title: Ribosomal frameshifting and dual-target antiactivation restrict quorum-sensing-activated transfer of a mobile genetic element.
abstract: Symbiosis islands are integrative and conjugative mobile genetic elements that convert nonsymbiotic rhizobia into nitrogen-fixing symbionts of leguminous plants. Excision of the Mesorhizobium loti symbiosis island ICEMlSym(R7A) is indirectly activated by quorum sensing through TraR-dependent activation of the excisionase gene rdfS. Here we show that a +1 programmed ribosomal frameshift (PRF) fuses the coding sequences of two TraR-activated genes, msi172 and msi171, producing an activator of rdfS expression named Frameshifted excision activator (FseA). Mass-spectrometry and mutational analyses indicated that the PRF occurred through +1 slippage of the tRNA(phe) from UUU to UUC within a conserved msi172-encoded motif. FseA activated rdfS expression in the absence of ICEMlSym(R7A), suggesting that it directly activated rdfS transcription, despite being unrelated to any characterized DNA-binding proteins. Bacterial two-hybrid and gene-reporter assays demonstrated that FseA was also bound and inhibited by the ICEMlSym(R7A)-encoded quorum-sensing antiactivator QseM. Thus, activation of ICEMlSym(R7A) excision is counteracted by TraR antiactivation, ribosomal frameshifting, and FseA antiactivation. This robust suppression likely dampens the inherent biological noise present in the quorum-sensing autoinduction circuit and ensures that ICEMlSym(R7A) transfer only occurs in a subpopulation of cells in which both qseM expression is repressed and FseA is translated. The architecture of the ICEMlSym(R7A) transfer regulatory system provides an example of how a set of modular components have assembled through evolution to form a robust genetic toggle that regulates gene transcription and translation at both single-cell and cell-population levels.
authors: Departments of Microbiology and Immunology and Curtin Health Innovation Research Institute Biosciences Precinct, Faculty of Health Sciences, Curtin University, Bentley, WA 6102, Australia; and joshramsay@gmail.com.
authors: Joshua P
authors: JP
authors: Ramsay
authors: Departments of Microbiology and Immunology and.
authors: Laura G L
authors: LG
authors: Tester
authors: Departments of Microbiology and Immunology and.
authors: Anthony S
authors: AS
authors: Major
authors: Departments of Microbiology and Immunology and.
authors: John T
authors: JT
authors: Sullivan
authors: Biochemistry, University of Otago, Dunedin 9016, New Zealand;
authors: Christina D
authors: CD
authors: Edgar
authors: Biochemistry, University of Otago, Dunedin 9016, New Zealand;
authors: Torsten
authors: T
authors: Kleffmann
authors: Curtin Health Innovation Research Institute Biosciences Precinct, Faculty of Health Sciences, Curtin University, Bentley, WA 6102, Australia; and.
authors: Jackson R
authors: JR
authors: Patterson-House
authors: Curtin Health Innovation Research Institute Biosciences Precinct, Faculty of Health Sciences, Curtin University, Bentley, WA 6102, Australia; and.
authors: Drew A
authors: DA
authors: Hall
authors: Biochemistry, University of Otago, Dunedin 9016, New Zealand;
authors: Warren P
authors: WP
authors: Tate
authors: Department of Biological Sciences, University of Calgary, Calgary, AB T2N 1N4, Canada.
authors: Michael F
authors: MF
authors: Hynes
authors: Departments of Microbiology and Immunology and.
authors: Clive W
authors: CW
authors: Ronson
mesh_terms: D001483:Base Sequence; D001665:Binding Sites; D018965:Frameshifting, Ribosomal; D018014:Gene Transfer Techniques; D044404:Genomic Islands; D020071:Interspersed Repetitive Sequences; D013058:Mass Spectrometry; D059128:Mesorhizobium / Q000378:metabolism; D010944:Plants / Q000382:microbiology; D010957:Plasmids / Q000378:metabolism; D011401:Promoter Regions, Genetic; D014176:Protein Biosynthesis; D053038:Quorum Sensing; D012231:Rhizobium / Q000378:metabolism; D012270:Ribosomes / Q000737:chemistry / Q000648:ultrastructure*; D013559:Symbiosis; D014157:Transcription Factors; D014158:Transcription, Genetic; D020798:Two-Hybrid System Techniques; D001616:beta-Galactosidase / Q000378:metabolism
chemical_list: D014157:Transcription Factors; D001616:beta-Galactosidase
keywords: ICE; antiactivator; horizontal gene transfer; quorum sensing; ribosomal frameshift