Strand breakage by the DNA untwisting enzyme results in covalent attachment of the enzyme to DNA.

Strands of DNA that have been broken by the DNA untwisting enzyme exhibit a reduced buoyant density in alkaline CsCl due to bound protein. A covalent linkage between the DNA and the enzyme was indicated by the stability of the complex in alkali (pH greaterthan 12.7), in 7 M guanidine-HCl, and at 90 degrees in 1% Sarkosyl for 5 min. The single-strand breaks generated by the enzyme are resistant to exonuclease III, indicating that the protein is attached to one of the ends of the broken strands. The free end of the broken strand bears a 5'-hydroxyl group, indicating attachment of the protein to the 3'-phosphoryl terminus. A nucleotide-peptide linkage involving a phosphoamide bond is unlikely since the complex is resistant to 3.5 M hydroxylamine at pH 4.75.