Complex control of the developmental regulatory locus brlA in Aspergillus nidulans

摘要

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 brlAα and brlAβ. 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 brlAα is regulated at the transcriptional level, brlAβ is regulated at both the transcriptional and translational levels. brlAα expression requires both abaA and brlA, but overexpression of brlAβ can induce brlAα in an abaA mutant. BrlAβμORF, a short ORF located upstream of the brlA initiator codon, regulates expression of brlA by damping translation of the brlAβ ORF, and translational repression of brlA expression prevents premature development in A. nidulans. Transcriptional control of brlAβ is apparently independent of BrlA. In order to understand better the transcriptional control of brlAα and brlAβ, we have made 5' deletions in the essential ～2-kb upstream control sequences that regulate brlAβ 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 brlAβ. As a complementary approach, we fused various fragments of the 2034-bp brlAβ and 754-bp brlAα 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 brlAβ fragments extending from -2901 to -2293 and -967 to -414, respectively, and a ～150-bp brlAα 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 brlAβ fragment. This indicates that abaA-mediated repression of brlA expression occurs through control of brlAβ, 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.