Characterization of the Single‐Strand‐Specific Nuclease S1 Activity on Double‐Stranded Supercoiled Polyoma DNA

The DNA of polyoma virus consists of double‐stranded supercoiled circular molecules. The single‐strand‐specific nuclease S 1 converts these molecules to unit length rods. The conversion occurs in a two‐step reaction, with nuclease S 1 cutting first one of the two strands and then the intact strand opposite to this nick. Cleavage of intact polyoma DNA by nuclease S 1 is a consequence of supercoiling. DNA molecules containing random single‐strand breaks introduced by physical or enzymatic means are also cut opposite to nicks.