Deprogrammed sporulation in Streptomyces.

The bacterial genus Streptomyces forms chains of spores by septation at intervals in aerial hyphae and subsequent maturation on solid medium. Substrate hyphae undergo extensive lysis, liberating nutrients on which aerial hyphae develop. Some mutant strains, however, ectopically form spores by septation in substrate hyphae on solid medium or in vegetative hyphae in liquid medium, which suggests that all hyphae have the potential to differentiate into spores. A Streptomyces griseus mutant strain NP4, which has a mutation in the regulatory system for an ATP-binding cassette (ABC) transporter gene, forms ectopic spores in substrate hyphae only on glucose-containing medium. In addition, overexpression of a substrate-binding protein of the ABC transporter in the wild-type strain causes ectopic septation in very young substrate hyphae and subsequent sporulation in response to glucose. These ectopic spores germinate normally. The ectopic sporulation is independent of A-factor, a microbial hormone that determines the timing of aerial mycelium formation during normal development. Thus, substrate hyphae of Streptomyces have a potential to develop into spores without formation of aerial hyphae. For programmed development, therefore, the strict repression of septum formation in substrate mycelium should be necessary, as well as the positive signal relay leading to aerial mycelium formation followed by septation and sporulation.