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Eukaryotic Cell doi:10.1128/EC.00085-07
Copyright (c) 2007, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

Response regulators SrrA and SskA are central components of a phosphorelay system involved in stress signal transduction and asexual sporulation in Aspergillus nidulans

Departamento de Genética Molecular, Instituto de Fisiolog'a Celular, Universidad Nacional Autónoma de México, Apartado Postal 70-242, 04510, México, D.F., México

^* To whom correspondence should be addressed. Email: jaguirre{at}ifc.unam.mx.

	Abstract

Among eukaryotes, only slime molds, fungi and plants contain signal transduction phosphorelay systems. In filamentous fungi, multiple sensor kinases appear to use a single HPt phosphotransfer protein to relay signals to two response regulators (RR). In Aspergillus nidulans RR SskA mediates activation of MAPK SakA in response to osmotic and oxidative stress whereas the functions of RR SrrA were unknown. Here we use a genetic approach to characterize the srrA gene as a new member of the skn7/prr1 family and to analyze SrrA roles in the phosphorelay system composed by RR SskA, HPt protein YpdA and the sensor kinase NikA. While mutants lacking HPt protein YpdA are unviable, mutants lacking SskA (sskA), SrrA (srrA) or both (srrA sskA) RRs, are viable and differentially affected in osmotic and oxidative stress responses. Both RRs are involved in osmostress resistance but sskA mutants are more sensitive to this stress, and only SrrA is required for H₂O₂ resistance and H₂O₂-mediated induction of catalase CatB. In contrast, both RRs are individually required for fungicide and calcofluor sensitivity, and for normal sporulation and conidiospore viability. The srrA and sskA sporulation defects appear to be related to decreased mRNA levels of the key sporulation gene brlA. In contrast, conidiospore viability defects do not correlate with the activity of the spore-specific catalase CatA. Our results support a model in which NikA acts upstream of SrrA and SskA to transmit fungicide signals and regulate asexual sporulation and conidiospore viability. In contrast, NikA appears dispensable for osmotic and oxidative stress signaling. These results highlight important differences in stress signal transmission among fungi and define a phosphorelay system involved in oxidative and osmotic stress, cell wall maintenance, fungicide sensitivity, asexual reproduction and spore viability.