Crystallization and preliminary X-ray analysis of the catalytic subunit ofthe ATP-dependent arsenite pump encoded by the Escherichia coli plasmid R773

The arsenical resistance (ars) operon of the Escherichia coli plasmid R773 encodes a system for the active extrusion from cells of the toxic oxyanions arsenite ((AsO2-)-O-III) and antimonite ((SbO2-)-O-III) via an ATP-driven pump. The arsA and arsB genes of the operon encode the catalytic subunit (A TPase) and the membrane subunit of the pump, respectively. The arsC gene co des for a reductase that converts arsenate ((AsO43-)-O-V) to arsenite, thus extending bacterial resistance to the pentavalent state of arsenic. Crysta ls diffracting beyond 2.0 Angstrom were obtained for the catalytic subunit of the pump (ArsA). These crystals belong to space group I222, with unit-ce ll parameters a similar or equal to 73. b similar or equal to 76, c similar or equal to 223 Angstrom. A single molecule of ArsA, composed of two homol ogous halves, occupies the asymmetric unit of the I222 crystals with a pred icted solvent content of 46%. Self-rotation function analysis suggests, how ever, that ArsA adopts a molecular packing corresponding to point group 422 . One possible explanation of this result is that the two homologous halves of ArsA are related by a twofold axis of local symmetry and that the two h alves of a 'pseudo'-tetramer are related by a crystallographic twofold axis .