Helicase-appended Topoisomerases: New Insight into the Mechanism of Directional Strand Transfer

Jody Plank,Tao-Shih Hsieh

doi:10.1074/jbc.r109.051268

Abstract

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.

Highlights

There are two types of topoisomerases, distinguishable on both mechanistic and structural grounds
The DNA transported through the reversible single-strand break in type I enzymes is usually single-stranded, the strand complementary to the scissile one, but it is possible to have a double strand transported as well
Deletion of the type IA enzymes generally results in viable cells, albeit with genomic instability phenotypes and, in the case of Saccharomyces cerevisiae, complete inhibition of sporulation [5]

Summary

Introduction

There are two types of topoisomerases, distinguishable on both mechanistic and structural grounds. On DNA that does not contain a permanently denatured region, these enzymes are most active on highly negatively supercoiled DNA, as these topological conditions favor the formation of the single-stranded regions required for protein binding.

Results

Conclusion