Pseudomonas aeruginosa uses a hierarchical quorum sensing (QS) network consisting of las, pqs and rhl regulatory elements to coordinate the expression of bacterial virulence genes. However, clinical isolates frequently contain loss-of-function mutations in the central las system. This motivated us to search for a mechanism that may functionally substitute las. Here we report identification of a new QS signal, IQS. Disruption of IQS biosynthesis paralyzes the pqs and rhl QS systems and attenuates bacterial virulence. Production of IQS is tightly controlled by las under normal culture conditions but is also activated by phosphate limitation, a common stressor that bacteria encounter during infections. Thus, these results have established an integrated QS system that connects the central las system and phosphate-stress response mechanism to the downstream pqs and rhl regulatory systems. Our discovery highlights the complexity of QS signaling systems and extends the gamut of QS and stress-response mechanisms.

In addition to producing lethal antibiotics, microorganisms may also use a new form of antagonistic mechanism in which signal molecules are exported to influence the gene expression and hence the ecological competence of their competitors. We report here the isolation and characterization of a novel signaling molecule, cis-2-dodecenoic acid (BDSF), from Burkholderia cenocepacia. BDSF is structurally similar to the diffusible signal factor (DSF) that is produced by the RpfF enzyme of Xanthomonas campestris. Deletion analysis demonstrated that Bcam0581, which encodes an RpfF homologue, was essential for BDSF production. The gene is highly conserved and widespread in the Burkholderia cepacia complex. Exogenous addition of BDSF restored the biofilm and extracellular polysaccharide production phenotypes of Xanthomonas campestris pv. campestris DSF-deficient mutants, highlighting its potential role in inter-species signaling. Further analyses showed that Candida albicans germ tube formation was strongly inhibited by either coculture with B. cenocepacia or by exogenous addition of physiological relevant levels of BDSF, whereas deletion of Bcam0581 abrogated the inhibitory ability of the bacterial pathogen. As B. cenocepacia and C. albicans are frequently encountered human pathogens, identification of the BDSF signal and its activity thus provides a new insight into the molecular grounds of their antagonistic interactions whose importance to microbial ecology and pathogenesis is now becoming evident.

Many bacterial pathogens produce diffusible signal factor (DSF)-type quorum sensing (QS) signals in modulation of virulence and biofilm formation. Previous work on Xanthomonas campestris showed that the RpfC/RpfG two-component system is involved in sensing and responding to DSF signals, but little is known in other microorganisms. Here we show that in Burkholderia cenocepacia the DSF-family signal cis-2-dodecenoic acid (BDSF) negatively controls the intracellular cyclic dimeric guanosine monophosphate (c-di-GMP) level through a receptor protein RpfR, which contains Per/Arnt/Sim (PAS)-GGDEF-EAL domains. RpfR regulates the same phenotypes as BDSF including swarming motility, biofilm formation, and virulence. In addition, the BDSF − mutant phenotypes could be rescued by in trans expression of RpfR, or its EAL domain that functions as a c-di-GMP phosphodiesterase. BDSF is shown to bind to the PAS domain of RpfR with high affinity and stimulates its phosphodiesterase activity through induction of allosteric conformational changes. Our work presents a unique and widely conserved DSF-family signal receptor that directly links the signal perception to c-di-GMP turnover in regulation of bacterial physiology.signal transduction | second messenger | molecular recognition | pathogenesis | cell-cell communication

Contents 1. Introduction 160 2. Chemistry of DSF-Family Signals 161 2.1. Detection of DSF-Family Signals 161 2.2. Purification and Structural Characterization of DSF-Family Signals 162 2.3. Synthesis of DSF-Family Signals 163 2.4. Nomenclature 163 2.5. Structural Features and Biological Activity 163 3. DSF Signaling Mechanisms in Xanthomonas campestris pv campestris 164 3.1. Enzymes Associated with DSF Biosynthesis 164 3.2. Dual Roles of the DSF Sensor Kinase RpfC 164 3.3. Response Regulator RpfG is a Novel DSF-Modulated c-di-GMP Degradation Enzyme 165 3.4. Clp is a Novel c-di-GMP Effector 166 3.5. Networking the DSF Signaling Pathways in Virulence Regulation 166 3.6. DSF-Dependent Biofilm Dispersal 167 4. DSF-Based QS Mechanisms in Other Organisms 168 4.1. Xylella fastidiosa 168 4.2. Xanthomonas axonopodis pv citri 168 4.3. Xanthomonas oryzae pv oryzae 168 4.4. Xanthomonas axonopodis pv glycines 169 4.5. Stenotrophomonas maltophilia 169 4.6. Burkholderia cepacia complex 169 4.7. Pseudomonas aeruginosa 170 5. Interaction with Other Signaling Mechanisms 170 6. Roles of DSF-Family Signals in Interspecies and Interkingdom Communication 171 7. Conclusions and Future Prospective 171 8. References 171