Molecular Biology

Ending with a Twist

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Science  24 Sep 2004:
Vol. 305, Issue 5692, pp. 1875
DOI: 10.1126/science.305.5692.1875b

The DNA double helix makes replication easy but creates topological problems. In both prokaryotes and eukaryotes, these problems are solved by enzymes called DNA topoisomerases, many of which relax supercoiling by nicking and resealing the DNA strands. While studying the replication of linear DNAs in the soil-dwelling bacteria Streptomyces, Bao and Cohen discovered a topoisomerase (TopA) with an intriguing new activity. TopA was identified as a component of a protein complex bound to the telomeric region (the ends) of DNA. Like the telomerase enzymes that synthesize the ends of DNA in eukaryotes, Streptomyces TopA exhibits reverse transcriptase (RT) activity; i.e., the ability to make DNA from an RNA template. The RT activity of TopA requires an Asp-Asp doublet that is also a requisite feature of eukaryotic reverse transcriptases. The presence of this sequence motif in other bacterial topoisomerases suggests that this surprising RT activity—whatever its precise role—may not be restricted to the Streptomyces enzyme. — PAK

Proc. Natl. Acad. Sci. U.S.A. 10.1073/pnas.0404386101 (2004).

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