The human tyrosinase gene is greater than 35kb and is organized in four introns and five exons [Tomita et al. (1989) Biochem. Biophys. Res. Commun., 164, 990-996]. Using the full-length cDNA encoding human tyrosinase and its exon-specific fragments as hybridization probes, we show that overall structural organization of the tyrosinase gene is unchanged in three patients affected with tyrosinase-negative oculocutaneous albinism (OCA). Moreover, we are able to show the presence of additional DNA segments cross-hybridizable to exon 4 or exon 5 of the tyrosinase gene in the genome of three OCA patients and a healthy individual. Namely, the exon 4-specific probe detected two bands in the DNA digested with EcoRI or HindIII and the exon 5-specific probe detected two bands in the Bgl II-digested DNA, in spite of the facts that no recognition sites for these enzymes are present in each exon. We have then isolated two phage clones harboring the distinct DNA segments, hybridizable to the exon 5 probe. Southern blotting analysis of each cloned DNA digested with Bgl II showed two different hybridization patterns as those detected in genomic DNA digested with Bgl II, confirming that there are at least two DNA segments hybridizing to the exon 5 sequence in human genome.
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