COMPLETE EXON-INTRON ORGANIZATION OF THE HUMAN GENE FOR THE ALPHA-1 CHAIN OF TYPE-XV COLLAGEN (COL15A1) AND COMPARISON WITH THE HOMOLOGOUS COL18A1 GENE

The human gene for the (yl chain of type XV collagen (COL15A1) is abou t 145 kilobases in size and contains 42 exons. The promoter is charact erized by the lack of a TATAA motif and the presence of several Spl bi nding sites, some of which appeared to be functional in transfected He La cells. Comparison with Co118a1, which encodes the alpha 1(XVIII) co llagen chain homologous with alpha 1(XV), indicates marked structural homology spread throughout the two genes. The mouse Co118a1 contains o ne exon more than COL15A1, due to the fact that COL15A1 lacks sequence s corresponding to exon 3 of Col18a1, which encodes a cysteine-rich se quence motif. Twenty-five of the exons of the two genes are almost ide ntical in size, six of them contain conserved split codons, and the lo cations of the respective exon-intron junctions are identical or almos t identical in the two genes. The homologous exons include the closely adjacent first pair of exons and the exons encoding a thrombospondin- l homology found in the N-terminal noncollagenous domain 1, which are followed by the most variable part of the two genes, covering the C-te rminal half of their noncollagenous domain 1 and the beginning of the collagenous portion, after which most of the exons are homologous. The lengths of the introns are not similar in these genes, with two excep tions, namely the first intron, which is very short, less than 100 bas e pairs, and the second intron, which is very large, about 50 kilobase s, in both genes. It can be concluded that COL15A1 and Col18a1 are der ived from a common ancestor.