Complex control of the developmental regulatory locus brlA in Aspergillus nidulans

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Abstract
brlA is a primary regulator of asexual development in Aspergillus nidulans. Activation of brlA is necessary and sufficient for conidiophore development. It is known that brlA produces two overlapping transcripts, designated brlA α and brlA β. We found that expression of brlA is subject to complex regulation, in that activation of the two brlA transcripts is regulated at different levels. While brlA α is regulated at the transcriptional level, brlA β is regulated at both the transcriptional and translational levels. brlA α expression requires both abaA and brlA, but overexpression of brlA β can induce brlA α in an abaA mutant. brlA β µORF, a short ORF located upstream of the brl Α initiator codon, regulates expression of brlA by damping translation of the brlA β ORF, and translational repression of brlA expression prevents premature development in A. nidulans. Transcriptional control of brlA β is apparently independent of BrlA. In order to understand better the transcriptional control of brlA α and brlA β, we have made 5′ deletions in the essential ~2-kb upstream control sequences that regulate brlA β transcription and fused them to the E. coli lacZ reporter gene. Various deletions in this region resulted in only minor changes in the regulation of β-galactosidase expression. The results of the deletion experiments indicate that there are probably several cis-acting control sequences involved in the regulation of brlA β. As a complementary approach, we fused various fragments of the 2034-bp brlA β and 754-bp brlA α control sequences to an otherwise inactive amdS::lacZ fusion, in order to search for regions that are sufficient to place the reporter under developmental control. We identified two ~600-bp brlA β fragments extending from –2901 to –2293 and –967 to –414, respectively, and a ~150-bp brlA α segment from –271 to –127, that confer activity on the inactive amdS promoter. brlA is overexpressed in an abaA null mutant and one site for abaA-dependent repression is apparently located in the –742 to –414 brlA β fragment. This indicates that abaA-mediated repression of brlA expression occurs through control of brlA β, but apparently involves a mechanism that does not require AbaA binding to brlA(p) sequences, because there are no AbaA binding sites in this region.

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