Enzymatic synthesis of duplex circular ΦX174 DNA containing phosphoramidate bonds in the (-) strand

Duplex circular ϕX174 DNA (RF I) containing some phosphoramidate links in the backbone chain of the (-) strand was synthesized by reaction of 5′-amino-5′-deoxythymidine 5′-triphosphate, dCTP, dGTP, and 3H-dATP with DNA polymerase I and DNA ligase (T4) on a($) strand ϕX174 amber 3 DNA template. The yield of duplex DNA was higher when dTTP was included along with the amino analog in the initial reaction system or was added late in the synthesis. RF I DNA was observed as a rapidly sedimenting species in an alkaline sucrose gradient, and the presence of phosphoramidate linkages was demonstrated by the unusual lability of the duplex DNA in a weakly acidic solution.