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.

A sensor protein ChvG is part of a chromosomally encoded twocomponent regulatory system ChvG͞ChvI that is important for the virulence of Agrobacterium tumefaciens. However, it is not clear what genes ChvG regulates or what signal(s) it senses. In this communication, we demonstrate that ChvG is involved in the regulation of acid-inducible genes, including aopB and katA, residing on the circular and linear chromosomes, respectively, and the tumor-inducing (Ti)-plasmid-harbored vir genes, virB and virE. ChvG was absolutely required for the expression of aopB and very important for the expression of virB and virE. However, it was responsible only for the responsiveness of katA and, to a limited extent, the vir genes to acidic pH. ChvG appears to play a role in katA expression by repressing katA at neutral pH. ChvG had no effect on the expression of two genes that were not acid-inducible. Because ChvG regulates unlinked acid-inducible genes encoding different functions in different ways, we hypothesize that ChvG is a global sensor protein that can directly or indirectly sense extracellular acidity. We also analyzed the re-sequenced chvG and found that ChvG is more homologous to its Sinorhizobium meliloti counterpart ExoS than was previously thought. Full-length ChvG is conserved in members of the ␣-proteobacteria, whereas only the C-terminal kinase domain is conserved in other bacteria. Sensing acidity appears to enable Agrobacterium to coordinate its coping with the environment of wounded plants to cause tumors.

SummaryMost plant pathogenic bacteria adopt the type III secretion systems to secrete virulence factors and/ or avirulence gene products, which trigger the plant hypersensitive response (HR) and the oxidative burst with hydrogen peroxide (H 2 O 2 ) as the main component. However, the soil-borne plant pathogen Agrobacterium tumefaciens uses the type IV secretion pathway to deliver its oncogenic T-DNA that causes crown gall tumours on many plant species. A. tumefaciens does not elicit a typical HR on those plants. Here, we report that inactivation of one of A. tumefaciens catalases (which converts H 2 O 2 to H 2 O and O 2 ) by a transposon insertion highly attenuated the bacterial ability to cause tumours on plants and to tolerate H 2 O 2 toxicity, but not the bacterial viability in the absence of exogenous H 2 O 2 . This provides the ®rst genetic evidence that the Agrobacterium ±plant interaction involves a plant defence response, such as H 2 O 2 production, and that catalase is a virulence factor for a plant pathogen.