Free and Membrane-bound Ribosomes of Rat Cerebral Cortex: METABOLISM OF RIBOSOMAL AND MESSENGER RIBONUCLEIC ACID

Abstract

Free and membrane-bound ribosomes were isolated from cerebral cortex of young rats at various times after they received simultaneous intracerebral injections of [14C]uridine and [methyl-3H]methionine. Using acid and alkaline extraction with phenol, two types of doubly labeled RNA were prepared from these ribosomes, one rich in ribosomal RNA (pH 6) and the other rich in messenger RNA (pH 8.3). The pH-6 RNA was further resolved into 18 S and 28 S RNA by sucrose density gradient centrifugation. An examination of the rate of incorporation of [14C]uridine into different RNA fractions showed that there was no difference in the 14C labeling of the 18 S and 28 S RNA's of free and membrane-bound ribosomes, indicating similar rates of polymerization and cytoplasmic transfer for these polynucleotides. The kinetics of 3H incorporation, however, revealed that the membrane-bound ribosomal RNA's had higher rates of methylation and might possess more methylated bases in comparison with their free ribosomal counterparts. The relative rates of incorporation of 14C and 3H indicated that the mRNA attached to the membrane-bound ribosomes may have a lower rate of turnover than the mRNA of free ribosomes. This view was supported by the comparatively slower incorporation in vivo of [3H]uridine into EDTA extract of membrane-bound polysomes, the lower rate of degradation subsequent to actinomycin D injection, and lower susceptibility to RNase of membrane-bound ribosomes as compared to free ribosomes.