alpha-Lactalbumin mRNA in 4-day lactating rat mammary gland

Rat alpha-Lactalbumin (alpha-LA) mRNA has been purified from the total RNA of 4-day lactating mammary gland by poly(U)-Sepharose chromatography and by fractionation of the poly(A)+ RNA on sucrose gradients. alpha-LA mRNA was homogeneous as judged by electrophoresis in urea/agarose gel and in 1.5% agarose gel under glyoxal-denaturation conditions, which gave a molecular weight of 210,000. The chymotryptic fingerprints of the protein synthesized with this mRNA, in a translational system derived from wheat germ, were similar to those of purified rat alpha-LA. In rat milk or lactating gland, the content of Mr 42,000 casein is 6 times greater than that of Mr 29,000 casein and 10 times greater than that of alpha-LA. However, alpha-LA mRNA (Mr 210,000) and two casein mRNas (Mr 460,000 and 390,000), coding for Mr 42,000 and 29,000 caseins, were present in equal proportions when measured with cDNA probes in the total RNA of 4- to 5-day lactating gland. Moreover, alpha-LA and total casein were synthesized in a ratio fo 1:2.5 in a wheat-germ translational system using the total RNA of the lactating gland. The results suggest post-transcription controls in the synthesis of these proteins.