Aspergillus asexual reproduction and sexual reproduction are differentially affected by transcriptional and translational mechanisms regulating stunted gene expression.

The Stunted protein (StuAp) is a member of a family of transcription factors that regulate fungal development and cell cycle progression. Regulated stuA gene expression is required for correct cell pattern formation during asexual reproduction (conidiation) and for initiation of the sexual reproductive cycle in Aspergillus nidulans. Transcriptional initiation from two different promoters yields overlapping mRNAs (stuA alpha and stuAbeta) that upon translation yield the same protein. Here we show that multiple regulatory mechanisms interact to control (i) developmental competence-dependent expression of both transcripts and (ii) induction-dependent expression of stuA alpha, but not stuAbeta, by the conidiation-specific Bristle (BrlAp) transcriptional activator. Quantitative levels of both mRNAs are further modulated by (i) an activator(s) located at a far-upstream upstream activation sequence, (ii) feedback regulation by StuAp, and (iii) positive translational regulation that requires the peptide product of a micro-open reading frame unique to the stuA alpha mRNA 5' untranslated region. Gradients in stuA alpha expression were most important for correct cell and tissue type development. Threshold requirements were as follows: metula-phialide differentiation < ascosporogenesis < cleistothecial shell-Hülle cell differentiation. Altered stuA expression affected conidiophore morphology and conidial yields quantitatively but did not alter the temporal development of cell types or conidiophore density. By contrast, the sexual cycle showed both temporal delay and quantitative reduction in the number of cleistothecial initials but normal morphogenesis of tissue types.