Histone Biosynthesis in the Mammary Gland during Development and Lactation

Abstract

Abstract The biosynthesis of DNA, histones, and other proteins has been studied in vivo in the rat mammary gland for the last 6 days of pregnancy and the first 6 days of lactation. In order to evaluate the incorporation of radioactive precursors, the pool of free amino acids in the gland was examined and was found to change both qualitatively and quantitatively as the gland developed. Furthermore, 15 min after the injection of lysine-14C the specific activity of lysine in the pool was observed to differ at different stages in pregnancy and lactation. The rate of incorporation of thymidine-3H into nuclear DNA showed a general downward trend during the period studied except for a very abrupt burst, consistent with a wave of cell division, on the 1st day of lactation. The rates of incorporation of lysine-14C followed qualitatively similar patterns for soluble and residual nuclear proteins and for histones. Like the synthesis of DNA, the synthesis of nuclear proteins reached a maximum rate on the 1st day postpartum and then declined. The rate of cytoplasmic protein synthesis remained high as lactation proceeded. Histones were separated chromatographically into lysine-rich, slightly lysine-rich, and arginine-rich classes; and the lysine-rich class was resolved further into its subfractions. The rates of synthesis for all of the histones followed similar patterns during late pregnancy and early lactation. However, at all times the lysine-rich and arginine-rich histones had higher rates of synthesis than the slightly lysine-rich histone. One of the subfractions within the lysine-rich class also had a higher rate of synthesis than the other subfractions.