The manganese peroxidase gene family (mnps) is a part of the ligninolytic system of Pleurotus ostreatus. This gene family is comprised of nine members, mnp1-9, encoding short manganese peroxidases (short-MnPs) or versatile peroxidases (VPs). We show that unlike in Mn2+-amended glucose-peptone (GP) medium, where redundancy among mnps was reported, in Mn2+-deficient GP medium mnp4 [encoding versatile peroxidase isoenzyme 4 (VP4)] has a key and nonredundant function. The abundance of mnps transcripts at time points corresponding to the tropophase (active growth), early idiophase, and idiophase indicates that mnp4 is the predominantly expressed mnp gene and that its relative predominance is dependent on the age of the culture. In this medium, azo dye, Orange II (OII) decolorization occurs only during the idiophase and a Delta mnp4 strain showed a drastic reduction in this decolorization. Three degradation metabolites were identified by liquid chromatography-mass spectroscopy (LC-MS), indicating both asymmetric and symmetric enzymatic cleavage of the azo-bond. In addition, the culture filtrate of Delta mnp4 showed negligible values of oxidation capability of four typical VP substrates: Mn2+, 2,6-dimethoxyphenol, phenol red, and Reactive Black 5 (RB5), compared to the wild-type strain PC9. We concluded that under Mn2+-deficient GP culture, VP4 (encoded by mnp4) is the main active ligninolytic enzyme able to oxidize Mn2+ as well as high and low redox potential aromatic substrate, including dyes. Furthermore, other VPs/MnPs do not compensate for the lack of VP4 activity.