Regulation of the expression of genes encoding types I, II, and III collagen during chick embryonic development

During the embryonic development of the chicken, stimulation of production of collagen-enriched tissue such as bone matrix, cartilage matrix, and skin dermis occurs between day 7 and day 15. We have examined the levels of the RNAs encoding the interstitial collagens (types I, II, and III) to determine if this developmental progress is associated with increased accumulation of collagen RNA. Using cell-free translation and DNA:RNA hybridization techniques, we have detected a coordinate enhancement of approximately 10-fold in the steady state levels of whole embryo RNAs encoding the alpha 1(I), alpha 2(I), alpha 1(II), and alpha 1(III) collagens between 5 and 10 days of chick embryonic development. The developmental pattern of expression of these collagen genes in whole embryos is in marked contrast to that of two noncollagenous proteins, cellular fibronectin and beta-actin, whose RNA levels were not found to change dramatically during early chicken development. In addition, we have observed that at least 3 of the 4 collagen genes examined were expressed at low levels beginning between 1-2 days. Both 5'- and 3'-specific alpha 2(I) collagen gene DNA probes hybridized to early stage and late stage chick embryo RNAs of identical sizes. DNase I-hypersensitive sites have been detected near the 5' end of the alpha 2(I) collagen gene in chromatin isolated from both 2-day and 5-day embryos, representing developmental time points well before and at the threshold of the onset of enhanced collagen RNA synthesis, respectively. These results suggest that the same gene is expressed in early and late chick embryos to yield alpha 2(I) collagen RNAs of similar structure.