Proximal DNA elements mediate repressor activity conferred by the distal portion of the chicken collagen X promoter.

Collagen X is expressed specifically in hypertrophic chondrocytes within cartilage that is undergoing endochondral ossification. The chicken collagen X gene is transcriptionally regulated, and under the control of multiple cis elements within the distal promoter region (-4,442 to -558 base pairs from the transcription start) as well as the proximal region (-558 to +1). Our previous data (LuValle et al., [1993] J. Cell Biol. 121:1173-1179) demonstrated that the proximal sequence directed high reporter gene activity in the three cell types tested (hypertrophic chondrocytes, immature chondrocytes, and fibroblasts), while distal elements acted in an additive manner to repress the effects of the proximal sequence on reporter gene activity in non-collagen X expressing cells only (immature chondrocytes and fibroblasts). We show here that elements within the proximal sequence (nucleotides -557 to -513) are necessary for the cell-specific expression of type X collagen by hypertrophic chondrocytes. These elements bind to proteins of 100 kDa in all three cell types, and 47 kDa in non-collagen X expressing cells. Reporter gene activity in hypertrophic chondrocytes is reduced to the levels seen in non-collagen X-expressing cells in the absence of these elements.