Translational repression of brlA expression prevents premature development in Aspergillus

The Aspergillus nidulans brlA developmental regulatory locus consists of two overlapping transcription units, brlA alpha and brlA beta, which encode functionally related polypeptides. We used translational fusions between each of the predicted brlA reading frames and the Escherichia coli lacZ gene to test the hypothesis that developmental regulation of brlA alpha and brlA beta expression occurs through different mechanisms. brlA alpha is transcriptionally controlled and a large portion of brlA alpha-directed beta-galactosidase activity is regulated in a brlA-dependent manner. In contrast, brlA beta mRNA is constitutively transcribed but translation of the brlA polypeptide is prevented by the presence of a short open reading frame (microORF) present in the 5' end of brlA beta mRNA. Removing the microORF initiation codon leads to deregulated brlA expression, resulting in an inappropriate activation of development. We propose that one mechanism for developmental induction in A.nidulans involves translational control.