The two Caenorhabditis elegans basement membrane (type IV) collagen genes are located on separate chromosomes

We have identified and characterized the two genes, clb-1 and clb-2, that encode basement membrane collagen (IV) in Caenorhabditis elegans. Both genes encode 5.5-kilobase mRNAs, similar in size to the mammalian and Drosophila type IV collagen gene transcripts but much larger than the cuticle collagen transcripts of C. elegans. The nucleotide sequences of the NC1 regions of both genes were determined. Comparisons of the clb-1 and clb-2 NC1 amino acid sequences with those of mouse and human show that clb-1 shares 72% identity with mammalian alpha 2(IV) and clb-2 shares 63% identity with mammalian alpha 1(IV), suggesting that clb-1 is the alpha 2(IV) homologue and clb-2 is the alpha 1(IV) homologue. The presence of two type IV collagen genes in C. elegans and the mammals, but a single gene in Drosophila, indicates that the primordial type IV collagen gene had already duplicated early in the evolution of the invertebrates and that one of the genes has been subsequently lost from Drosophila. The mouse and human alpha 1(IV) and alpha 2(IV) genes are separated by only about 130 base pairs and are transcribed in opposite directions from overlapping promoters. We show that the C. elegans genes are located on separate chromosomes, clb-1 on X and clb-2 on III, demonstrating that the mammalian arrangement is not a requirement for all type IV collagen genes. We have identified a candidate genetic locus for the clb-2 basement membrane collagen gene, which will allow us to pursue a genetic analysis of basement membrane structure and function in C. elegans.