Baruch, A., Institute of Animal Science, Volcani Center, Bet Dagan 50250, Israel
Shani, M., Institute of Animal Science, Volcani Center, Bet Dagan 50250, Israel
Barash, I., Institute of Animal Science, Volcani Center, Bet Dagan 50250, Israel

A dramatic uncoupling of the expression of chimaeric β-lactoglobulin (BLG)/human serum albumin (HSA) gene constructs at the RNA and protein levels was observed in cultured mammary explants of virgin transgenic mice. Upon explantation, both HSA RNA and protein were expressed at high levels. However, when the explants were grown in hormone-free medium, HSA RNA continued to accumulate, whereas the synthesis of the corresponding protein was dependent on the presence of insulin and prolactin with a minor contribution of hydrocortisone. The untranslated HSR RNA was indistinguishable from its translatable counterpart in its mobility on agarose gels, was transported normally from the nucleus to the cytoplasm and was translated efficiently in rabbit reticulocyte lysate. In the presence of cycloheximide, HSR RNA rapidly disappeared, suggesting a dependency on ongoing protein synthesis. Its estimated half-life of 5-6 h in hormone-free medium increased significantly in the presence of insulin, hydrocortisone and prolactin and was comparable to that of β-casein RNA. The uncoupling of the expression of the BLG/HSA transgenes at the RNA and protein levels was also confirmed by in situ hybridization and immunohystochemistry on sections from virgin mammary explants. HSA synthesis was initiated within 13 h of higher than that observed with insulin alone. Addition of hydrocortisone contributed to an additional 20% in HSA synthesis. We believe this is the first demonstration of translational control of exogenous milk protein gene expression in the mammary gland of transgenic animals.