Interaction of DNA-binding proteins with a milk protein gene promoter in vitro: identification of a mammary gland-specific factor.

The minimal 5' regulatory region of the sheep beta-lactoglobulin gene (BLG), as defined in transgenic mice, was used to identify nuclear factors which may be involved in milk protein gene expression in the lactating mammary gland. This 406bp promoter region was dissected into short, overlapping, double-stranded oligonucleotides to facilitate identification of the bound proteins. A variety of sites, for both known and previously undescribed DNA-binding proteins, are occupied in vitro. Some of these factors were investigated in detail. Two forms of nuclear factor I (NFI), which have different recognition site affinities, are present in nuclear extracts from lactating mammary gland and bind to at least 5 sites in this BLG control element. In addition, a factor (milk protein binding factor, MPBF) which is specific to extracts from both mouse and sheep lactating mammary gland binds to 3 BLG promoter sites and may be a milk protein gene transcription factor.