FLBD ENCODES A MYB-LIKE DNA-BINDING PROTEIN THAT COORDINATES INITIATION OF ASPERGILLUS-NIDULANS CONIDIOPHORE DEVELOPMENT

The timing of asexual fruiting body formation during Aspergillus nidul ans colony development is precisely regulated so that conidiophores ar e typically produced 1-2 mm behind the growing edge of the colony. Mut ations in any of four A. nidulans genes, flbB, flbC, flbD, or flbE, re sult in colonies that are delayed at least 24 hr in their ability to i nitiate conidiophore development resulting in fluffy colonies with con idiophores forming in the center, at least 12-15 mm behind the growing edge. The requirement for each of these four genes in determining the timing of developmental initiation precedes transcriptional activatio n of the primary developmental regulatory gene brlA, indicating a poss ible role for each gene in developmentally regulated activation of brl A expression. The wild-type flbD gene was isolated and shown to encode an similar to 1.6-kb mRNA that is present throughout the A. nidulans life cycle. The deduced FlbD protein sequence predicts a 314-amino-aci d polypeptide with significant identity at its amino terminus to the D NA-binding domain of the Myb family of transcription factors indicatin g that FlbD probably functions as a sequence-specific transcriptional activator. Although conidiophore development does not normally occur i n submerged culture, forced overexpression of flbD in submerged hyphae caused inappropriate activation of brlA expression and resulted in pr oduction of complex conidiophores that produced all of the distinct ce ll types observed in wild-type conidiophores including viable spores. This ability of flbD overexpression to activate conidiation requires b rlA, flbB, and flbA (another early developmental regulator) but does n ot require flbC or flbE. We propose that FlbD functions during normal development by activating transcription of other genes required for de velopment (such as brlA) and that FlbD activity is normally controlled post-transcriptionally by an unknown mechanism.