title: A bacterial antirepressor with SH3 domain topology mimics operator DNA in sequestering the repressor DNA recognition helix.
abstract: Direct targeting of critical DNA-binding elements of a repressor by its cognate antirepressor is an effective means to sequester the repressor and remove a transcription initiation block. Structural descriptions for this, though often proposed for bacterial and phage repressor-antirepressor systems, are unavailable. Here, we describe the structural and functional basis of how the Myxococcus xanthus CarS antirepressor recognizes and neutralizes its cognate repressors to turn on a photo-inducible promoter. CarA and CarH repress the carB operon in the dark. CarS, produced in the light, physically interacts with the MerR-type winged-helix DNA-binding domain of these repressors leading to activation of carB. The NMR structure of CarS1, a functional CarS variant, reveals a five-stranded, antiparallel beta-sheet fold resembling SH3 domains, protein-protein interaction modules prevalent in eukaryotes but rare in prokaryotes. NMR studies and analysis of site-directed mutants in vivo and in vitro unveil a solvent-exposed hydrophobic pocket lined by acidic residues in CarS, where the CarA DNA recognition helix docks with high affinity in an atypical ligand-recognition mode for SH3 domains. Our findings uncover an unprecedented use of the SH3 domain-like fold for protein-protein recognition whereby an antirepressor mimics operator DNA in sequestering the repressor DNA recognition helix to activate transcription.
authors.affiliation: Instituto de Química-Física Rocasolano, Consejo Superior de Investigaciones Científicas (CSIC), Madrid, Spain.
authors.forename: Esther
authors.initials: E
authors.lastname: León
authors.forename: Gloria
authors.initials: G
authors.lastname: Navarro-Avilés
authors.forename: Clara M
authors.initials: CM
authors.lastname: Santiveri
authors.forename: Cesar
authors.initials: C
authors.lastname: Flores-Flores
authors.forename: Manuel
authors.initials: M
authors.lastname: Rico
authors.forename: Carlos
authors.initials: C
authors.lastname: González
authors.forename: Francisco J
authors.initials: FJ
authors.lastname: Murillo
authors.forename: Montserrat
authors.initials: M
authors.lastname: Elías-Arnanz
authors.forename: María Angeles
authors.initials: MA
authors.lastname: Jiménez
authors.forename: S
authors.initials: S
authors.lastname: Padmanabhan
mesh_terms: D000595:Amino Acid Sequence; D001426:Bacterial Proteins / Q000737:chemistry* / Q000235:genetics; D001665:Binding Sites; D004247:DNA / Q000737:chemistry; D004268:DNA-Binding Proteins / Q000737:chemistry; D008958:Models, Molecular; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D019906:Nuclear Magnetic Resonance, Biomolecular; D009875:Operator Regions, Genetic; D017433:Protein Structure, Secondary; D012097:Repressor Proteins / Q000737:chemistry*; D014157:Transcription Factors / Q000737:chemistry* / Q000235:genetics; D018909:src Homology Domains
chemical_list: D001426:Bacterial Proteins; C091094:CarS protein, Myxococcus xanthus; D004268:DNA-Binding Proteins; D012097:Repressor Proteins; D014157:Transcription Factors; D004247:DNA
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