| Abstract: | A helix-destabilizing protein, HD40 (Mr 40,000), isolated from the cytoplasm of Artemia salina (Marvil, D.K., Nowak, L., and Szer, W. (1980) J. Biol. Chem. 255, 6466-6472) stoichiometrically disrupts the secondary structures of synthetic single-stranded and helical polynucleotides (e.g. poly(rA), poly(dA), poly(rC), poly(dC), and poly(rU)) as well as those of natural polynucleotides (e.g. MS2 RNA and phi X174 viral DNA). The conformations of double-stranded DNA and double- or triple-stranded synthetic polynucleotides are not affected by the protein. Formation of duplexes, e.g. poly(rA . rU), is prevented by HD40 at 25 to 50 mM but not at 100 to 140 mM NaCl. The unwinding of the residual secondary structure of RNA and DNA by HD40 is not highly cooperative and has a stoichiometry of one HD40 per 12 to 15 nucleotides. The addition of HD40 in excess of 1 molecule per 12 to 15 nucleotides results in the cooperative formation of distinct bead-like structures along the nucleic acid strand. The beads are about 20 nm in diameter with a center to center distance of about 40 nm. The appearance of the beads is not accompanied by any spectral changes (CD and UV) beyond those obtained at a stoichiometry of one HD40 molecule per 12 to 15 nucleotides. |
