Complex hormonal regulation of rat casein gene expression.

Cloned cDNAs for three rat casein mRNAs (CY, p, and y) and a fourth abundant mammary gland mRNA, designated pX32 mRNA, have been used to isolate pure single-stranded cDNA probes from total cDNA synthesized using poly(A+) mRNA.These probes have been employed in a sensitive cDNA titration hybridization assay to measure the levels of these mRNAs in total RNA extracts both during normal mammary gland development and in organ culture in response to the hormones, prolactin and hydrocortisone.The results of the solution hybridization experiments were compared to those of a dot hybridization assay in which doublestranded cloned cDNA probes were hybridized to RNA immobilized on diazobenzyloxymethyl-ceIlulose filters.A significant level of each of the mRNAs was found in virgin glands.During the first 6 days after conception, the relative proportions of these mRNAs changed considerably.The levels of all four mRNAs then increased coordinately during the remainder of pregnancy and lactation, In organ culture, both prolactin and hydrocortisone were found to be essential for maximum induction of all four mRNAs.By 24 h after prolactin ' J. M. Rosen and J. Rodgers, unpublished observations.