XRAY CRYSTAL STRUCTURE DETERMINATION OF RUVB DNA HELICASE

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Data was collected on tetragonal crystals of the bacterial hexameric RuvB DNA helicase in May 1996 at CHESS beam line A-1. These crystals are in the space group P41212 (or the enantiomorph P43212) with unit cell parameters a = b = 86.7
, c = 362.5
, a = b = g = 900. The solvent content calculations and the calculated self-rotation function are consistent with three molecules in the asymmetric unit for an overall solvent content of 62%. Optimal crystallization conditions produce crystals of size 0.7 mm x 0.4 mm x 0.4 mm. A cryosolvent was successful for flash cooling these crystals for data collection to a resolution of 3.6
. Non-crystallographic symmetry calculations on the native data set are inconsistent with models that suggest that hexameric helicases, such as the bacterial Rho RNA helicase, adopt a D3 point group symmetry with the tops of every other molecule in the ring points in an opposite direction. Rather, they suggest that the ring orients the tops towards the same side. Significantly, the conformation of RuvB is not six-fold symmetric, but rather three-fold symmetric. This is one of the states observed for the DnaB helicase by electron microscopy and suggests that the crystals represent one of two alternate conformations of the helicase during the reaction cycle.