Protelomerase uses a topoisomerase IB/Y-recombinase type mechanism to generate DNA hairpin ends.

Protelomerases are enzymes responsible for the generation of closed hairpin ends in linear DNA. It is proposed that they use a breaking-and-rejoin type mechanism to affect DNA rearrangement on specific DNA sequences. In doing so, one strand turns around and becomes the complementary strand. Using the purified enzyme from the Escherichia coli phage N15 and the Klebsiella phage phiKO2 and synthetic oligonucleotide substrates, we directly demonstrate the location where the cutting/re-ligation occurs. We identified a pair of transient staggered cleavages six base-pairs apart centered around the axis of dyad symmetry of the target site. Two molecules of the protelomerase form a pair of protein-linked DNA intermediates at each 3' end of the cleaved openings leaving a 5'-OH. Then, in a process not yet clearly defined, the partners of the two initial openings are exchanged, and the transient breaks are resealed to generate hairpin ends. The formation of 3'-covalent DNA-protein intermediates is a hallmark of the topoisomerase IB type reaction, and we have thus shown experimentally that protelomerase is a member of the tyrosine-recombinase superfamily. In addition, by introducing single nicks in the substrates as perturbation, we found that the integrity of the nucleotide chain 4 bp away from the cutting site as well as this nucleotide's complementary location on the stem if the strands were to fold into a cruciform structure are required for activity, suggesting that these locations may be important substrate-protein contacts. We determined that N15 and phiKO2 protelomerases are monomers in solution and two molecules are needed to interact with the substrate to form two closed hairpin products. The target sites of protelomerases invariably consist of inverted repeats. Comparative studies using the related target sites of different protelomerases suggest that these proteins may require both sequence-specific and structure (possibly cruciform)-specific recognition for activity.