The alternative sigma factor RpoN regulates the quorum sensing gene rhlI in Pseudomonas aeruginosa

School of Bio-technology and Bio-molecular Sciences, University of New South Wales, Sydney, NSW 2052, Australia.
FEMS Microbiology Letters (Impact Factor: 2.12). 04/2003; 220(2):187-95. DOI: 10.1016/S0378-1097(03)00097-1
Source: PubMed


The rhl quorum sensing (QS) circuit of Pseudomonas aeruginosa is known to regulate the expression of a number of virulence factors. This study investigates the regulation of rhlI, encoding the auto-inducer synthase RhlI responsible for the synthesis of N-butryl-L-homoserine lactone (BHL). A putative RpoN binding site was located upstream, in the promoter region of rhlI. Utilising a rhlI-lacZ transcriptional reporter, we demonstrate that under certain media conditions RpoN is a positive regulator of rhlI transcription. Measurements of BHL in extracted supernatant showed that the transcriptional patterns were reflected in the BHL levels, which were reduced in the rpoN mutant. Elastase and pyocyanin, known to be regulated by the rhl QS circuit, were shown to be reduced in a RpoN deficient strain. However, exogenous addition of BHL to the rpoN mutant did not restore these phenotypes suggesting that other regulatory factors apart from BHL are involved. Consistent with other rhl regulated phenotypes, we found that a rpoN mutant strain forms a biofilm that is different from that of the wild-type but similar to that displayed by a rhlI mutant.

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    • "The expression of genes encoding these cellular functions is modulated during biofilm development [7], [8]. The role of global regulator proteins on biofilm formation has been studied in different species, for example, HhA and RpoS in Escherichia coli, CcpA in Bacillus subtilis and Streptococcus mutans and RpoN in P. aeruginosa [9], [10], [11], [12]. "
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    • "In low cell densities, RpoN was shown to downregulate the expression of gacA, whilst the rpoN null mutant presents upregulation of lasI and lasR in P. aeruginosa (Heurlier et al., 2003). The rhl QS system is directly activated by RpoN at high cell density (Thompson et al., 2003). In addition, RpoN increasingly activates the expression of virulence-related vfr gene during exponential growth phase, Fig. 1. "
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    • "Overexpression of RsmA also resulted in reduced expression of the AHL synthase genes lasI and rhlI (Pessi et al., 2001). Furthermore, it has been shown that RpoN controls the expression of rhlI (Heurlier et al., 2003; Thompson et al., 2003). "
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