FlbD, a Myb Transcription Factor of Aspergillus nidulans, Is Uniquely Involved in both Asexual and Sexual Differentiation

In the fungus Aspergillus nidulans, inactivation of the flbA to -E, fluG, fluF, and tmpA genes results in similar phenotypes, characterized by a delay in conidiophore and asexual spore production. flbB to -D encode transcription factors needed for proper expression of the brlA gene, which is essential for asexual development. However, recent evidence indicates that FlbB and FlbE also have nontranscriptional functions. Here we show that fluF1 is an allele of flbD which results in an R47P substitution. Amino acids C46 and R47 are highly conserved in FlbD and many other Myb proteins, and C46 has been proposed to mediate redox regulation. Comparison of Delta flbD and flbD(R47P) mutants uncovered a new and specific role for flbD during sexual development. While flbD(R47P) mutants retain partial function during conidiation, both Delta flbD and flbD(R47P) mutants are unable to develop the peridium, a specialized external tissue that differentiates during fruiting body formation and ends up surrounding the sexual spores. This function, unique among other fluffy genes, does not affect the viability of the naked ascospores produced by mutant strains. Notably, ascospore development in these mutants is still dependent on the NADPH oxidase NoxA. We generated R47K, C46D, C46S, and C46A mutant alleles and evaluated their effects on asexual and sexual development. Conidiation defects were most severe in Delta flbD mutants and stronger in R47P, C46D, and C46S strains than in R47K strains. In contrast, mutants carrying the flbD(C46A) allele exhibited conidiation defects in liquid culture only under nitrogen starvation conditions. The R47K, R47P, C46D, and C46S mutants failed to develop any peridial tissue, while the flbD(C46A) strain showed normal peridium development and increased cleistothecium formation. Our results show that FlbD regulates both asexual and sexual differentiation, suggesting that both processes require FlbD DNA binding activity and that FlbD is involved in the response to nitrogen starvation.