Tyrosinase activity in Streptomyces glaucescens is controlled by three chromosomal loci

In Streptomyces glaucescens the production of the enzyme tyrosinase (EC 1.14.18.1) is controlled by at least three loci: MELA, MELB, and MELC. Mutations in MELA and MELB are rare. They both map on the S. glaucescens chromosome in the interval between the markers leu-2 and ura-3. MELC, however, shows an exceptionally high mutation rate. MelC strains arise spontaneously with a frequency of about 0.1%, and they can be specifically induced with ethidium bromide, acridine orange, or by prolonged storage of spores or mycelium at 4 degrees C. Ultraviolet (UV) irradiation or treatment with N-methyl-N'-nitro-N-nitrosoguanidine under conditions which induce auxotrophic mutations, however, do not specifically induce the appearance of tyrosinase-negative strains. Most, but not all, of the melC strains acquire additional mutations which make them sensitive to streptomycin (strS) and unable to produce hydroxystreptomycin. Mapping of the mutations classified as melC proves to be difficult. Some of the matings indicate a map position between his-2 and pro-1, while others, all highly unbalanced crosses, can not be analyzed in a meaningful way because of abnormal segregation of the reference markers. This is probably due to some of the additional mutations which occur frequently with the melC mutations and not to extrachromosomal inheritance of MELC.