Helicase-appended topoisomerases: new insight into the mechanism of directional strand transfer.

DNA strand passage through an enzyme-mediated gate is a key step in the catalytic cycle of topoisomerases to produce topological transformations in DNA. In most of the reactions catalyzed by topoisomerases, strand passage is not directional; thus, the enzyme simply provides a transient DNA gate through which DNA transport is allowed and thereby resolves the topological entanglement. When studied in isolation, the type IA topoisomerase family appears to conform to this rule. Interestingly, type IA enzymes can carry out directional strand transport as well. We examined here the biochemical mechanism for directional strand passage of two type IA topoisomerases: reverse gyrase and a protein complex of topoisomerase III alpha and Bloom helicase. These enzymes are able to generate vectorial strand transport independent of the supercoiling energy stored in the DNA molecule. Reverse gyrase is able to anneal single strands, thereby increasing linkage number of a DNA molecule. However, topoisomerase III alpha and Bloom helicase can dissolve DNA conjoined with a double Holliday junction, thus reducing DNA linkage. We propose here that the helicase or helicase-like component plays a determinant role in the directionality of strand transport. There is thus a common biochemical ground for the directional strand passage for the type IA topoisomerases.