title: Saccharomyces cerevisiae replication protein A binds to single-stranded DNA in multiple salt-dependent modes.
abstract: We have examined the single-stranded DNA (ssDNA) binding properties of the Saccharomyces cerevisiae replication protein A (scRPA) using fluorescence titrations, isothermal titration calorimetry, and sedimentation equilibrium to determine whether scRPA can bind to ssDNA in multiple binding modes. We measured the occluded site size for scRPA binding poly(dT), as well as the stoichiometry, equilibrium binding constants, and binding enthalpy of scRPA-(dT)L complexes as a function of the oligodeoxynucleotide length, L. Sedimentation equilibrium studies show that scRPA is a stable heterotrimer over the range of [NaCl] examined (0.02-1.5 M). However, the occluded site size, n, undergoes a salt-dependent transition between values of n = 18-20 nucleotides at low [NaCl] and values of n = 26-28 nucleotides at high [NaCl], with a transition midpoint near 0.36 M NaCl (25.0 degrees C, pH 8.1). Measurements of the stoichiometry of scRPA-(dT)L complexes also show a [NaCl]-dependent change in stoichiometry consistent with the observed change in the occluded site size. Measurements of the deltaH(obsd) for scRPA binding to (dT)L at 1.5 M NaCl yield a contact site size of 28 nucleotides, similar to the occluded site size determined at this [NaCl]. Altogether, these data support a model in which scRPA can bind to ssDNA in at least two binding modes, a low site size mode (n = 18 +/- 1 nucleotides), stabilized at low [NaCl], in which only three of its oligonucleotide/oligosaccharide binding folds (OB-folds) are used, and a higher site size mode (n = 27 +/- 1 nucleotides), stabilized at higher [NaCl], which uses four of its OB-folds. No evidence for highly cooperative binding of scRPA to ssDNA was found under any conditions examined. Thus, scRPA shows some behavior similar to that of the E. coli SSB homotetramer, which also shows binding mode transitions, but some significant differences also exist.
authors.affiliation: Department of Biochemistry and Molecular Biophysics, Washington University School of Medicine, 660 South Euclid Avenue, St. Louis, Missouri 63110, USA.
authors.forename: Sangaralingam
authors.initials: S
authors.lastname: Kumaran
authors.forename: Alexander G
authors.initials: AG
authors.lastname: Kozlov
authors.forename: Timothy M
authors.initials: TM
authors.lastname: Lohman
mesh_terms: D004277:DNA, Single-Stranded / Q000737:chemistry* / Q000378:metabolism; D004268:DNA-Binding Proteins / Q000737:chemistry / Q000378:metabolism; D029968:Escherichia coli Proteins / Q000737:chemistry / Q000378:metabolism; D011485:Protein Binding; D051716:Replication Protein A / Q000737:chemistry* / Q000378:metabolism; D012441:Saccharomyces cerevisiae / Q000737:chemistry* / Q000378:metabolism; D029701:Saccharomyces cerevisiae Proteins / Q000737:chemistry* / Q000378:metabolism; D012965:Sodium Chloride / Q000737:chemistry*; D013816:Thermodynamics
chemical_list: D004277:DNA, Single-Stranded; D004268:DNA-Binding Proteins; D029968:Escherichia coli Proteins; D051716:Replication Protein A; C476430:SSB protein, E coli; D029701:Saccharomyces cerevisiae Proteins; D012965:Sodium Chloride