OVEREXPRESSION OF FLBA, AN EARLY REGULATOR OF ASPERGILLUS ASEXUAL SPORULATION, LEADS TO ACTIVATION OF BRLA AND PREMATURE INITIATION OF DEVELOPMENT

Aspergillus nidulans reproduces asexually by forming thousands of mito tically derived spores atop highly specialized multicellular organs te rmed conidiophores. We have identified a gene called flbA (for fluffy low brlA expression) that is required for initiation of A. nidulans co nidiophore development, flbA mutants form abnormal colonies that have a distinct fluffy phenotype characterized by tightly interwoven aerial hyphae that autolyse as the colony matures. The requirement for flbA in conidiophore development precedes activation of brlA, a primary reg ulator of conidiophore development. The wild-type flbA gene was isolat ed and found to encode a 3.0 kb mRNA that is expressed throughout the A. nidulans asexual life cycle. Overexpression of flbA using an induci ble promoter resulted in misscheduled expression of brlA in vegetative cells and caused hyphal tips to differentiate into spore-producing st ructures. Sequence analysis of a nearly full-length flbA cDNA clone sh owed that flbA is predicted to encode a 717-amino-acid polypeptide wit h 30% identity to the Saccharomyces cerevisiae SST2 protein. SST2 is r equired by yeast cells for resuming growth following prolonged exposur e to yeast mating pheromone and far mating partner discrimination. We propose that flbA plays a related role in a signalling pathway for Asp ergillus conidiophore development.