Grabbing a Helping Strand

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Science  24 Nov 2006:
Vol. 314, Issue 5803, pp. 1219
DOI: 10.1126/science.314.5803.1219c

Helicases are a highly conserved class of enzymes that use ATP to unwind or destabilize DNA and RNA double helices. These enzymes are thought to latch onto a single-stranded (ss) region of the duplex, the “loading strand,” and then to motor along the strand, either in the 5′ or 3′ direction, peeling apart the duplex as they go. Puzzlingly, some RNA helicases can unwind duplexes regardless of which strand they start from. Yang and Jankowsky have analyzed the unwinding activity of the yeast RNA helicase Ded1, which is involved in translation initiation. Although Ded1 cannot unwind DNA-DNA duplexes, it can load onto ssDNA (of either polarity), “travel” across a short region of double-stranded DNA (without unwinding it), and tease apart a DNA-RNA duplex on the far side. Indeed, the loading strand need only be nearby and not necessarily covalently linked to the target duplex. Thus, the loading strand may serve to increase the concentration of Ded1 in the vicinity of the target. An unwinding mechanism in which the enzyme doesn't travel extensively may be well suited for local conformational changes in protein-nucleic acid complexes, something this class of helicases specializes in. — GR

Nat. Struct. Mol. Biol. 13, 981 (2006).

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