Mapping of Mutational Alterations in DNA with S1 Nuclease: The Location of Deletions, Insertions and Temperature-sensitive Mutations in SV40

S1 nuclease from Aspergillus oryzae degrades single- but not double-stranded DNA (Ando 1966); nevertheless, superhelical SV40 (Beard et al. 1973) and polyoma (Germond et al. 1974) DNAs are converted to unit-length linear molecules by this enzyme. Presumably this occurs because unpaired, or weakly hydrogen-bonded regions, susceptible to S1 nuclease, occur or can be induced in the strained superhelical molecule. S1 nuclease can also cleave duplex DNA at or near single-strand nicks under conditions where intact duplex DNA is relatively resistant (Beard et al. 1973; Germond et al. 1974).