Polysomes and polysomal RNA of rat brain

Two 5S genes are expressed in chicken somatic cells

Two 5S RNA species were detected in chicken cells. 5S I RNA has the nucleotide sequence of chicken 5S RNA previously published by Brownlee et al. (1) and 5S II RNA differs from it by 10 mutations. The secondary structure of both species is compatible with that proposed for other eukaryotic 5S RNAs. 5S II RNA represents 50-60% of 5S I RNA. Both species were found in total chicken liver and brain and were present in polysomes in the same relative proportions. Only one 5S RNA species could be detected in rat liver and HeLa cells. Chicken is the first vertebrate described so far in which two 5S RNA genes are expressed in somatic cells.

Labelling kinetics of ribosomal RNA's in rat brain

Slices of cerebral cortex and cerebellum from two-week-old rats were incubated in the presence of [14C] uridine and [methyl-3H] methionine. Incorporation of 14C- and 3H-radioactivity into 18S and 28S RNA's of the two tissues was analysed by sucrose-density-gradient centrifugation. The results show that the rates of synthesis of ribosomal RNA's as well as the pattern of methylation in the two tissues were different. Some of these differences may be ascribed to factors such as pool sizes, intracellular rate of transport of the precursors by other pathways, etc. Examination of the results indicates that some differences may consist in the actual biosynthesis and maturation of ribosomal RNA's.

The limiting factors of a cell-free protein-synthesizing system from rat brain

The limiting factors of a cell-free system from rat brain for incorporating amino acids into protein were studied. The initial more rapid incorporation by microsomes, as opposed to that by ribosomes, is suggested to be due to damage of the ribosomes by detergent. The defect is rectifiable by incubation of the ribosomes in cell sap, so that ribosomes eventually incorporate more amino acid than do microsomes. This may be because ribonuclease, which is associated with the microsomes but removed by detergent treatment, inactivates the microsomal system. The factor that causes incorporation by microsomes to cease abruptly within 1h is not the lack of any precursor or of adenosine triphosphate, of the inactivation of cell-sap factors or the accumulation of inhibitory substances, but is a deficiency of usable messenger ribonucleic acid. Chain initiation in the system is negligible. Ribosomes also become jammed at the end of messenger ribonucleic acid molecules, unable to terminate protein chains. This eventually leads to jammed polyribosomes, which can be partially relieved by very low concentrations of puromycin. A study of the release of polypeptides synthesized in response to the addition of synthetic messengers did not provide any conclusive information on chain-termination sequences, but did indicate some phenomena that were artifacts. It is concluded that ribonuclease action is sufficient to account for all the deficiencies of the cell-free system, but a lack of chain initiation may be a contributory factor.

Relationship between nuclear giant-size dRNA and microsomal dRNA of rat brain

(1) Among the DNA-like RNA (dRNA) species of rat brain, 35 per cent remain nuclear, whereas 65 per cent are found in the microsomes and may represent functional messenger RNA. It was estimated that 30,000-300,000 cistrons would thus code for messenger RNA in rat brain.(2) No nuclear-specific dRNA with sedimentation characteristics similar to microsomal dRNA are detected.(3) The nuclear large dRNA contain all the dRNA species of the brain.(4) The presence of microsomal dRNA precursors and of nuclear restricted dRNA in large dRNA suggests the existence of a limited cleavage process and of a selection among the fragments for their transport from the nucleus to the cytoplasm. Such a selective mechanism could represent an important regulation step in the transfer of genetic information.

Synthesis of ribonucleic acid in normal bone in vitro

1. The incorporation of [2-(14)C]uridine into nucleic acids of bone cells was studied in rat and pig trabecular-bone fragments surviving in vitro. 2. The rapid uptake of uridine into trichloroacetic acid-soluble material, and its subsequent incorporation into a crude nucleic acid fraction of bone or purified RNA extracted from isolated bone cells, was proportional to uridine concentration in the incubation medium over a range 0.5-20.0mum. 3. During continued exposure to radioactive uridine, bulk RNA became labelled in a curvilinear fashion. Radioactivity rapidly entered nuclear RNA, which approached its maximum specific activity by 2hr. of incubation; cytoplasmic RNA, and particularly microsomal RNA, was more slowly labelled. The kinetics of labelling and rapid decline of the nuclear/microsomal specific activity ratio were consistent with a precursor-product relationship. 4. Bulk RNA preparations were resolved by zonal centrifugation in sucrose density gradients into components with approximate sedimentation coefficients 28s, 18s and 4s. 5. Rapidly labelled RNA, predominantly nuclear in location, demonstrated a polydisperse sedimentation pattern that did not conform to the major types of stable cellular RNA. Material of highest specific activity, sedimenting in the 4-18s region and insoluble in 10% (w/v) sodium chloride, rapidly achieved its maximum activity during continued exposure to radioactive precursor and decayed equally rapidly during ;chase' incubation, exhibiting an average half-life of 4.3hr. 6. Ribosomal 28s and 18s RNA were of lower specific activity, which increased linearly for at least 6hr. in the continued presence of radioactive uridine. There was persistent but variable incorporation into ribosomal RNA during ;chase' incubation despite rapid decline in total radioactivity of the acid-soluble pool containing RNA precursors.