Two overlapping cDNA clones that cover the complete length of the mRNA for human type III procollagen were characterized. The data provided about 2500 base pairs of sequence not previously defined for human type III procollagen. Two tripeptide sequences of -Gly-Xaa-Yaa- were identified that were not detected previously by amino acid sequencing of human type III collagen. The two additional tripeptide units, together with three previously detected, establish that the alpha 1 (III) chain is 15 amino acids longer than either the alpha 1 (I) or alpha 2 (I) chains of type I collagen. The additional tripeptide units made hydropathy plots of the N-terminal and C-terminal regions of type III collagen distinctly different from those of type I collagen. The data also demonstrated that human type III procollagen has the same third base preference in codons for glycine, proline and alanine that was previously found with human and chick type I procollagen. In addition, comparison of two cDNA clones from the same individual revealed a variation in structure in that the codon for amino acid 880 of the alpha 1 (III) chain was -CTT- for leucine in one clone and -TTT- for phenylalanine in the other.

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