Interaction of the HB protein of Bacillus globigii with nucleic acids

Fluorescence of the phenylalanine residues of the HB protein of Baccillus globigii was found to be quenched upon binding to nucleic acids. Using this phenomenon, the binding properties were investigated on the basis of an approach published recently. The stoichiometric numbers were about 10 basepairs per bound protein molecule for double-stranded DNA and about 10 bases for single-stranded polynucleotides, independent of salt concentration. Cooperativity parameters were in the range of 50-250. Binding constants were about 5 X 10(6) M-1 (at 0.1 M NaCl) and decreased with increasing salt concentration. From the salt dependence it is inferred that about one Na+ ion is displaced upon binding of a protein molecule to DNA. Therefore the binding site should contain one positively charged amino acid residue. The protein was found to bind with comparable strength to double-stranded and single-stranded DNA as well as to poly(rA). Hence it is concluded that the HB protein does not belong to the category of 'melting proteins'.