title: Isovaleryl-homoserine lactone, an unusual branched-chain quorum-sensing signal from the soybean symbiont Bradyrhizobium japonicum.
abstract: Many species of Proteobacteria communicate by using LuxI-LuxR-type quorum-sensing systems that produce and detect acyl-homoserine lactone (acyl-HSL) signals. Most of the known signals are straight-chain fatty acyl-HSLs, and evidence indicates that LuxI homologs prefer fatty acid-acyl carrier protein (ACP) over fatty acyl-CoA as the acyl substrate for signal synthesis. Two related LuxI homologs, RpaI and BtaI from Rhodopseudomonas palustris and photosynthetic stem-nodulating bradyrhizobia, direct production of the aryl-HSLs p-coumaroyl-HSL and cinnamoyl-HSL, respectively. Here we report that BjaI from the soybean symbiont Bradyrhizobium japonicum USDA110 is closely related to RpaI and BtaI and catalyzes the synthesis of isovaleryl-HSL (IV-HSL), a branched-chain fatty acyl-HSL. We show that IV-HSL induces expression of bjaI, and in this way IV-HSL functions like many other acyl-HSL quorum-sensing signals. Purified histidine-tagged BjaI was an IV-HSL synthase, which was active with isovaleryl-CoA but not detectably so with isovaleryl-ACP. This suggests that the RpaI-BtaI-BjaI subfamily of acyl-HSL synthases may use CoA- rather than ACP-linked substrates for acyl-HSL synthesis. The bjaI-linked bjaR(1) gene is involved in the response to IV-HSL, and BjaR(1) is sensitive to IV-HSL at concentrations as low as 10 pM. Low but sufficient levels of IV-HSL (about 5 nM) accumulate in B. japonicum culture fluid. The low levels of IV-HSL synthesis have likely contributed to the fact that the quorum-sensing signal from this bacterium has not been described elsewhere.
authors.affiliation: Department of Microbiology, University of Washington, Seattle, WA 98195, USA.
authors.forename: Andrea
authors.initials: A
authors.lastname: Lindemann
authors.forename: Gabriella
authors.initials: G
authors.lastname: Pessi
authors.forename: Amy L
authors.initials: AL
authors.lastname: Schaefer
authors.forename: Margrith E
authors.initials: ME
authors.lastname: Mattmann
authors.forename: Quin H
authors.initials: QH
authors.lastname: Christensen
authors.forename: Aline
authors.initials: A
authors.lastname: Kessler
authors.forename: Hauke
authors.initials: H
authors.lastname: Hennecke
authors.forename: Helen E
authors.initials: HE
authors.lastname: Blackwell
authors.forename: E Peter
authors.initials: EP
authors.lastname: Greenberg
authors.forename: Caroline S
authors.initials: CS
authors.lastname: Harwood
mesh_terms: D015107:4-Butyrolactone / Q000031:analogs & derivatives* / Q000378:metabolism; D000214:Acyl Coenzyme A / Q000378:metabolism; D000595:Amino Acid Sequence; D001426:Bacterial Proteins / Q000235:genetics / Q000378:metabolism; D020369:Bradyrhizobium / Q000378:metabolism* / Q000502:physiology; D016000:Cluster Analysis; D019295:Computational Biology; D015964:Gene Expression Regulation, Bacterial / Q000502:physiology*; D010802:Phylogeny; D053038:Quorum Sensing / Q000502:physiology*; D020133:Reverse Transcriptase Polymerase Chain Reaction; D016415:Sequence Alignment; D013025:Glycine max / Q000382:microbiology*; D014157:Transcription Factors / Q000235:genetics / Q000378:metabolism
chemical_list: D000214:Acyl Coenzyme A; D001426:Bacterial Proteins; C085427:LuxI protein, Bacteria; D014157:Transcription Factors; C088386:homoserine lactone; C017447:isovaleryl-coenzyme A; D015107:4-Butyrolactone