Developmental regulation of the sheep beta-lactoglobulin gene in the mammary gland of transgenic mice.

beta-Lactoglobulin (BLG) is the most abundant whey protein in sheep milk but it is not present in mouse milk. We have previously shown that transgenic mice carrying the BLG gene express it specifically in the mammary gland and secrete BLG into milk at high concentrations. Here we demonstrate that BLG transcription is correctly initiated in mice and that BLG synthesis is restricted to the secretory epithelial cells of the mammary gland. We have also determined the temporal pattern of milk protein gene expression and find that the BLG transgene is regulated coordinately with mouse beta-casein and that the patterns of regulation of BLG in mouse and sheep share some similarities.