Objective: The amelogenin gene is essential for dental enamel formation. Two amelogenin genes are generally found in both sex chromosomes of many eutherian mammals, but only a single amelogenin mRNA in some lower mammals and reptiles. However, an amphibian, Xenopus laevis, has two types of amelogenin mRNAs in both sexes. In this study, we determined the amelogenin mRNA in Xenopus (Silurana) tropicalis, of which the genome size is much smaller than that of X. laevis, and investigated the structure of its amelogenin gene. Methods: The total mRNA was extracted from the maxilule, mandibule and leg muscle of X. tropicalis, and their cDNA templates were prepared for RT-PCR. In order to detect the 5' and 3' ends of the mRNA, the 5'- and 3'-Full RACE Core Set (TaKaRa) were used. Results: In both sexes of X. tropicalis, two amelogenin mRNAs were strongly expressed in the toothed maxillule, hardly expressed in the thoothless mandible, and not in the leg muscle. They had two different 5' non coding regions followed by a single entire amino acid coding region. Regarding the structure of the amelogenin gene, a total of eight exons were found. The first three exons were the promotor region, and the coding region was from the middle of the 4th to the end of the 7th exon. The 8th exon was the 3' non coding region. Among them, the 5th, 6th and 7th exons corresponded to exons 3, 5 and 6 in the mammal gene. The nucleotid sequence of the coding region of X. tropicalis was very similar to the amalogenin mRNA clone 2 of X. laevis. Conclusion: The existence of two types of promotor regions and no nucleotid variation of the coding region in both sexes of X. tropicalis amelogenin mRNA were confirmed.

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