ISOLATION OF ASPERGILLUS-NIDULANS MUTANTS THAT OVERCOME BRLA-INDUCED GROWTH ARREST

The Aspergillus nidulans brlA gene is a primary regulator of developme nt-specific gene expression during conidiation, Forced activation of b rlA in vegetative cells leads to inappropriate induction of conidiopho re formation and causes growth to stop, In fact, when conidia containi ng a nutritionally inducible brlA gene fusion are placed on inducing m edium, they fail to germinate. We used this phenotype to select 174 mu tants that continue growing following such forced brlA activation, For ty-six of these mutants also produced abnormal developmental structure s during air-induced conidiation as expected if the mutations resulted in an altered response to BrlA (designated sbr mutants for suppressor s of brlA response), The predominant mutant class identified was defec tive in a known developmental regulatory gene, abaA, We also identifie d mutants with defects in the previously characterized early acting de velopmental regulatory genes flbB and flbD and in four previously unde scribed loci designated sbrA-D, sbrA mutants represent the second larg est group and are characterized by production of conidiophore stalks t hat lack a normal vesicle and form branching sterigmata that rarely ma ke spores. Because abaA expression could not be detected in sbrA mutan ts following brlA activation we propose that sbrA functions as a devel opmental modifier, participating in brlA-dependent activation of other developmental regulators. (C) 1997 Academic Press.