Aspergillus nidulans FlbE is an upstream developmental activator of conidiation functionally associated with the putative transcription factor FlbB

Aspergillus nidulans switches from vegetative growth to conidiation when aerial hyphae make contact with the atmosphere, or are subjected to specific environmental stress. The activation of the central conidiation pathway led by the transcription factor brlA is a critical milestone in this morphogenetic transition. A number of upstream developmental activators (UDAs), expressed in vegetative cells, are required for this process to occur in conjunction with cessation of vegetative growth. Mutants affected in these factors remain aconidial (fluffy) with low brlA expression levels (flb). In this report, we describe FlbE as a UDA containing two conserved but hitherto uncharacterized domains, which functions in close association with putative transcription factor FlbB. Both UDAs are functionally interdependent, and colocalize at the hypha tip in an actin cytoskeleton-dependent manner. Moreover, bimolecular fluorescence studies show that they physically interact in vivo. These findings add evidence in favour of the existence of a signalling complex at or near the Spitzenkorper as an important part of the machinery controlling the morphogenetic transition between vegetative growth and conidiation.