Isolation and characterization of naturally occurring hairpin structures in single-stranded DNA of coliphage M13

With precise conditions of digestion with single-strand-specific nucleases, namely, endonuclease S1 of $\text{Aspergillus oryzae}$ and exonuclease I of $\text{Escherichia coli}$, nuclease-resistant DNA cores can be obtained reproducibly from single-stranded M13 DNA. The DNA cores are composed almost exclusively of two sizes (60 and 44 nucleotides long). These have high (G + C)-contents relative to that of intact M13 DNA, and arise from restricted regions of the M13 genome. The resistance of these fragments to single-strand-specific nucleases and their nondenaturability strongly suggest the presence of double-stranded segments in these core pieces. That the core pieces are only partially double-stranded is shown by their lack of complete base complementarity and their pattern of elution from hydroxyapatite.