title: The structure of the cofactor-binding fragment of the LysR family member, CysB: a familiar fold with a surprising subunit arrangement.
abstract: BACKGROUND
  CysB is a tetrameric protein of identical subunits (M(r) = 36,000) which controls the expression of genes associated with the biosynthesis of cysteine in bacteria. CysB is both an activator and a repressor of transcription whose activity is responsive to the inducer N-acetylserine; thiosulphate and sulphide act as anti-inducers. CysB is a member of the LysR family of prokaryotic transcriptional regulatory proteins which share sequence similarities over approximately 280 residues including a putative helix-turn-helix DNA-binding motif at their N terminus. The aims of the present study were to explore further the complex molecular biology and curious ligand binding properties of CysB and to provide structural insights into the LysR family of proteins.
  
  
  RESULTS
  The crystal structure of a dimeric chymotryptic fragment of Klebsiella aerogenes CysB comprising residues 88-324, has been solved by multiple isomorphous replacement and multi-crystal averaging and refined against data extending to 1.8 A resolution. The protein comprises two alpha/beta domains (I and II) connected by two short segments of polypeptide. The two domains enclose a cavity lined by polar sidechains, including those of two residues whose mutation is associated with constitutive expression of the cysteine regulon. A sulphate anion and a number of well ordered water molecules have been modelled into discrete electron-density peaks within this cavity. In the dimer, strands beta B from domain I and strands beta G from domain II come together so that a pair of antiparallel symmetry-related 11-stranded twisted beta-pleated sheets is formed.
  
  
  CONCLUSIONS
  The overall structure of CysB(88-324) is strikingly similar to those of the periplasmic substrate-binding proteins. A similar fold has also been observed in the cofactor-binding domain of Lac repressor, implying a structural relationship between the Lac repressor and LysR families of proteins. In contrast to Lac repressor, in CysB the twofold axis of symmetry that relates the monomers in the dimer is perpendicular rather than parallel to the long axis of the cofactor-binding domain. This seems likely to place the DNA-binding domains at opposite extremes of the molecule possibly accounting for CysB's extended DNA footprints.
authors.affiliation: Department of Chemistry, University of York, Heslington, UK.
authors.forename: R
authors.initials: R
authors.lastname: Tyrrell
authors.forename: K H
authors.initials: KH
authors.lastname: Verschueren
authors.forename: E J
authors.initials: EJ
authors.lastname: Dodson
authors.forename: G N
authors.initials: GN
authors.lastname: Murshudov
authors.forename: C
authors.initials: C
authors.lastname: Addy
authors.forename: A J
authors.initials: AJ
authors.lastname: Wilkinson
mesh_terms: D000595:Amino Acid Sequence; D001426:Bacterial Proteins / Q000737:chemistry*; D001665:Binding Sites; D018360:Crystallography, X-Ray; D004268:DNA-Binding Proteins / Q000737:chemistry*; D019281:Dimerization; D007711:Klebsiella pneumoniae / Q000737:chemistry; D008958:Models, Molecular; D008969:Molecular Sequence Data; D008970:Molecular Weight; D010446:Peptide Fragments / Q000737:chemistry; D011487:Protein Conformation; D016415:Sequence Alignment
chemical_list: D001426:Bacterial Proteins; C062886:CysB protein, Bacteria; D004268:DNA-Binding Proteins; D010446:Peptide Fragments