A versatile peroxidase able to oxidize Mn(2+) as well as phenolic and nonphenolic aromatic compounds is produced in peptone-containing liquid cultures of Pleurotus eryngii encoded by the gene mnpl. The regulation of its transcript levels was investigated by Northern blotting of total RNA. High-peroxidase transcripts and activity were found in cultures grown in glucose-peptone medium, whereas only basal levels were detected in glucose-ammonium medium. The addition of more than 25 microM Mn(2+) to the former medium did not result in detectable peroxidase transcripts or activity. Potential regulators were also added to isolated mycelium. In this way, it was shown that high transcript levels (in peroxidase-expressing mycelium) were maintained on peptone, whereas expression was not induced in short-term incubation experiments. Similar results were obtained with Mn(2+) ions. Strong induction of mnpl expression was caused by exogenous H(2)O(2) or by continuous H(2)O(2) generation during redox cycling of menadione. By the use of the latter system in the presence of Fe(3+), which catalyzes the reduction of H(2)O(2) to hydroxyl radical, it was shown for the first time that the presence of this strong oxidant causes a rapid increase of the transcripts of a ligninolytic peroxidase. In conclusion, peptone and Mn(2+) affect the levels of transcripts of this versatile peroxidase in culture, and reduced oxygen species induce short-term expression in isolated mycelium, probably via a stress response mechanism.