Pka, Ras and RGS Protein Interactions Regulate Activity of AflR, a Zn(II)2Cys6 Transcription Factor in Aspergillus nidulans

Sterigmatocystin (ST) is a carcinogenic polyketide produced by several filamentous fungi including Aspergillus nidulans. Expression of ST biosynthetic genes (stc genes) requires activity of a Zn(II)2Cys6 transcription factor, AflR. aflR is transcriptionally and post-transcriptionally regulated by a G-protein/cAMP/protein kinase A (PkaA) signaling pathway involving FlbA, an RGS (regulator of G-protein signaling) protein. Prior genetic data showed that FlbA transcriptional regulation of aflR was PkaA dependent. Here we show that mutation of three PkaA phosphorylation sites in AflR allows resumption of stc expression in an overexpression pkaA background but does not remediate stc expression in a (delta) flbA background. This demonstrates negative regulation of AflR activity by phosphorylation and shows that FlbA post-transcriptional regulation of aflR is PkaA independent. AflR nucleocytoplasmic location further supports PkaA-independent regulation of AflR by FlbA. GFP-tagged AflR is localized to the cytoplasm when pkaA is overexpressed but nuclearly located in a (delta) flbA background. aflR is also transcriptionally and posttranscriptionally regulated by RasA. RasA transcriptional control of aflR is PkaA independent but RasA posttranscriptional control of AflR is partially mediated by PkaA.