Identification of a cartilage-specific promoter within intron 2 of the chick alpha 2(I) collagen gene.

Chick cartilages contain type I collagen mRNAs but do not synthesize type I collagen. The 5' end of the mRNA derived from the alpha 2 type I collagen gene (alpha 2(I] in cartilage differs from the 5' end of the mRNA in cells and tissues that actively synthesize alpha 2(I) collagen. This difference in mRNA structure results from the use of a cartilage-specific transcription start site within intron 2 of the alpha 2(I) collagen gene. The use of the cartilage transcription start site replaces exons 1 and 2 with a 96-base exon contained within intron 2. The resulting transcripts contain several small open reading frames, all of which appear out of frame with the collagen coding sequence. The cartilage form of the mRNA no longer encodes alpha 2(I) collagen, thus explaining the absence of alpha 2(I) collagen in cartilage. Transcription of the alpha 2(I) collagen gene initiates at the previously described (bone/tendon) promoter in prechondrogenic limb mesenchyme, which synthesizes alpha 2(I) collagen. Thus, the cessation of alpha 2(I) collagen synthesis which occurs during differentiation of prechondrogenic mesenchymal cells into chondrocytes apparently results from the switch in promoter utilization from the bone/tendon promoter to the cartilage promoter.

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