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.

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

Dickeya zeae is the causal agent of the rice foot rot disease, but its mechanism of infection remains largely unknown. In this study, we identified and characterized a novel gene designated as zmsA. The gene encodes a large protein of 2,346 amino acids in length, which consists of multidomains arranged in the order of N-terminus, β-ketoacyl synthase, acyl transferase, acyl carrier protein, β-ketoacyl reductase, dehydratase. This multidomain structure and sequence alignment analysis suggest that ZmsA is a member of the polyketide synthase family. Mutation of zmsA abolished antimicrobial activity and attenuated the virulence of D. zeae. To determine the relationship between antimicrobial activity and virulence, active compounds were purified from D. zeae EC1 and were structurally characterized. This led to identification of two polyamino compounds, i.e., zeamine and zeamine II, that were phytotoxins and potent antibiotics. These results have established the essential role of ZmsA in zeamine biosynthesis and presented a new insight on the molecular mechanisms of D. zeae pathogenicity.

Recently, rhenium disulfide (ReS 2) monolayers were experimentally extracted by conventional mechanical exfoliation technique from as-grown ReS 2 crystals. Unlike the well-known members of transition metal dichalcogenides (TMDs), ReS 2 crystallizes in a stable distorted-1T structure and lacks an indirect to direct gap crossover. Here we present an experimental and theoretical study of the formation, energetics, and stability of the most prominent lattice defects in monolayer ReS 2. Experimentally, irradiation with 3-MeV He +2 ions was used to break the strong covalent bonds in ReS 2 flakes. Photoluminescence measurements showed that the luminescence from monolayers is mostly unchanged after highly energetic α particle irradiation. In order to understand the energetics of possible vacancies in ReS 2 we performed systematic first-principles calculations. Our calculations revealed that the formation of a single sulfur vacancy has the lowest formation energy in both Re and S rich conditions and a random distribution of such defects are energetically more preferable. Sulfur point defects do not result in any spin polarization whereas the creation of Re-containing point defects induce magnetization with a net magnetic moment of 1-3µ B. Experimentally observed easy formation of sulfur vacancies is in good agreement with first-principles calculations.