EFFLUX OF THE NATURAL POLYAMINE SPERMIDINE FACILITATED BY THE BACILLUS-SUBTILIS MULTIDRUG TRANSPORTER BLT

Multidrug transporters pump structurally dissimilar toxic molecules ou t of cells. It is not known, however, if detoxification is the primary physiological function of these transporters. The chromosomal organiz ation of the gene encoding the Bacillus subtilis multidrug transporter Bit suggests a specific function for this protein; it forms a single operon with another gene, bltD, whose protein product is identified he re as a spermine/spermidine acetyltransferase, an enzyme catalyzing a key step in spermidine degradation. Overexpression of the Blt transpor ter in B. subtilis leads not only to the multidrug-resistance phenotyp e but also to the efflux of large amounts of spermidine into the mediu m; this efflux is supressed by an inhibitor of Blt, reserpine. Taken t ogether, these results strongly suggest that the natural function of t he Blt transporter is the efflux of spermidine, whereas multiple drugs may be recognized by Blt merely opportunistically.