Isolation of Aspergillus nidulans mutants that overcome brlA-induced growth arrest

The Aspergillus nidulans brlA gene is a primary regulator of development-specific gene expression during conidiation. Forced activation of brlA in vegetative cells leads to inappropriate induction of conidiophore formation and causes growth to stop. In fact, when conidia containing a nutritionally inducible brlA gene fusion are placed on inducing medium, they fail to germinate. We used this phenotype to select 174 mutants that continue growing following such forced brlA activation. Forty-six of these mutants also produced abnormal developmental structures during air-induced conidiation as expected if the mutations resulted in an altered response to BrlA (designated sbr mutants for suppressors of brlA response). The predominant mutant class identified was defective in a known developmental regulatory gene, abaA. We also identified mutants with defects in the previously characterized early acting developmental regulatory genes flbB and flbD and in four previously undescribed loci designated sbrA-D. sbrA mutants represent the second largest group and are characterized by production of conidophore stalks that lack a normal vesicle and form branching sterigmata that rarely make spores. Because abaA expression could not be detected in sbrA mutants following brlA activation we propose that sbrA functions as a developmental modifier, participating in brlA-dependent activation of other developmental regulators.