5′ Phosphorylation of DNA in mammalian cells: Identification of a polymin P‐precipitable polynucleotide kinase

Proteins that catayze 5′ phosphorylation of an oligodeozyribonucleotide substrate can be fractionated by polumin P treatment of whole cell extrats of calf thymus glands. Anion exchange chromatography on Q‐Sepharose revealed three separable peaks of activity in the polymin P supernatant fraction, and one peak of activity in the Polymin P pellet fraction. The latter activity, polymin P‐precipitable polynucleotide kinase (PP‐PNK), was futher purified with a 1,500‐fold increase of specific activity compared to the crude polymin fraction. Oligonucleotides, a dephosphorylated 2.9‐kb EcoRI fragment, and poly(A) were phosphorylated by the enzyme preparation, but thymidine 3′monophosphate was not a substrate. PP‐PNk preparations exhibited an apparent K M of 52 μM for ATP and 8 μM for oligo dT 25 . The enzyme preparation displayed no detectable 3′ phosphatase or cyclic 2′,3′ phosphohydrolase activities. The sedimentation coefficient of the PP‐Pnk activity was 3.85 as determined by sucrose density gradient analysis; the stokes radius was 45 Å, leading to an estimated molecular mass of 72 kDa. The enzyme had a pH optimun in the neutral to alkaline range in several buffer systems and is distinct from the DNA Kinase with an acidic pH optimum previously described in calf thymus. © Wiley‐Liss, Inc.