Aspergillus nidulans transcription factor AtfA interacts with the MAPK SakA to regulate general stress responses, development and spore functions

Feb 16, 2011Molecular microbiology

The Aspergillus nidulans stress regulator AtfA works with the MAPK SakA to control stress responses, growth, and spore activity

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Abstract

SakA phosphorylation is active in asexual spores and remains phosphorylated in dormant conidia.

The response regulator SskA transmits signals related to osmotic and oxidative stress to the stress MAPK SakA.
AtfA is permanently located in the nucleus and interacts with SakA, indicating a collaborative role in stress responses.
AtfA is necessary for the expression of multiple genes and the accumulation of SakA in conidia.
SakA's phosphorylation in spores relies on SskA and not on other components of the phosphorelay system.
Constitutive phosphorylation of SakA caused by the fungicide fludioxonil inhibits germ tube formation and nuclear division.
Phosphorylation and dephosphorylation of SakA during spore dormancy and germination suggest a mechanism for regulating growth states.

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Fungi utilize a phosphorelay system coupled to a MAP kinase module for sensing and processing environmental signals. In Aspergillus nidulans, response regulator SskA transmits osmotic and oxidative stress signals to the stress MAPK (SAPK) SakA. Using a genetic approach together with GFP tagging and molecular bifluorescence we show that SakA and ATF/CREB transcription factor AtfA define a general stress-signalling pathway that plays differential roles in oxidative stress responses during growth and development. AtfA is permanently localized in the nucleus, while SakA accumulates in the nucleus in response to oxidative or osmotic stress signals or during normal spore development, where it physically interacts with AtfA. AtfA is required for expression of several genes, the conidial accumulation of SakA and the viability of conidia. Furthermore, SakA is active (phosphorylated) in asexual spores, remaining phosphorylated in dormant conidia and becoming dephosphorylated during germination. SakA phosphorylation in spores depends on certain (SskA) but not other (SrrA and NikA) components of the phosphorelay system. Constitutive phosphorylation of SakA induced by the fungicide fludioxonil prevents both, germ tube formation and nuclear division. Similarly, Neurospora crassa SakA orthologue OS-2 is phosphorylated in intact conidia and gets dephosphorylated during germination. We propose that SakA-AtfA interaction regulates gene expression during stress and conidiophore development and that SAPK phosphorylation is a conserved mechanism to regulate transitions between non-growing (spore) and growing (mycelia) states.

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Aspergillus nidulans transcription factor AtfA interacts with the MAPK SakA to regulate general stress responses, development and spore functions

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