EXPRESSION OF THE TYROSINASE-ENCODING GENE IN A COLORLESS MELANOPHOREMUTANT OF THE MEDAKA FISH, ORYZIAS-LATIPES

In the medaka fish Oryzias latipes many mutants for body colors have b een isolated. Among them, a colorless melanophore mutant b, carrying b alleles homozygously, has pigmented black eyes but orange-colored ski n with amelanotic melanophores, suggesting the presence of a tissue-sp ecific mechanism of melanin formation. To cast light on the molecular basis of the mechanism, we have cloned cDNAs for tyrosinase (Tyr), a k ey enzyme in melanin biosynthesis, from the wild-type (wt) fish. DNA s equence analysis revealed that all clones encode a protein of 540 amin o acids, having five potential glycosylation sites and two copper-bind ing sites that are characteristic features of Tyr. Genomic DNA blot an alysis disclosed that the Tyr gene is present as a single copy in the fish genome. Using a cDNA clone as a probe, RNA blot analysis was carr ied out. In the wt, the 2.2-kb Tyr mRNA was expressed in eyes and skin but not in liver, corresponding to tissue-specific melanin formation, In the b mutant, contrary to expectation, the mRNA was detected not o nly in eyes but also in amelanotic skin. Therefore, pigmentation of th e skin controlled by b is not directly related to expression of the Ty r gene.