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Helicases are ATP-driven enzymes essential for DNA unwinding. The broad host range plasmid RSFI010 harbours a gene (repA) encoding for one of the smallest known oligomeric helicases, RepA, a homo-hexamer with 30 kDa subunits. Electron micrographs indicate that the overall shape of RepA resembles a hexagon with globular monomers at the corners, diameter 140 A, and a central channel. Below pH 6, the molecules aggregate into tubular structures. The enzyme has been purified and crystallized using the hanging-drop vapour-diffusion method with polyethyleneglycol monomethylether as precipitating agent. The crystals exhibit the monoclinic space group P2(1) with unit-cell parameters a = 105.8, b = 180.3, c = 115.4 A, beta = 95.2 degrees, and diffract to 3.5 A resolution using rotating-anode Cu Kalpha radiation. Assuming two 180 kDa molecules per asymmetric unit, the volume per unit weight is V(m) = 3.06 A Da(-1), equivalent to a solvent content of 60%. A self-rotation search indicates that the sixfold axis of the hexamer is parallel to the ac plane and inclined at about 2 degrees to the c axis. The two hexamers are oriented head-to-head with point-group symmetry D(6).

Crystals of the adduct Hg(SCN)2 · 2HgCl2·2C6H12N4 were obtained from an aqueous solution containing 0.1 mol/l hexamethylenetetramine and 0.05 mol/l potassium thiocyanate and mercury(II) chloride. The compound crystallizes monoclinically; space group P21/n, a = 767.3(4), b = 1854.7(7), c = 872.5(7) pm, and β = 101.48(5)°. The structure consists of a net­ work of chains -Cl-Hg-Cl-Hg- and -(Cl-Hg-)2NCS-Hg-SCN-(Hg-Cl)2-which share common Hg atoms and form distorted hexagonal and trigonal channels. The hexamethylenetetramine molecules lie in the hexagonal channels. They act as μ3 bridging ligands, forming bonds to two Hg atoms in a given layer, and an additional bond to a Hg atom a neighbouring layer.

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