Base composition of rapidly sedimenting nuclear ribonucleic acid of the regenerating liver

Developmental and regional variations in ribonucleic acid synthesis on cerebral chromatin

1. Chromatin was prepared from purified nuclei isolated from liver and cerebral regions of the rat. 2. The capacity of these preparations to promote RNA synthesis in the presence of bacterial RNA polymerase was determined. 3. The rate of RNA synthesis on chromatin was normally 12-21% of the rate observed with native DNA, but was markedly stimulated on addition of 200mm-ammonium sulphate. 4. At physiological concentrations (80mug./ml.), the brain-specific S-100 protein inhibited RNA synthesis on DNA and chromatin. 5. Cerebral chromatin from foetal and newborn animals was more active in RNA synthesis than were the analogous preparations from liver. 6. Cerebellar chromatin maintained a high rate of RNA synthesis during brain maturation. In contrast, RNA synthesis on chromatin from other brain regions and liver declined with age of the rat. 7. RNA synthesized on chromatin stimulated amino acid incorporation in an Escherichia coli ribosomal system and hybridized with homologous DNA. 8. RNA synthesized on chromatin from adult cortex or hindbrain hybridized with DNA to a greater extent than that synthesized on cerebellar chromatin. 9. The proportion of RNA formed on cerebral-cortical chromatin that hybridized with DNA increased with age of the rat. 10. The results indicate that the total amount and the types of RNA synthesized on cerebral chromatin vary regionally and during development.

Sequential changes in classes of RNA during compensatory growth of the kidney

The synthesis of nuclear precursors of rRNA and of cytoplasmic mRNA has been estimated in mouse kidneys following contralateral nephrectomy. 1. The production of nuclear rRNA parallels mitotic activity in the proximal tubule cells. Its peak is 2 days after nephrectomy; there may be a second peak at 8 days. 2. The production of mRNA is almost the reciprocal of rRNA production. It is least at 2 days and most at 4 days after nephrectomy. It may also decrease immediately after nephrectomy. 3. Growing kidney cells may build a store of ribosomes before they elaborate mRNA. The sequential relation between rRNA and mRNA synthesis raises the possibility that production of rRNA could regulate the production of mRNA and that the stimulus to compensatory hypertrophy might need to act only long enough to set into motion the machinery for making rRNA, either directly or as a consequence of exhausting mRNA.