FLUG AND FLBA FUNCTION INTERDEPENDENTLY TO INITIATE CONIDIOPHORE DEVELOPMENT IN ASPERGILLUS-NIDULANS THROUGH BRLA-BETA ACTIVATION

The Aspergillus nidulans fluG gene is necessary for the synthesis of a small diffusible factor that is required for the endogenously regulat ed induction of asexual sporulation that takes place during the develo pment of an air-exposed colony, Previous work established that FluG is present at nearly constant levels throughout the Aspergillus life cyc le, leading to the hypothesis that FluG factor is constitutively produ ced and development initiates after its concentration surpasses a fixe d threshold, Here we show that overexpression of fluG can overcome the developmental block normally imposed on vegetative cells in submerged culture and leads to the formation of complex conidiophores that are remarkably similar to wild-type conidiophores made by air-exposed colo nies, This fluG-induced sporulation requires the activities of other e arly developmental regulatory genes including, flbA, flbB, flbC, flbD, flbE and brlA. The requirement for flbA in fluG-induced sporulation i s particularly interesting because overexpression of flbA can also ind uce sporulation in submerged culture and this flbA activity requires f luG, The interdependence of fluG and flbA activities suggests a close relationship between the products of these two genes in controlling co nidiophore development, In addition to the endogenous sporulation sign al provided by fluG, several environmental factors, including air expo sure, carbon or nitrogen stress, and increased osmolarity, can influen ce developmental activation, We demonstrate that each of these signals requires the brlA beta gene, but not brlA alpha, to initiate conidiop hore development, We present a model to account for the complex geneti c and environmental controls leading to the activation of brlA beta an d sporulation.