An oligopeptide permease responsible for the import of an extracellular signal governing aerial mycelium formation in Streptomyces coelicolor

Morphological differentiation in the filamentous bacterium Streptomyces coelicolor is believed to involve a mechanism of extracellular signalling that culminates with the formation of an aerial mycelium. We have identified a gene cluster designated bldK in which insertional and deletion mutations cause a block in aerial mycelium formation. Extracellular 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 bldK 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-spanning transporters. Because bldK mutations confer resistance to the toxic 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 control of the wild-type product of the bld261 gene. The BldK-imported signal, in turn, causes the production of a second extracellular signal molecule that depends on the products of bldA and bldH for its action.