Effects of ssDNA sequences on non-sequence-specific protein binding

The circular dichroism (CD) spectra of single-stranded DNAs (ssDNAs) are significantly perturbed by the binding of single-stranded DNA binding proteins such as the Ff bacteriophage gene 5 protein (g5p) and the A domain of the 70 kDa subunit of human replication protein A (RPA70-A). These two proteins have similar OB-fold secondary structures, although their CD spectra at wavelengths below 250 nm differ greatly. The spectrum of g5p is dominated by a tyrosyl La band at 229 nm, while that of RPA70-A is dominated by its β secondary structure. Despite differences in their inherent spectral properties, these two proteins similarly perturb the spectra of bound nucleic acid oligomers. CD spectra of free, non-protein-bound ssDNAs are dependent on interactions of the nearest-neighboring nucleotides in the sequence. The CD spectra (per mol of nucleotide) of simple repetitive sequences 48 nucleotides in length and containing simple combinations of A and C are related by nearest-neighbor equations. For example, 3 × Δε[d(AAC)₁₆] = 3 × Δε[d(ACC)₁₆] + Δε[d(A)₄₈] - Δε[d(C)₄₈]. Moreover, nearest-neighbor equations relate the spectra of ssDNAs when they are bound by g5p, indicating that each type of perturbed nearest neighbor has a similar average structure within the binding site of the protein.