AN OLIGOPEPTIDE PERMEASE RESPONSIBLE FOR THE IMPORT OF AN EXTRACELLULAR SIGNAL GOVERNING AERIAL MYCELIUM FORMATION IN STREPTOMYCES-COELICOLOR

Morphological differentiation in the filamentous bacterium Streptomyce s coelicolor is believed to involve a mechanism of extracellular signa lling that culminates with the formation of an aerial mycelium. We hav e identified a gene cluster designated bldK in which insertional and d eletion mutations cause a block in aerial mycelium formation. Extracel lular complementation experiments indicate that bldK defines a step in a cascade of extracellular signals; colonies of a bldK-mutant strain extracellularly complement bld261-mutant colonies, and are themselves extracellularly complemented by bldA- and bldH-mutant colonies. The bl dK locus, which is located at 5 o'clock on the genetic map and within Asel fragment 'N' on the physical map, consists of five adjacent open reading frames. These genes specify homologues of the subunits of the oligopeptide-permease family of ATP-binding cassette (ABC) membrane-sp anning transporters. Because bldK mutations confer resistance to the t oxic tripeptide bialaphos, it is inferred that BldK is an oligopeptide importer. We propose that the BldK transporter is responsible for the import of an extracellular signalling molecule produced under the con trol of the wild-type product of the bld261 gene. The BldK-imported si gnal, in turn, causes the production of a second extracellular signal molecule that depends on the products of bldA and bldH for its action.