Part of a multicomponent enzyme which catalyzes the degradation of phenol and some of its methylated derivatives (PubMed:2254259). DmpM is a regulatory subunit that stimulates the phenol hydroxylase activity of the complex (PubMed:10451366, PubMed:12186554). The steady-state rate of phenol hydroxylase turnover is dependent on the DmpM concentration, with a maximum observed rate at about 1.5 DmpM per oxygenase monomer. Higher concentrations of DmpM inhibit phenol hydroxylase activity (PubMed:12186554). May act by altering the redox potential of the oxygenase (PubMed:12186554). Required for growth on phenol and for in vitro phenol hydroxylase activity (PubMed:2254258, PubMed:2254259). H(+) + NADH + O2 + phenol = catechol + H2O + NAD(+) Aromatic compound metabolism; phenol degradation. Active as a monomer (PubMed:10451366). Formation of dimers inactivates the protein (PubMed:10451366). The multicomponent enzyme phenol hydroxylase is formed by DmpL (P1 component), DmpM (P2 component), DmpN (P3 component), DmpO (P4 component) and DmpP (P5 component) (PubMed:2254259). Cells lacking this gene cannot grow on phenol. Belongs to the TmoD/XamoD family.