Abstract
BackgroundQuorum-sensing regulation of gene expression in Pseudomonas aeruginosa is complex. Two interconnected acyl-homoserine lactone (acyl-HSL) signal-receptor pairs, 3-oxo-dodecanoyl-HSL-LasR and butanoyl-HSL-RhlR, regulate more than 300 genes. The induction of most of the genes is delayed during growth of P. aeruginosa in complex medium, cannot be advanced by addition of exogenous signal, and requires additional regulatory components. Many of these late genes can be induced by addition of signals early by using specific media conditions. While several factors super-regulate the quorum receptors, others may co-regulate target promoters or may affect expression posttranscriptionally.ResultsTo better understand the contributions of super-regulation and co-regulation to quorum-sensing gene expression, and to better understand the general structure of the quorum sensing network, we ectopically expressed the two receptors (in the presence of their cognate signals) and another component that affects quorum sensing, the stationary phase sigma factor RpoS, early in growth. We determined the effect on target gene expression by microarray and real-time PCR analysis. Our results show that many target genes (e.g. lasB and hcnABC) are directly responsive to receptor protein levels. Most genes (e.g. lasA, lecA, and phnAB), however, are not significantly affected, although at least some of these genes are directly regulated by quorum sensing. The majority of promoters advanced by RhlR appeared to be regulated directly, which allowed us to build a RhlR consensus sequence.ConclusionThe direct responsiveness of many quorum sensing target genes to receptor protein levels early in growth confirms the role of super-regulation in quorum sensing gene expression. The observation that the induction of most target genes is not affected by signal or receptor protein levels indicates that either target promoters are co-regulated by other transcription factors, or that expression is controlled posttranscriptionally. This architecture permits the integration of multiple signaling pathways resulting in quorum responses that require a "quorum" but are otherwise highly adaptable and receptive to environmental conditions.
Highlights
Quorum-sensing regulation of gene expression in Pseudomonas aeruginosa is complex
LasR, RhIR and RpoS levels increase in the stationary phase of growth Our previous transcriptome analyses [8,15], in addition to other studies with reporter fusions [20,21,22], showed that the transcript levels of lasR, rhlR, and rpoS increase at the transition from logarithmic to stationary phase during batch culture growth of P. aeruginosa
Expression strategy If LasR, RhlR or RpoS levels were limiting the expression of QS-controlled genes during logarithmic growth of wildtype P. aeruginosa, early expression of these factors should advance quorum-controlled gene expression from stationary to logarithmic phase
Summary
Quorum-sensing regulation of gene expression in Pseudomonas aeruginosa is complex. Two interconnected acyl-homoserine lactone (acyl-HSL) signal-receptor pairs, 3-oxododecanoyl-HSL-LasR and butanoyl-HSL-RhlR, regulate more than 300 genes. The induction of most of the genes is delayed during growth of P. aeruginosa in complex medium, cannot be advanced by addition of exogenous signal, and requires additional regulatory components. In Pseudomonas aeruginosa, two acyl-homoserine lactone quorum sensing (acyl-HSL QS) systems, LasR-LasI and RhlR-RhlI, control the expression of partially overlapping sets of genes. In our study [8] we noted that the expression of most quorum-controlled genes increased in the stationary phase of growth and could not be advanced to logarithmic phase by the addition of exogenous signal, confirming earlier observations with individual quorum-controlled genes [9,10,11] This suggested that the activation of most quorum-controlled genes is not triggered by the accumulation of signal, and seems to require additional factors
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