Nucleic Acids Research: VOLUME 37 WEB SERVER ISSUE JULY 1, 2009

Rat liver chromatin contains a 3'-phosphatase/5'-OH kinase which may be involved in the repair of DNA strand breaks limited by 3'-phosphate/5'-OH ends. In order to determine whether the phosphate group can be transferred directly from the 3' to the 5' position, a polynucleotide duplex was synthesized between poly(dA) and oligo(dT) segments which had 3'-[32P]phosphate and 5'-OH ends. The oligo(dT) segments were separated by simple nicks as shown by the ability of T4 DNA ligase to seal the nick after the 3'-phosphate was removed by a phosphatase and the 5' end was phosphorylated with a kinase. The chromatin 3'phosphatase/5'-OH kinase was unable to transfer phosphate directly from the 3' to the 5' end of the oligo(dT) segments in the original duplex; ATP was needed to phosphorylate the 5'-OH end. It is concluded that the chromatin 3'-phosphatase/5'-OH kinase is unable to convert a 3'-phosphate/5'-OH nick which cannot be repaired by DNA ligase directly into a 3'-OH/5'-phosphate nick which can be repaired by DNA ligase; the chromatin enzyme rather acts in two steps : hydrolysis of the 3'-phosphate followed by ATP-mediated phosphorylation of the 5'-OH end.