A Single-stranded Nucleic Acid-binding Protein from Artemia salina 11

A helix-destabilizing protein, HD40 (Mr 40,000), isolated from the cytoplasm of ArtemM salina (Marvil, D. K., Nowak, L., and Szer, W. (1980) J. BioL Chem 256, 6466-6472) stoichiometrically disrupts the secondary structures of synthetic single-stranded and helical polynucleotides (ag. poly(rA), poly(&), poly(&), poly(dC), and poly(rU)) as well as those of natural polynucleotides (ag. MS2 RNA and $X174 viral DNA). The conformations of double-stranded DNA and doubleor triple-stranded synthetic polynucleotides are not aected by the protein. Formation of duplexes, ag. poly(rA-rU), is prevented by HD40 at 25 to 50 m~ but not at 100 to 140 m~ NaC1. 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 W) beyond those obtained at a stoichiometry of one HD40 molecule per 12 to 15 nucleotides.