Polyadenylic acid sequences in ribonucleic acid of free and membrane-bound polyribosomes of cultured thyroid cells

Subcytoplasmic distribution of thyroglobulin mRNA in normal sheep thyroid

The thyroglobulin 33-S mRNA was isolated from sheep thyroid total polysomes. The 33-S RNA, twice purified on a 1% sodium dodecylsulfate/sucrose gradient, was 30-fold enriched in thyroglobulin messenger activity and was estimated as 50% pure by its messenger activity and 80% pure by the electrophoretic profile. It was used as template for complementary DNA synthesis and hybridized up to 85% of the DNA copy with pseudo-first-order kinetics. Back-hybridization kinetics showed that the purified mRNA corresponds to a major kinetic component with a base sequence complexity of 10000 nucleotides as determined by comparison to globin mRNA. Cross-reactivity of [3H]cDNA with liver RNA is less than 10%. Restriction endonuclease digestion of [3H]cDNA yielded a discrete band pattern. The distribution of thyroglobulin mRNA among free polysomes, membrane-bound polysomes and extrapolysomal pools was analyzed using hybridization to the specific [3H]cDNA probe. Free particles were recovered in the supernatant and membrane-bound particles in the pellet after a brief centrifugation of detergent-free homogenate (5 min at 27000 x g: procedure A; 12 min at 130000 x g: procedure B) with precautions taken to avoid cross-contamination. Using procedure A, 80% of thyroglobulin mRNA sequences were found in the membrane-bound fraction. Using procedure B, where contamination of free particles by membrane-bound particles was avoided by high-speed initial centrifugation and further isolation through a discontinuous sucrose gradient, 95-98% of thyroglobulin mRNA sequences were recovered in membrane-bound polysomes. In total polysomes, 89% of thyroglobulin mRNA sequences were in the polysomal area and shifted to ribosomal subunits after EDTA treatment.

[RNA metabolism in chick embryo skin]

Explants from 7, 8, 9, 11, 13-day chick embryonic skin incorporating (3H) Uridine for different periods 1 hr, 3 or 4 hr and a chase with actinomycin) are studied with respect to free (F) or membrane bound (B) cytoplasmic polysomes and to RNA extracted from them. Polysome specific activity decreases at older stages but the amount of polysomes increases due to increased protein synthesis. At each stage B polysomes are less abundant but more radioactive than F polysomes. RNA extracted from each kind is analysed on sucrose gradients: one half of each fraction is precipitated by TCA to estimate total radioactivity, the other is retained on millipore at high salt concentration to estimate radioactivity in messenger-like RNAs due to their poly-A sequences. The pattern of the labelling of the different fractions of RNA changes with the length of incorporation, the stages of explants and the kind of polysomes (F or B); at 11-13 days the incorporation is slow, radioactivity is low and distributed among several peaks of poly-A RNA; at 7-8 days the incorportion is rapid, dispersed throughout the gradient; at 9 days, a midway stage, incorporation is particularly high into 12S and 24S fractions from B RNA. In the 5 studied stages the labelling of this 12S occurs early, remains for a longer time and cannot be chased. These observations suggest stability of the 12S RNA. Since, in 14-day chick embryos, feather keratin m RNA has been shown to sediment at 12S and although our experiments have been done with total skin because this differentiating tissue is the site of extensive interactions between dermis and epidermis, they suggest that this 12S RNA is the actual keratin m RNA and might be synthesised some days before the onset of keratin synthesis. Its template ability will be investigated at earlier stages.

Thyroglobulin messenger RNA: translation of a 33-S mRNA into a peptide immunologically related to thyroglobulin

Poly(UC)--Sepharose chromatography of the RNA extracted from a thyroid fraction sedimenting between 800 X g and 27000 X g allows the purification of two RNA fractions amounting each to 1% of the applied material. The first one is loosely bound to the column from which it is eluted at 25 degrees C. It is mainly composed of 16-S and 12-S RNA comprising no poly(A) sequences. This could correspond to mitochondrial rRNA. The second one, which is eluted at 50 degrees C, is poly(A)-rich and represents 33-S and 17--18-S RNA species. The 33-S RNA resists heating at 80 degrees C, suggesting that it is composed of one polynucleotide chain. When injected into Xenopus oocytes, the 33-S RNA specifically promotes the synthesis of a peptide with an apparent molecular weight of 185000 and an apparent sedimentation coefficient of 10-S. This peptide is immunologically related to thyroglobulin and could represent its main precursor. Under the conditions tested it does not polymerize spontaneously into 19-S thyroglobulin, suggesting that assembly of the molecule could require specific, post-translational alterations of the precursor and/or the presence of additional lighter subunits.