title: Intricate interactions within the ccd plasmid addiction system.
abstract: The ccd addiction system plays a crucial role in the stable maintenance of the Escherichia coli F plasmid. It codes for a stable toxin (CcdB) and a less stable antidote (CcdA). Both are expressed at low levels during normal cell growth. Upon plasmid loss, CcdB outlives CcdA and kills the cell by poisoning gyrase. The interactions between CcdB, CcdA, and its promoter DNA were analyzed. In solution, the CcdA-CcdB interaction is complex, leading to various complexes with different stoichiometry. CcdA has two binding sites for CcdB and vice versa, permitting soluble hexamer formation but also causing precipitation, especially at CcdA:CcdB ratios close to one. CcdA alone, but not CcdB, binds to promoter DNA with high on and off rates. The presence of CcdB enhances the affinity and the specificity of CcdA-DNA binding and results in a stable CcdA*CcdB*DNA complex with a CcdA:CcdB ratio of one. This (CcdA(2)CcdB(2))(n) complex has multiple DNA-binding sites and spirals around the 120-bp promoter region.
authors.affiliation: Vlaams interuniversitair Instituut voor Biotechnologie, Vrije Universiteit Brussel, Laboratorium voor Ultrastructuur, Paardenstraat 65, B-1640 Sint-Genesius-Rode, Belgium.
authors.forename: Minh-Hoa
authors.initials: MH
authors.lastname: Dao-Thi
authors.forename: Daniel
authors.initials: D
authors.lastname: Charlier
authors.forename: Remy
authors.initials: R
authors.lastname: Loris
authors.forename: Dominique
authors.initials: D
authors.lastname: Maes
authors.forename: Joris
authors.initials: J
authors.lastname: Messens
authors.forename: Lode
authors.initials: L
authors.lastname: Wyns
authors.forename: Jan
authors.initials: J
authors.lastname: Backmann
mesh_terms: D001426:Bacterial Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D001427:Bacterial Toxins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D003603:Cytotoxins / Q000235:genetics; D004926:Escherichia coli / Q000235:genetics*; D005144:F Factor / Q000502:physiology*; D007700:Kinetics; D008958:Models, Molecular; D011487:Protein Conformation; D013056:Spectrophotometry, Ultraviolet
chemical_list: D001426:Bacterial Proteins; D001427:Bacterial Toxins; C068355:CcdA protein, Bacteria; C068356:CcdB protein, Plasmid F; D003603:Cytotoxins