Albinism, due to a lack of melanin pigment, is one of the oldest known mutations in mice. Tyrosinase (monophenol oxygenase, EC 1.14.18.1) is the first enzyme in the pathway for melanin synthesis, and the gene encoding this enzyme has been mapped to the mouse albino (c) locus. We have used mouse tyrosinase cDNA clones and genomic sequencing to study the albino mutation in laboratory mice. Within the tyrosinase gene coding sequences, a G to C transversion at nucleotide 308, causing a cysteine to serine mutation at amino acid 103, is sufficient to abrogate pigment production in transgenic mice. This same base pair change is fully conserved in classical albino strains of laboratory mice. These results indicate that a conserved mutation in the tyrosinase coding sequences is responsible for the classical albino mutation in laboratory mice, and also that most albino laboratory mouse strains have been derived from a common ancestor.

An attempt was made in the present study to express mouse tyrosinase cDNAs fused with the authentic genomic 5' non-coding flanking sequence in cultured albino melanocytes. One of the cDNA sequences, which expressed successfully and produced melanin pigments, was analyzed with respect to deduced amino acid sequence. Sequencing of the tyrosinase genomic gene revealed the existence of several sets of a characteristic structure which consists of a chain of two successive stem structures, CCAAT-homology and TATA box at its 5' non-coding region.It seems possible that this region represents the regulatory element of the tyrosinase gene. Unusually long GA cluster at 5' upstream region was also found.

A cDNA library was constructed from poly (A)+ mRNA from mouse melanocytes and screened using anti-tyrosinase antiserum and oligonucleotide probes corresponding to amino acid sequence of tyrosinase. Sequencing of some cDNA clones positive in these screenings gave a nucleotide sequence of 1838 nucleotides including a open reading frame of 1344 nucleotides.

Murine albinism is characterized by complete lack of melanin pigments in skin and retina. In order to study the molecular basis of albinism, we have cloned and characterized the tyrosinase gene of BALB/c mice (c/c). Sequence analysis of this gene reveals a point mutation at nucleotide residue 387 (G+C transversion) causing a Cys+Ser substitution at position 85 in one of the cysteine-rich domains of the tyrosinase molecule. Since this G-C transversion creates an additional DdeI site, we were able to confirm that this mutation is actually present in BALB/c genomic DNA using DNA amplification techniques. In contrast, both C57BL/6 (C/c> and DBA/2 (C/C) mouse strains carry the G residue at the same position, suggesting that this point mutation is specific for the albino mutation at the c locus. Moreover, we were able to show that the tyrosinase containing Ser-85 is not functional in transient expression of its cDNA. We therefore suggest that a G+C transversion at nucleotide residue 387 of the tyrosinase gene could lead to the albino phenotype of BALB/c mouse.Albinism is an inherited disorder of melanin formation and is characterized by decrease or absence of melanin in the skin, hair and eyes (reviewed in [l]). Murine albinism is also characterized by complete absence of melanin pigments, although it is not clear whether albino animals lack tyrosinase, a multi-functional copper-containing enzyme that is essential for melanin biosynthesis in pigment cells [2]. It catalyzes the conversion of tyrosine to dihydroxyphenylalanine (dopa) and dopa to dopaquinone. Previous studies showed virtually no tyrosinase activity [3,4] or a low but detectable activity in albino mouse skin [5]. Recently, Tamate et al. [6] showed the absence of immunoreactive tyrosinase in albino mouse skin, whereas others have shown the presence of immunoreactive proteins [7]. More recently, Halaban et al. [8] demonstrated that their anti-tyrosinase polyclonal antibodies precipitated peptides from BALB/c mouse tyrosinase of about 42 kDa and 47 kDa, smaller than the normal 80-kDa tyrosinase. They therefore suggested that BALB/c mouse tyrosinase is more sensitive to proteolytic degradation than the wild-type enzyme [8].In order to study the molecular basis of murine albinism, we have isolated a pigment-cell-specific mouse cDNA, pMT4,

Vesiculo-globular bodies, 40 nm in diameter, are present in melanosomes. The mode of their involvement in melanosomal differentiation was studied by ultrastructural comparison of eu- and pheomelanogenesis occurring in retinal and follicular melanocytes. We found that the number and distribution of these bodies differ significantly with types of melanogenesis and tissues. They were not affected by physical stimuli nor by embryonic origin of melanocytes. The earliest form of melanosomes is identical in eu- and pheomelanogenesis. The vesiculo-globular bodies are involved in organization of melanosomal constituents. In eumelanogenesis, they are more numerous in feather than in retina and hair. They are least numerous in white hair and pink eyes where melanization is blocked. During melanosomal development, they become associated with melanosomal inner lamellae and their outer surface becomes melanized, but their core is hardly melanized, thus leaving small vesicular structures. In pheomelanogenesis, their number is almost equal in feather and hair. Lamellae are not formed, but these bodies fuse with each other to form an amorphous matrix on complete differentiation of melanosomes.