Site-directed mutagenesis was used to determine the functional role of several residues of Streptomyces glaucescens tyrosinase. Replacement of His-37, -53, -193 or -215 by glutamine yields albino phenotypes, as determined by expression on melanin-indicator plates. The purified mutant proteins display no detectable oxy-enzyme and increased Cu lability at the binuclear active site. The carbonyl derivatives of H189Q and H193Q luminesce, with lambda max. displaced more than 25 nm to a longer wavelength compared with native tyrosinase. The remaining histidine mutants display no detectable luminescence. The results are consistent with these histidine residues (together with His-62 and His-189 reported earlier) acting as Cu ligands in the Streptomyces glaucescens enzyme. Conservative substitution of the invariant Asn-190 by glutamine also gives an albino phenotype, no detectable oxy-enzyme and labilization of active-site Cu. The luminescence spectrum of carbonyl-N190Q, however, closely resembles that of the native enzyme under conditions promoting double Cu occupancy of the catalytic site. A critical role for Asn-190 in active-site hydrogen-bonding interactions is proposed.