Submitted to: Mycotoxin Research
Publication Type: Review Article
Publication Acceptance Date: 1/10/2006
Publication Date: 3/1/2006
Citation: Cary, J.W., Ehrlich, K., Kale, S.P., Calvo, A.M., Bhatnagar, D., Cleveland, T.E. 2006. Regulatory Elements in Aflatoxin Biosynthesis. Mycotoxin Research. 22(2):105-109.
Technical Abstract: Aflatoxin (AF) biosynthesis in fungi is responsive to environmental cues, such as carbon and nitrogen source, stress, plant constituents (i.e. volatiles and tannins), and physical factors such as pH and temperature. These environmental stimuli are transduced via complex signaling cascades that control the expression of both global-acting and AF pathway-specific transcription factors. Two known AF pathway-specific regulator genes, aflR and aflJ, have been localized to the AF biosynthetic gene cluster in Aspergillus flavus and A. parasiticus. These genes are also present in the sterigmatocystin (ST) cluster of A. nidulans. The aflR gene was first characterized in A. flavus by Payne et al. and in A. parasiticus by Chang et al. AflR protein is a Gal4-type, Zn-finger binuclear cluster, positive-acting transcription factor that is required for expression of all known AF biosynthetic genes. AflR activates transcription of AF pathway genes following binding to the conserved 11 bp palindromic sequence, 5’-TCGN5CGR-3’, present in the promoter regions of most AF genes. Fungal isolates in which the aflR gene has been inactivated no longer produce AF nor express AF pathway genes while fungi engineered to over-express aflR produce increased levels of AF transcripts leading to increased levels of AF production. AflJ is a second gene in the AF pathway cluster that appears to play a role in regulation of AF biosynthesis. The aflJ gene was first characterized by Meyers et al. AflJ resides adjacent to aflR in the AF gene cluster and the two genes are divergently transcribed. AflJ protein does not demonstrate significant identity to other proteins of known function in the databases. Interestingly, it was shown that aflJ mutants do not produce AF (or only very minute amounts) but they still express AF pathway genes though at reduced levels. So it appears that AflJ is not functioning like AflR as a transcriptional activator of AF pathway genes. Chang et al. used a yeast two-hybrid system to show that AflJ can bind to the C-terminal region of AflR. Based on AflJ’s ability to bind AflR, it has been proposed that AflJ is either acting as a transcriptional enhancer or co-activator of AflR. However, more studies will be needed to determine the exact role of AflJ in AF biosynthesis. An intergenic region of 758 bp is located between the bidirectionally transcribed aflR and aflJ genes. To analyze promoter function of the aflR gene, the entire 758-bp intergenic region as well as truncated forms of this region were used to drive expression of the Escherichia coli-' glucuronidase (GUS)-encoding gene uidA. Removal of sequences in the promoter from nucleotide -758 to -280 (with respect to the aflR translational start site) had no apparent effect on promoter activity, but further truncation to -118 enhanced gene expression nearly 5-fold. Therefore, there appears to be a negative regulatory element in the region from -280 to -118. Further removal of bases -118 to -100 almost entirely eliminated GUS gene expression. When the region from -118 to -107 was deleted, two-thirds of this activity was lost. Therefore, sequences in the 18-bp region from -100 to -118 appear to be critical for aflR promoter activity. This region overlaps a 10-bp palindrome (-120 to -111) and a purine-rich region. EMSA using nuclear extracts from A. parasiticus and oligonucleotide ligands covering the region from -81 to -173 revealed the presence of a putative PacC-binding site (5’-GCCARG-3’). Binding to the aflR PacC site is consistent with the function of this protein in repressing transcription of acid-expressed genes under alkaline conditions. AF biosynthesis in A. flavus occurs in acidic media, but is inhibited in alkaline media. It is possible that PacC binding to the -148 to -173 site has a negative effect on aflR expression. In a different study Chang et al.