Synthesis and breakdown of rat liver RNA during the neonatal period

Abstract

The incorporation of radioactivity from labeled pyrimidine precursors (6- 14C-orotic acid and 2- 14C-uridine) and from 32P into liver RNA has been evaluated in fetuses and newborn rats. Very low values were recorded in the last days of intrauterine life and at birth. Soon after birth, there was a very sharp increase of the incorporation, and maximum values were attained at three days of life. Nuclear DNA-dependent RNA polymerase activity was also found to be very low in fetuses, but increased gradually after birth and reached the adult values only at about 25 days of of life. Ribonuclease activity of isolated nuclei and microsomal fractions were very high at birth and decreased in the first days of extrauterine life, attaining the lowest values in adult life. Similarly, autohydrolysis of endogenous nuclear or microsomal RNA was higher in young than in adult animals but did not undergo appreciable change during the first 3 weeks of life. Newborn liver nuclei and microsomes contained ribonuclease essentially in a free form, whereas in adult ones the latent form of enzyme was predominant. The significance of these findings in relation to the labeling of RNA, and with the changes in enzyme activities, liver size, and liver protein content in the perinatal period, is discussed.