THE PROXIMAL MILK PROTEIN-BINDING FACTOR-BINDING SITE IS REQUIRED FORTHE PROLACTIN RESPONSIVENESS OF THE SHEEP BETA-LACTOGLOBULIN PROMOTERIN CHINESE-HAMSTER OVARY CELLS

To identify cis-acting prolactin (PRL) response elements within the sh eep beta-lactoglobulin (BLG) promoter, CHO cells were cotransfected wi th a rabbit PRL-receptor (PRL-R) expression plasmid and a number of BL G-CAT constructs. Resection through the 4200 bp BLG promoter diminishe d the PRL response. Mutation of the proximal binding site for milk pro tein binding factor (MPBF), a previously described mammary gland trans cription factor, abolished the PRL inducibility of full length and sho rter forms of the promoter. MPBF was shown to be similar to the Stat p rotein mammary gland factor (MGF) which has been shown to mediate PRL responsiveness of the rat beta-casein gene in mammary cells. MPBF bind ing activity was detected in the nucleus of CHO cells and was increase d 2-6-fold in cells stably transfected with the PRL-R. The lactating m ammary gland has high levels of MPBF binding activity and it is likely that this has an important role in the PRL induction of a variety of milk protein genes.