TRANSLATIONAL REPRESSION OF BRLA EXPRESSION PREVENTS PREMATURE DEVELOPMENT IN ASPERGILLUS

The Aspergillus nidulans brlA developmental regulatory locus consists of two overlapping transcription units, brlAalpha and brlAbeta, which encode functionally related polypeptides. We used translational fusion s between each of the predicted brlA reading frames and the Escherichi a coli lacZ gene to test the hypothesis that developmental regulation of brlAalpha and brlAbeta expression occurs through different mechanis ms. brlAalpha is transcriptionally controlled and a large portion of b rlAalpha-directed beta-galactosidase activity is regulated in a brlA-d ependent manner. In contrast, brlAbeta mRNA is constitutively transcri bed but translation of the brlA polypeptide is prevented by the presen ce of a short open reading frame (muORF) present in the 5' end of brlA beta mRNA. Removing the muORF initiation codon leads to deregulated br lA expression, resulting in an inappropriate activation of development . We propose that one mechanism for developmental induction in A.nidul ans involves translational control.