Stabilisation of DNA by Incorporation of Phosphothioate Groups

The synthesis of Rp- and Sp-d (GGsAATTCC), an octanucleotide containing the recognition sequence for the restriction endonuclease Eco R1 as well as a phosphorothioate group at the cleavage site is described. Only the Rp-diastereomer is a substrate for Eco R1. Viral DNA, containing exclusively phosphate groups in the (+)strand, and phosphorothioate groups 5′-to deoxyadenosine in the (-)strand yields nicked DNA on cleavage by Eco R1 as an isolatable intermediate. Other restriction enzymes show a similar pattern of hydrolysis. - The influence of phosphorothioate groups on the B Z conformational change is demonstrated on phosphorothioate analogues of d(C-G)4 and d(G-C)4.