Multiple copies of beta-lactoglobulin promoter do not function as LCR

Increasing the number of transcription factor binding sites within a constr uct can enhance expression. In an attempt to create a synthetic locus contr ol region for mammary expression, we have generated beta-lactoglobulin-repo rter constructs with multiple copies of the cluster of transcription sites normally located within the proximal promoter. These constructs were functi onally tested by stable transfection of mammary epithelial cells in vitro a nd in transgenic mice in vivo. Rather than enhancing expression, multimeris ation of the promoter region acted neither in vivo nor in vitro to enhance expression. Indeed, its presence reduced expression. This failure to enhanc e expression was reflected in the inability of this region to form a DNaseI hypersensitive site autonomously in mammary chromatin in vivo. It is impli cit from our study that not all combinations of transcription factor bindin g sites will enhance transcription. (C) 2000 Academic Press.