CD of single-stranded, double-stranded, and G-quartet nucleic acids in complexes with a single-stranded DNA-binding protein.

We review CD studies of a single-stranded DNA binding protein, g5p, of the Ff group of bacterial viruses. The CD spectrum of the g5p is dominated by a positive tyrosine La band at 229 nm, to which all five of the protein tyrosines contribute. The La band becomes much less positive upon binding of g5p to nucleic acids. CD spectra of mutant proteins identified a single tyrosine, Y34, that is largely responsible for this CD perturbation. At >250 nm, CD perturbations of nucleic acids can be monitored during g5p binding, and CD titrations have identified two distinct modes of binding of the g5p at physiological ionic strength (0.2 M NaCl). SELEX selection of sequences bound preferentially by g5p yielded a G-rich sequence that is closely related to telomere sequences and has CD properties of a G-tetraplex. CD spectroscopy showed that the presumed G-quadruplex form is maintained within saturated g5p x DNA complexes.