ORPHAN ENZYME OR PATRIARCH OF A NEW TRIBE - THE ARSENIC RESISTANCE ATPASE OF BACTERIAL PLASMIDS

The plasmid-determined arsenite and antimonite efflux ATPase of bacter ia differs from other membrane transport ATPases, which are classified into several families (such as the F0F1-type H+-translocating ATP syn thases, the related vacuolar H+-translocating ATPases, the P-type cati on-translocating ATPases, and the superfamily which includes the perip lasmic binding-protein-dependent systems in Gram-negative bacteria, th e human multidrug resistance P-glycoprotein, and the cystic fibrosis t ransport regulator). The amino acid sequences of the components of the arsenic resistance system are not similar to known ATPase proteins. N ew findings with the arsenic resistance operons of bacterial plasmids suggest that instead of being an orphan the Ars system will now be the first recognized member of a new class of ATPases. Furthermore, funda mental questions of energy-coupling (ATP-driven or chemiosmotic) have recently been raised and the finding that the arsC gene product is a s oluble enzyme that reduces arsenate to arsenite changes the previous p icture of the functioning of this widespread bacterial system.