Functional Role of Aspergillus carbonarius AcOTAbZIP Gene, a bZIP Transcription Factor within the OTA Gene Cluster

Aspergillus carbonarius is the principal fungal species responsible for ochratoxin A (OTA) contamination of grapes and derived products in the main viticultural regions worldwide. In recent years, co-expressed genes representing a putative-OTA gene cluster were identified, and the deletion of a few of them allowed the partial elucidation of the biosynthetic pathway in the fungus. In the putative OTA-gene cluster is additionally present a bZIP transcription factor (AcOTAbZIP), and with this work, A. carbonarius Delta AcOTAbZIP strains were generated to study its functional role. According to phylogenetic analysis, the gene is conserved in the OTA-producing fungi. A Saccharomyces cerevisiae transcription factor binding motif (TFBM) homolog, associated with bZIP transcription factors was present in the A. carbonarius OTA-gene cluster no-coding regions. AcOTAbZIP deletion results in the loss of OTA and the intermediates OTB and OT beta. Additionally, in Delta AcOTAbZIP strains, a down-regulation of AcOTApks, AcOTAnrps, AcOTAp450, and AcOTAhal genes was observed compared to wild type (WT). These results provide evidence of the direct involvement of the AcOTAbZIP gene in the OTA biosynthetic pathway by regulating the involved genes. The loss of OTA biosynthesis ability does not affect fungal development as demonstrated by the comparison of Delta AcOTAbZIP strains and WT strains in terms of vegetative growth and asexual sporulation on three different media. Finally, no statistically significant differences in virulence were observed among Delta AcOTAbZIP strains and WT strains on artificially inoculated grape berries, demonstrating that OTA is not required by A. carbonarius for the pathogenicity process.

Automated summary

Aspergillus carbonarius is the main fungal species responsible for ochratoxin A (OTA) contamination in grapes. The bZIP transcription factor AcOTAbZIP plays a crucial role in the biosynthesis of OTA, and its deletion results in loss of OTA production and down-regulation of other related genes. Furthermore, OTA is not required for the pathogenicity process of A. carbonarius.