Crystallization and preliminary x-ray analysis of the catalytic subunit of the 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 (AsIIIO2-) and antimonite (SbIIIO2-) via an ATP-driven pump. The arsA and arsB genes of the operon encode the catalytic subunit (ATPase) and the membrane subunit of the pump, respectively. The arsC gene codes for a reductase that converts arsenate (AsVO43-) to arsenite, thus extending bacterial resistance to the pentavalent state of arsenic. Crystals diffracting beyond 2.0 A were obtained for the catalytic subunit of the pump (ArsA). These crystals belong to space group I222, with unit-cell parameters a approximately 73, b approximately 76, c approximately 223 A. A single molecule of ArsA, composed of two homologous halves, occupies the asymmetric unit of the I222 crystals with a predicted solvent content of 46%. Self-rotation function analysis suggests, however, 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 halves of a 'pseudo'-tetramer are related by a crystallographic twofold axis.