RNA polymerase sigma factor that blocks morphological differentiation by Streptomyces coelicolor

The filamentous bacterium Streptomyces coelicolor undergoes a complicated p rocess of morphological differentiation that begins with the formation of a n aerial mycelium and culminates in sporulation. Genes required for the ini tiation of aerial mycelium formation have been termed bld (bald), describin g the smooth, undifferentiated colonies of mutant strains. By using an inse rtional mutagenesis protocol that relies on in vitro transposition, we have isolated a bld mutant harboring an insertion in a previously uncharacteriz ed gene, SCE59.12c, renamed here rsuA. The insertion mutant exhibited no me asurable growth defect but failed to produce an aerial mycelium and showed a significant delay in the production of the polyketide antibiotic actinorh odin. The rsuA gene encodes an apparent anti-sigma factor and is located im mediately downstream of SCE59.13c, renamed here sigU, whose product is infe rred to be a member of the extracytoplasmic function subfamily of RNA polym erase sigma factors. The absence of rsuA in a strain that contained sigU ca used a block in development, and the overexpression of sigU in an otherwise wild-type strain caused a delay in aerial mycelium formation. However, a s train in which both rsuA and sigU had been deleted was able to undergo morp hological differentiation normally. We conclude that the rsuA-encoded anti- sigma factor is responsible for antagonizing the function of the sigma fact or encoded by sigU. We also conclude that the sigU-encoded sigma factor is not normally required for development but that its uncontrolled activity ob structs morphological differentiation at an early stage.