Expression and novel structure of a collagen gene in Drosophila

We report the structure and developmental expression of collagen gene sequences in Drosophila melanogaster. Collagen-like genomic clones were isolated by screening a Drosophila genomic library with a chicken pro alpha 2(I) cDNA clone as a hybridization probe. A 1.5-kilobase (kb) DNA sequence from a 9.2-kb DNA clone (pDCg1) is presented. Unlike the highly fragmented genes for vertebrate type I collagen, there is no evidence of a 54-base-pair primordial unit within this gene segment. Instead, the fragment is composed of two large coding sequences. Together they specify a sequence of 469 amino acids. This collagen product is composed almost entirely of the Gly-X-Y repeat characteristic of peptides involved in triple helix formation. Within the polypeptide there are four minor discontinuities in the Gly-X-Y pattern. Similar interruptions have been observed in a mouse basement membrane collagen protein sequence. Therefore, the Drosophila collagen gene may encode a nonfibrous collagen such as a basement membrane or cuticle collagen or a novel collagenous protein. By using the DNA segment of known sequence as a hybridization probe, a developmental sequence of polyadenylylated RNA samples was screened for the presence of homologous sequences. A RNA species 6.4 kb in length was detected as a prominent band only in the first- and second-instar larval stages. This pattern of developmental hybridization correlates with the production of the cuticle and basement membranes, and the large size of the RNA is consistent with its identification as a collagen-encoding RNA.