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.