Interplay between minor and major groove-binding transcription factors Sox2 and Oct1 in translocation on DNA studied by paramagnetic and diamagnetic NMR

Abstract

The pathways whereby Sox2 scans DNA to locate its specific binding site are investigated by NMR in specific and nonspecific Sox2·DNA complexes and in a specific ternary complex with Oct1 on the Hoxb1 regulatory element. Direct transfer of Sox2 between nonspecific sites on different DNA molecules occurs without dissociation into free solution at a rate of ∼106 M -1 s-1, whereas one-dimensional sliding proceeds with a diffusion constant of ≥0.1 μm2·s-1. Translocation of Sox2 from one specific DNAsite to another occurs via jumping, involving complete dissociation into free solution (kd∼5-6 s-1) followed by reassociation (ka∼5×108 M -1 s-1). In the presence of Oct1 bound to an adjacent specific site, kd is reduced by more than 10-fold. Paramagnetic relaxation measurements, however, demonstrate that sparsely populated (<1%), transient states involving nonspecifically bound Sox2 in rapid exchange with specifically bound Sox2 are sampled in both binary Sox2·DNA- and ternary Oct1·Sox2·Hoxb1-DNA-specific complexes. Moreover, Sox2 modulates the mechanism of translocation of Oct1. Both Sox2 and the Oct1 POUHD domain are transiently released from the specific ternary complex by sliding to an adjacent nonspecific site, followed by direct transfer to another DNA molecule, whereas the Oct1 POUS domain is fixed to its specific site through direct interactions with Sox2. Intermolecular translocation of POU HD results in the formation of a bridged intermediate spanning two DNA molecules, enhancing the probability of complete intermolecular translocation of Oct1. By way of contrast, in the specific Oct1·DNA binary complex, POUS undergoes direct intermolecular translocation, whereas POUHD scans the DNA by sliding.