Molecular Biology

Untangling Linked DNA

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Science  07 Sep 2012:
Vol. 337, Issue 6099, pp. 1150-1151
DOI: 10.1126/science.337.6099.1150-c

To access the information stored in the genome during replication, repair, and recombination, the two strands of DNA must be unwound and sometimes cut and rejoined. Such synthesis and repair work can result in DNA strands becoming catenated, or topologically linked to one another, and topoisomerase enzymes have evolved to deal with such problems. Cejka et al. show that a topoisomerase complex in yeast, consisting of topoisomerase III (Top3), the helicase Sgs1, and Rmi1, works hand in hand with the single-stranded (ss)DNA binding protein RPA to unlink catenated and hemicatenated dsDNA rings, arising from late or partially replicated DNA intermediates, double holliday junctions, and so forth. They show that Sgs1 acts to unwind the dsDNA and that RPA captures the ssDNA to provide a substrate for Top3, which prefers single strands. Sgs1 delivers Top3 to the ssDNA, where it first cuts a DNA strand, forming a gap, and then passes intact ssDNA strands through the gap to unlink the DNA. Rmi1 stimulates strand passage by stabilizing the Top3 reaction intermediate, thereby lengthening the time for strand passage to occur.

Mol. Cell 47, 10.1016/j.molcel.2012.06.032 (2012).

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