Characterization of the conserved phosphorylation site in the Aspergillus nidulans response regulator SrrA

Ssk1- and Skn7-type response regulators are widely conserved in fungal His-Asp phosphorelay (two-component) signaling systems. SrrA, a Skn7-type RR of Aspergillus nidulans, is implicated not only in oxidative stress responses but also in osmotic adaptation, conidia production (asexual development), inhibition by fungicides, and cell wall stress resistance. Here, we characterized SrrA, focusing on the role of the conserved aspartate residue in the receiver domain, which is essential for phosphorelay function. We constructed strains carrying an SrrA protein in which aspartate residue D385 was replaced with either asparagine (N) or alanine (A). These mutants exhibited normal conidiation and partial oxidative stress resistance. In osmotic adaptation, mutants with substitution at SrrA D385 showed as much sensitivity as ΔsrrA strains, suggesting that SrrA plays a role in osmotic stress adaptation in a phosphorelay-dependent manner. The SrrA D385 substitution mutants showed significant resistance to fungicides and cell wall stresses. These results together led us to conclude that the conserved aspartate residue has a substantial impact on SrrA function, and that SrrA plays a role in several aspects of cellular function via His-Asp phosphorelay circuitry in Aspergillus nidulans.