Changes in chromatin template activity for in vitro transcription during the development of the rat mammary gland.

The template activities of chromatin isolated from virgin, pregnant, lactating, and regressed rat mammary glands were compared by using exogenously added bacterial and mammalian RNA polymerases. Template activity increased during the stage of pregnancy, then decreased during the stage from early lactation to regression. It showed the highest level at early lactation when intracellular levels of rRNA and mRNA are known to be at their maximums. These results suggest that transcriptional activity in the mammary gland is regulated, at least in part, by a modulation of chromatin template activity.