Transcriptomic profiling of Aspergillus flavus in response to 5-azacytidine

Aspergillus flavus is a common saprophyte and opportunistic pathogen producing aflatoxin (AF) and many other secondary metabolites. 5-Azacytidine (5-AC), a derivative of the nucleoside cytidine, is widely used for studies in epigenetics and cancer biology as an inactivator of DNA methyltransferase and is also used for studying secondary metabolism in fungi. Our previous studies showed that 5-AC affects development and inhibits AF production in A. flavus, and that A. flavus lacks DNA methylation. In this study, an RNA-Seq approach was applied to explore the mechanism of 5-AC's effect on A. flavus. We identified 240 significantly differentially expressed (Q-value < 0.05) genes after 5-AC treatment, including two backbone genes respectively in secondary metabolite clusters #27 and #35. These two clusters are involved in development or survival of sclerotia. GO functional enrichment analysis showed that these significantly differentially expressed genes were mainly involved in catalytic activity and proteolytic functions. The expressed transcripts of most genes in the AF biosynthetic gene cluster in A. flavus showed no significant changes after treatment with 5-AC and were expressed at low levels, and the transcription regulator genes aflR and aflS in this cluster did not show differential expression relative to the sample without 5-AC treatment. We found that the veA gene, which encodes protein bridges VelB and LaeA, decreased profoundly the expressed transcripts, and brlA, which encodes an early regulator of development, increased its transcripts in A. flavus after 5-AC treatment. Our data support a model whereby 5-AC affects development through increasing the expression of brlA by depressing the expression of veA and AF production through suppressing veA expression and dysregulating carboxypeptidase activity, which then prevents the aflatoxisomes (vesicles) from performing their normal function in AF formation. Furthermore, the suppressed veA expression weakens or even interrupts the connection between VelB and LaeA, leading to dysregulation of the expression pattern of genes involved in development and secondary metabolism in A. flavus. The RNA-seq data presented in this work were also served to improve the annotation of the A. flavus genome. This work provides a comprehensive view of the transcriptome of A. flavus responsive to 5-AC and supports the conclusion that fungal development and secondary metabolism are co-regulated.