title: A ring-opening mechanism for DNA binding in the central channel of the T7 helicase-primase protein.
abstract: We have investigated the mechanism of binding single-stranded DNA (ssDNA) into the central channel of the ring-shaped T7 gp4A' helicase-primase hexamer. Presteady-state kinetic studies show a facilitated five-step mechanism and provide understanding of how a ring-shaped helicase can be loaded on the DNA during the initiation of replication. The effect of a primase recognition sequence on the observed kinetics suggests that binding to the helicase DNA-binding site is facilitated by transient binding to the primase DNA-binding site, which is proposed to be a loading site. The proposed model involves the fast initial binding of the DNA to the primase site on the outside of the helicase ring, a fast conformational change, a ring-opening step, migration of the DNA into the central channel of the helicase ring, and ring closure. Although an intermediate protein-DNA complex is kinetically stable, only the last species in the five-step mechanism is poised to function as a helicase at the unwinding junction.
authors.affiliation: Department of Biochemistry, Robert Wood Johnson Medical School, Piscataway, NJ 08854-5635, USA.
authors.forename: P
authors.initials: P
authors.lastname: Ahnert
authors.forename: K M
authors.initials: KM
authors.lastname: Picha
authors.forename: S S
authors.initials: SS
authors.lastname: Patel
mesh_terms: D017123:Bacteriophage T7 / Q000201:enzymology*; D001483:Base Sequence; D019915:DNA Primase / Q000378:metabolism*; D004277:DNA, Single-Stranded / Q000737:chemistry / Q000378:metabolism*; D004279:DNA, Viral / Q000737:chemistry / Q000378:metabolism*; D007700:Kinetics; D009690:Nucleic Acid Conformation; D011485:Protein Binding
chemical_list: D004277:DNA, Single-Stranded; D004279:DNA, Viral; D019915:DNA Primase