Cleavage of phosphorothioate-substituted DNA by restriction endonucleases.

M13 RF DNA was synthesized in vitro in the presence of various single deoxynucleoside 5'-O-(1-thiotriphosphate) phosphorothioate analogues, and the three other appropriate deoxynucleoside triphosphates using a M13 (+)-single-stranded template, Escherichia coli DNA polymerase I and T4 DNA ligase. The resulting DNAs contained various restriction endonuclease recognition sequences which had been modified at their cleavage points in the (-)-strand by phosphorothioate substitution. The behavior of the restriction enzymes AvaI, BamHI, EcoRI, HindIII, and SalI towards these substituted DNAs was investigated. EcoRI, BamHI, and HindIII were found to cleave appropriate phosphorothioate-substituted DNA at a reduced rate compared to normal M13 RF DNA, and by a two-step process in which all of the DNA is converted to an isolable intermediate nicked molecule containing a specific discontinuity at the respective recognition site presumably in the (+)-strand. By contrast, SalI cleaved substituted DNA effectively without the intermediacy of a nicked form. AvaI, however, is only capable of cleaving the unsubstituted (+)-strand in appropriately modified DNA.