StuAp is a sequence-specific transcription factor that regulates developmental complexity in Aspergillus nidulans

The Aspergillus nidulans Stunted protein (StuAp) regulates multicellular complexity during asexual reproduction by moderating the core developmental program that directs differentiation of uninucleate, terminally differentiated spores from multinucleate, vegetative hyphae. StuAp is also required for ascosporogenesis and multicellular development during sexual reproduction. StuAp is a member of a family of fungal transcription factors that regulate development or cell cycle progression. Further, StuAp characterizes a sub-family possessing the conserved APSES domain. We demonstrate for the first time that the APSES domain is a sequence-specific DNA-binding domain that can be modeled as a basic helix-loop-helix (bHLH)-like structure. We have found that StuAp response elements ((A)/(T)CGCG(T)/(A)N(A)/(C)) are located upstream of both critical developmental regulatory genes and cell cycle genes in A. nidulans. StuAp is shown to act as a transcriptional repressor in A. nidulans, but as a weak activator in budding yeast. Our data suggest that the differentiation of pseudohyphal-like sterigmatal cells during multicellular conidiophore development requires correct StuAp-regulated expression of both developmental and cell cycle genes in A. nidulans. The budding pattern of sterigmata may involve processes related to those controlling pseudohyphal growth in budding yeast.