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The single stranded DNA tail is bound in a groove over the red and green RecA-like domains. The cleft between the red and green domain is closed in response to ADPNP binding. This cleft closure is shown in a movie of the transition between substrate and product complexes. The blue and yellow domains create a surface for duplex DNA binding which results in distortion of the five base pairs nearest the ss/dsDNA junction. The distortion is suggestive of an "active" mechanism for PcrA helicase since some of the free enegy associated with ATP hydrolysis is used to directly destabilise the DNA duplex. The structure is inconsistent with the active rolling model for helicase activity, since it demonstrates that a monomeric PcrA molecule is competent to bind both single- and double-stranded DNA simultaneously. Instead, we propose a modified inchworm model for PcrA catalysed helicase activity.