title: Aryl-homoserine lactone quorum sensing in stem-nodulating photosynthetic bradyrhizobia.
abstract: Many Proteobacteria possess LuxI-LuxR-type quorum-sensing systems that produce and detect fatty acyl-homoserine lactone (HSL) signals. The photoheterotroph Rhodopseudomonas palustris is unusual in that it produces and detects an aryl-HSL, p-coumaroyl-HSL, and signal production requires an exogenous source of p-coumarate. A photosynthetic stem-nodulating member of the genus Bradyrhizobium produces a small molecule signal that elicits an R. palustris quorum-sensing response. Here, we show that this signal is cinnamoyl-HSL and that cinnamoyl-HSL is produced by the LuxI homolog BraI and detected by BraR. Cinnamoyl-HSL reaches concentrations on the order of 50 nM in cultures of stem-nodulating bradyrhizobia grown in the presence or absence of cinnamate. Acyl-HSLs often reach concentrations of 0.1-30 μM in bacterial cultures, and generally, LuxR-type receptors respond to signals in a concentration range from 5 to a few hundred nanomolar. Our stem-nodulating Bradyrhizobium strain responds to picomolar concentrations of cinnamoyl-HSL and thus, produces cinnamoyl-HSL in excess of the levels required for a signal response without an exogenous source of cinnamate. The ability of Bradyrhizobium to produce and respond to cinnamoyl-HSL shows that aryl-HSL production is not unique to R. palustris, that the aromatic acid substrate for aryl-HSL synthesis does not have to be supplied exogenously, and that some acyl-HSL quorum-sensing systems may function at very low signal production and response levels.
authors.affiliation: Department of Microbiology, University of Washington, Seattle, WA 98195, USA.
authors.forename: Nathan A
authors.initials: NA
authors.lastname: Ahlgren
authors.forename: Caroline S
authors.initials: CS
authors.lastname: Harwood
authors.forename: Amy L
authors.initials: AL
authors.lastname: Schaefer
authors.forename: Eric
authors.initials: E
authors.lastname: Giraud
authors.forename: E Peter
authors.initials: EP
authors.lastname: Greenberg
mesh_terms: D001426:Bacterial Proteins / Q000378:metabolism*; D020369:Bradyrhizobium / Q000166:cytology / Q000378:metabolism*; D007783:Lactones / Q000494:pharmacology*; D053038:Quorum Sensing / Q000187:drug effects / Q000502:physiology*; D012241:Rhodopseudomonas / Q000166:cytology / Q000378:metabolism*
chemical_list: D001426:Bacterial Proteins; D007783:Lactones