A strain producing an inhibitor for pleiotropic drug resistant 5 (Pdr5) was
isolated using our original screening system in yeast. The strain was clas
sified and named as Kitasatospora sp. E-420. The purified inhibitor (molecu
lar weight = 1,193 by FAB-MS) inhibited a Pdr5-mediated efflux of cyclohexi
mide and cerulenin. The intracellular accumulation of a fluorescent dye, rh
odamine 123, by the inhibitor was also confirmed. Some physicochemical data
suggested that the Pdr5-specific inhibitor was different from an immunosup
pressant, FK506, reported as the only inhibitor of Pdr5 in vivo.