Chemical and enzymatic properties of bridging 5'-S-phosphorothioester linkages in DNA

We describe physicochemical and enzymatic properties of 5' bridging phosphorothioester linkages at specific sites in DNA oligonucleotides. The susceptibility to hydrolysis at various pH values is examined and no measurable hydrolysis is observed at pH 5-9 after 4 days at 25 degrees C. The abilities of three 3'- and 5'-exonuclease enzymes to hydrolyze the DNA past this linkage are examined and it is found that the linkage causes significant pauses at the sulfur linkage for T4 DNA polymerase and calf spleen phosphodiesterase, but not for snake venom phosphodiesterase. Restriction endonuclease (Nsi I) cleavage is also attempted at a 5'-thioester junction and strong resistance to cleavage is observed. Also tested is the ability of polymerase enzymes to utilize templates containing single 5'-S-thioester linkages; both Klenow DNA polymerase and T7 RNA polymerase are found to synthesize complementary strands successfully without any apparent pause at the sulfur linkage. Finally, the thermal stabilities of duplexes containing such linkages are measured; results show that T m values are lowered by a small amount (2 degrees C) when one or two thioester linkages are present in an otherwise unmodified duplex. The chemical stability and surprisingly small perturbation by the 5' bridging sulfur make it a good candidate as a physical and mechanistic probe for specific protein or metal interactions involving this position in DNA.