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, gyp, of the Ff group of bacterial viruses. The CD spectrum of the gyp is dominated by a positive tyrosine L-a band at 229 rim, to which all five of the protein tyrosines contribute. The L-a band becomes much less positive upon binding of gyp 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 gyp binding, and CD titrations have identified two distinct modes of binding of the gyp at physiological ionic strength (0.2 M NaCl). SELEX selection of sequences bound preferentially by gyp 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. DNA complexes.