Characterization of the DNA-binding properties of the origin-binding domain of simian virus 40 large T antigen by fluorescence anisotropy

The affinity of the origin-binding domain (OBD) of simian virus 40 large T antigen for its cognate origin was measured at equilibrium using a DNA binding assay based on fluorescence anisotropy. At a near-physiological concentration of salt, the affinities of the OBD for site II and the core origin were 31 and 50 nM, respectively. Binding to any of the four 5'-GAGGC-3' binding sites in site II was only slightly weaker, between 57 and 150 nM. Although the OBD was shown previously to assemble as a dimer on two binding sites spaced by 7 bp, we found that increasing the distance between both binding sites by 1 to 3 bp had little effect on affinity. Similar results were obtained for full-length T antigen in absence of nucleotide. Addition of ADP-Mg, which promotes hexamerization of T antigen, greatly increased the affinity of full-length T antigen for the core origin and for nonspecific DNA. The implications of these findings for the assembly of T antigen at the origin and its transition to a non-specific DNA helicase are discussed.