Response Regulators SrrA and SskA Are Central Components of a Phosphorelay System Involved in Stress Signal Transduction and Asexual Sporulation in Aspergillus nidulans

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 histidine-containing phosphotransfer (HPt) protein to relay signals to two response regulators (RR). In Aspergillus nidulans, the RR SskA mediates activation of the mitogen-activated protein kinase SakA in response to osmotic and oxidative stress, whereas the functions of the RR SrrA were unknown. We used a genetic approach to characterize the srrA gene as a new member of the skn7/prr1 family and to analyze the roles of SrrA in the phosphorelay system composed of the RR SskA, the HPt protein YpdA, and the sensor kinase NikA. While mutants lacking the HPt protein YpdA are unviable, mutants lacking SskA (⌬sskA), SrrA (⌬srrA), or both RR (⌬srrA ⌬sskA) are viable and differentially affected in osmotic and oxidative stress responses. Both RR are involved in osmostress resistance, but ⌬sskA mutants are more sensitive to this stress, and only SrrA is required for H 2 O 2 resistance and H 2 O 2 -mediated induction of catalase CatB. In contrast, both RR are individually required for fungicide sensitivity and calcofluor resistance 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 to 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.

Characterization of NikA Histidine Kinase and Two Response Regulators with Special Reference to Osmotic Adaptation and Asexual Development inAspergillus nidulans

“…These observations are consistent with those of previous studies. 11,27) ÁnikA, however, showed a poor growth rate even on low osmolarity CD-agar plates, similarly to previous reports. 12) This phenomenon was also observed on another standard complex medium (and on YPD-agar plates) (data not shown).…”

“…1A and B), as has also been found in previous studies. 12,27) To investigate the function of NikA, we constructed a pair of ÁnikAÁsskA and ÁnikAÁsrrA double mutants capable of growing as vigorously as the reference cells (Fig. 3A).…”

“…11,27) Hence it was also of interest to examine ÁnikA with particular reference to asexual development. For this purpose, the numbers of conidia harvested from CD-agar plates were counted ( Fig.…”

“…22) In this context, it has been demonstrated that the A. nidulans RRs of SskA and SrrA also additively contribute to osmotic adaptation. 27) The fungal His-Asp phosphorelay system appears to be implicated not only in stress responses for growth, but also in the control of developmental processes. In N. crassa, an rrg-1 mutant is unable to produce protoperithecia, a female reproductive structure.…”

Mitogen-activated protein kinase signaling pathways of the tangerine pathotype of Alternaria alternata

“…[108][109][110][111] In fungi, Skn7 has been demonstrated to be required for sporulation, fungicide sensitivity, cell wall biosynthesis, oxidative stress adaptation, osmotic stress response, cell cycle, sexual mating and pathogenicity/virulence. 101,109,[112][113][114][115][116][117][118][119][120] The A. alternata Skn7 homolog was cloned and functionally inactivated, revealing a close association of Skn7 response regulator and resistance to osmotic and oxidative stress, conidiation, conidial morphology and fungicide sensitivity. 44 The Ssk1-Hog1 pathway, independent of Hsk1 (a Sln1p ortholog), confers resistance primarily to salts and oxidative stress, whereas the Hsk1-Skn7 pathway is responsible for sugar-induced osmotic and oxidative stress ( Figure 2).…”