Cadmium resistance from Staphylococcus aureus plasmid pI258

Cadmium resistance specified by the cadA determinant of Staphylococcus aureus plasmid p1258 results from the functioning of a cadmium-efflux system. In the nucleotide sequence of the DNA fragment containing the cadA determinant, two open reading frames were identified. The larger one, corresponding to a predicted polypeptide of 727 amino acid residues, is necessary and sufficient for expression of cadmium resistance. Comparison of the CadA amino acid sequence with known protein sequences suggested that CadA is a member of the EjE2 cation-translocating ATPases, similar to the K+-uptake ATPases of Gram-positive and Gram-negative bacteria. The sequence homology is lower but significant with other EjE2-type ATPases, including the H+-efflux ATPases of eukaryotic microbes and the Ca2+and Na+/K+-ATPases of animals. A frame-shift mutation in the middle of the gene destroys the Cd2+-resistance phenotype. A detailed model for the putative CadA ATPase based on homologies to other EjE2 ATPases is presented and discussed. Resistance to Cd2+ is widespread in Staphylococcus aureus (1). There are two separate Cd2+-resistance determinants, cadA and cadB, located on plasmids (2). The cadB gene product may protect the cell by binding Cd2+ (3). The resistance function coded by the cadA determinant results from decreased intracellular accumulation of Cd2+ (4), mediated by an energy-dependent efflux mechanism (5). In S. aureus, Cd2' enters the cell by the Mn2' active transport system (4, 6), but cells that have the cadA gene have lower net accumulation. The CadA-efflux system is sensitive to metabolic inhibitors such as uncouplers but is not affected by agents that eliminate the membrane potential (5). Therefore, it was proposed (5) that the cadA product is an electroneutral antiporter that ejects one Cd2+ while accumulating two protons. We have cloned the cadA determinant from S. aureus plasmid pI258 and expressed it in Bacillus subtilis. The DNA sequence was determined.* It consists of a single open reading frame (ORF) very likely coding for an E1E2 cationtranslocating ATPase.