Differing ability to transport non-metal substrates by two RND-type metal exporters

摘要

The metal-exporting systems CusCFBA of Escherichia coli and GesABC of Salmonella are RND-type multi-protein systems responsible for detoxification during metal stress. In this study the substrate range was determined for each metal transport system and possible amino acid residues important in substrate specificity identified. The Ges system, previously identified as a gold-efflux system, conferred resistance to the greatest number and variety of organic chemicals including chloramphenicol, not previously recognized as a substrate. Phylogenetic analysis showed that GesB is most closely related to a class of RND transporters including MexF that have been shown to be responsible for exporting fluoroquinolones, chloramphenicol, and biocides. However, many of the closest homologs of GesABC appear to have a role in metal resistance judging from the genetic context. In contrast, CusCFBA belongs to a distinct family of RND-type monovalent metal exporter systems containing a number of essential metal-binding methionines resulting in a much narrower substrate range.