title: Genetic control of quorum-sensing signal turnover in Agrobacterium tumefaciens.
abstract: A signal turnover system is an essential component of many genetic regulatory mechanisms. The best-known example is the ubiquitin-dependent protein degradation system that exists in many organisms. We found that Agrobacterium tumefaciens adopts a unique signal turnover system to control exiting from a quorum-sensing mode. A. tumefaciens regulates Ti plasmid conjugal transfer by a quorum-sensing signal, N-3-oxo-octanoyl homoserine lactone (3OC8HSL), also known as Agrobacterium autoinducer. By using Tn5 mutagenesis and a functional cloning approach, we identified two genes that are involved in switching from a conjugal quorum-sensing mode to a nonconjugal mode at the onset of stationary phase. First, we located attJ, which codes for an IclR-type suppressor that regulates the second gene attM. The latter encodes a homologue of N-acylhomoserine lactone (AHL)-lactonase. Mass spectrometry analysis shows that the enzyme encoded by attM is an AHL-lactonase that hydrolyzes the lactone ring of 3OC8HSL. In wild-type A. tumefaciens, attM expression is initially suppressed by AttJ but significantly elevated at the stationary phase accompanied a sharp decline in 3OC8HSL. DNA gel retardation analysis shows that AttJ specifically binds to the promoter that controls AHL-lactonase expression. Mutation of attJ resulted in constitutive production of AHL-lactonase that abolishes 3OC8HSL accumulation and Ti plasmid transfer. These data suggest that A. tumefaciens has a sophisticated multicomponent quorum-sensing signal turnover system, allowing the cell to sense a change in growth and adjust cellular activities accordingly.
authors.affiliation: Laboratory of Biosignals and Bioengineering, Institute of Molecular Agrobiology, National University of Singapore, 1 Research Link, Singapore 117604.
authors.forename: Hai-Bao
authors.initials: HB
authors.lastname: Zhang
authors.forename: Lian-Hui
authors.initials: LH
authors.lastname: Wang
authors.forename: Lian-Hui
authors.initials: LH
authors.lastname: Zhang
mesh_terms: D015107:4-Butyrolactone / Q000031:analogs & derivatives* / Q000378:metabolism; D016960:Agrobacterium tumefaciens / Q000235:genetics* / Q000378:metabolism*; D001483:Base Sequence; D002265:Carboxylic Ester Hydrolases / Q000235:genetics; D003001:Cloning, Molecular; D015964:Gene Expression Regulation, Bacterial; D008969:Molecular Sequence Data; D009876:Operon; D015398:Signal Transduction
chemical_list: C088386:homoserine lactone; D002265:Carboxylic Ester Hydrolases; C432003:N-acyl homoserine lactonase; D015107:4-Butyrolactone