Interaction between developmental and cell cycle regulators is required for morphogenesis in Aspergillus nidulans

In Aspergillus nidulans, mutation of the transcriptional regulator brlA arrests formation of asexual spore-forming structures called conidiophores but does not hinder vegetative hyphal growth. During conidiophore development a 6-fold, brlA-dependent increase in the kinase activities of NIMX(cdc2) and NIMA occurs. A similar level of kinase induction was promoted by ectopic expression of brlA. Northern and Western analysis revealed marked induction of nimX(cdc2) mRNA after ectopic expression of brlA and increased amounts of NIMX(cdc2). Therefore, nimX(cdc2) is developmentally regulated by brlA indicating a direct role for brlA in the regulation of cell cycle genes. That correct regulation of nimX(cdc2) is important for normal development was further supported by analysis of conidiophore development and septation in cell cycle specific mutants. Most noticeably, the nimX(cdc2AF) mutation promoted inappropriate septation and hindered the switch from filamentous growth to budding growth seen during conidiophore development. Therefore, in contrast to the situation previously reported for other multicellular eukaryotes, interaction between developmental regulators and cell cycle regulators is essential for normal morphogenesis in A.nidulans.