The internal chondrocyte-specific promoter of the chick type III collagen gene is activated by AP1 and is repressed in fibroblasts by a complex containing an LBP1-related protein.

Abstract

The chick type III collagen gene contains an internal promoter in intron 23 in addition to the promoter preceding exon 1. The internal promoter, which is used preferentially in cultured chondrocytes, directs production of an alternative transcript that cannot encode type III collagen. This promoter is used ineffic-iently in skin fibroblasts, which transcribe the gene from the upstream promoter. We show below that the internal promoter is regulated by an activation element containing a potential activator protein 1 (AP1) site and a repressor element containing a potential binding site for leader binding protein 1 (LBP1). Electro-phoretic mobility shift assays indicate that the activation and repressor elements are bound by AP1 and an LBP1-related protein, respectively. Replacement of the AP1 site resulted in substantially decreased promoter activity in both chondrocytes and fibroblasts, indicating that this site is required for promoter function, but the low level of promoter activity in fibro-blasts is not due to loss of functional AP1. In contrast, replacement of the LBP1-like site increased activity only in fibroblasts, suggesting that this site is responsible in part for repression of promoter activity in fibroblasts.