Expression of ptxR and its effect on toxA and regA expression during the growth cycle of Pseudomonas aeruginosa strain PAO1.

The expression of the toxA and regA genes in Pseudomonas aeruginosa is negatively regulated by iron at the transcriptional level. We have previously described ptxR, an exotoxin A regulatory gene which appears to enhance toxA expression through regA. In this study, we have tried to determine if ptxR expression correlates with its effect on toxA and regA expression throughout the growth cycle of P. aeruginosa strain PAO1. This was done using Northern blot hybridization experiments (with toxA, regA, and ptxR probes), and ptxR transcriptional fusion studies. To avoid problems related to the presence of multiple copies of ptxR in PAO1, we have constructed a PAO1 strain (PAO1-XR) that carries only two ptxR genes in its chromosome. Our results showed that when PAO1-XR was grown in iron-limited conditions, the increase in exotoxin A activity and the accumulation of toxA mRNA appeared at about mid- to late-exponential phase. A similar increase in the accumulation of regA mRNA was detected. Both regA transcripts, T1 and T2, were enhanced in PAO1-XR. In iron-sufficient medium, neither toxA nor regA mRNA was detected at any time point in the growth cycle of PAO1-XR. In contrast, the accumulation of ptxR mRNA was detected throughout the growth cycle of PAO1-XR under both iron-deficient and iron-sufficient conditions. The presence of iron in the growth medium also had no effect on the level of beta-galactosidase activity produced by a ptxR-lacZ fusion in PAO1. These results suggest that (i) the enhancement in toxA expression by ptxR correlates with the enhancement in regA expression; (ii) ptxR affects the expression of the regA P1 and P2 promoters; (iii) ptxR expression precedes its effect on toxA and regA expression; and (iv) unlike toxA and regA, the overall expression of ptxR throughout the growth cycle of PAO1 is not negatively regulated by iron.