Rapidly-Labelled Rna in the Mouse Mammary Gland Before and During Lactation

SUMMARY Linear sucrose density gradient analysis showed that the synthesis of rapidly-labelled high molecular weight RNA was virtually absent in the mammary glands of virgin mice. The rapidly-labelled RNA was first evident in the mammary gland in pregnancy and was also present during lactation. The bulk of this newly-made nuclear RNA sedimented as 45S and 32S fractions after a 15-min [3H]uridine pulse period in vivo. No labelled 18S RNA was detectable in the nuclear fraction after the 15-min pulse but it was present in the cytoplasm, suggesting that the 18S RNA migrates to the cytoplasm almost immediately after its formation. Thirty minutes after injection of [3H]uridine, the initial radioactivity of the 45S region migrated to the 32S fraction and a labelled 28S peak was also present in the cytoplasmic RNA at 60 min, suggesting that the processed 28S ribosomal RNA in the mammary gland began to migrate to the cytoplasm between 30 and 60 min after the nuclear synthesis of the precursor molecule.