of AHLs might not necessarily re-establish the conditions present in the wild-type
strain. The Selleck SRT1720 expression levels of ppoR in P. putida WCS358 IBE2 & IBE3 (ppuR and rsaL Ion Channel Ligand Library datasheet mutant respectively), and P. putida RD8MR3PPRI and RD8MR3PPRR although higher were not statistically significant (Figures 4e &4f). These results suggest that ppoR interaction with the endogenous QS systems
in these two P. putida strains may not be similar; in strain WCS358 negative regulation (albeit not very strong) of ppoR gene expression occurred in response to AHLs via a mechanism which could be independent of the cognate PpuR AHL sensor/regulator. ppoR expression is growth phase regulated In order to understand if PpoR expression patterns showed any correlation to its role in interacting with the endogenous QS system, ppoR expression levels
were measured as β-galactosidase activities at different growth phases. Importantly, it was observed for both P. putida WCS358 and RD8MR3 that at low cell densities ppoR transcription showed minimal expression but was found to increase sharply when the culture enters the logarithmic Fossariinae phase of growth (Figure 5). This pattern of expression level was maintained even in WCS358PPOR and RD8MR3PPOR indicating a lack of regulation by PpoR of its own expression. To find out if ppoR expression is under the control of well known growth phase dependent global regulators, its expression level was monitored in P. putida WCS358 MKO1 (rpoS), M17 (psrA) and IBE1 (gacA). There was no significant difference in the expression pattern levels of ppoR promoter in the three mutants when compared to wild type suggesting that these three global growth-phase regulators were not involved in modulating ppoR expression levels (Figure 5). It was therefore concluded that ppoR gene expression is stringently growth phase regulated via a yet unidentified regulator. Figure 5 ppoR promoter activities in wild type and various mutant strains of P. putida WCS358 and RD8MR3. Bacterial cultures were started with an initial inoculum of 5 × 106 CFU per ml in 20 ml of minimal medium (M9-Cas) and β-galactosidase activities were measured at different stages of growth.