The amelogenins are highly conserved enamel-matrix proteins that are e
ssential for proper mineral formation. Transcriptionally active genes
encoding the bovine amelogenin proteins reside on both the X and Y chr
omosomes. Comparison of relative levels of amelogenin mRNAs at various
stages of development indicated that the X-chromosomal amelogenin mes
sage is at least six fold more abundant than the Y. Alternative splici
ng generates at least seven messages, five from the X primary transcri
pt, and two from the Y. The two most abundant X-chromosomal amelogenin
messages are approx. 850 and 450 nucleotides long, and nearly 10-fold
more 850-nucleotide mRNA can be measured than 450 nucleotide, which h
as lost most of exon 6 by splicing. The predominant small message enco
des leucine-rich amelogenin protein (LRAP), and amounts of LRAP messag
e are relatively constant during development. However, the amelogenin
message from which exon 3 has been spliced declines approximately 2.3-
fold, when compared to total X chromosomal amelogenin transcripts, sug
gesting differential regulation of alternative splicing. In addition,
a new exon was identified within genomic DNA, which was shown to be ex
pressed by the use of reverse transcriptase-polymerase chain reaction,
and the exons were renamed accordingly. This new exon-4 sequence is u
nusual in that it is not highly conserved between species.