Poly(A)containing RNA was isolated from the polyribosomal and post-ribosomal fractions of the livers of normal and iron-treated rats. These RNA fractions were then translated in a wheat germ system to provide a measure of the amount of ferritin mRNA present in each fraction. Following iron administration, there was a 2-fold increase in the amount of ferritin mRNA in the polyribosomal fraction. This increase was not inhibited by prior treatment of the rats with actinomycin D or cordycepin, suggesting a cytoplasmic control mechanism. In normal rats, the post-ribosomal fraction contained an amount of ferritin mRNA equal to that in the polyribosomes. When iron was administered, this untranslated ferritin mRNA became reduced to negligible quantities, thus accounting for the doubling of the ferritin mRNA content of the polyribosomal fraction. A scheme is proposed in which translation of the ferritin mRNA in the post-ribosomal fraction is prevented by adhering ferritin subunits. Iron administration removes this inhibition of the translation of ferritin mRNA by promoting aggregation of these subunits into ferritin.Evidence from many tissues shows that not all cytoplasmic mRNA is associated with polyribosomes (1-19). This pool of free untranslated cytoplasmic mRNA constitutes 10-30% of the total cytoplasmic mRNA content of a number of tissues (8,10,15,(17)(18)(19) and in some others even accounts for 50-80% (1,3,4,11). It is generally believed to represent a storage of precursor form of the polyribosomal mRNA. Recently, several individual mRNAs, coding for specific proteins, have been identified in the post-ribosomal supernatant of a variety of tissues, amongst them the mRNAs coding for actin (10), ferritin (18), the a-chain of hemoglobin (14, 15), myosin (9, 11), and vesicular stomatitis virus proteins (8). Nevertheless, no precise function for any one of these post-ribosomal mRNAs has been demonstrated so far.Since the early observations of Granick et al. (20), it has been shown by many authors that iron increases the synthesis of ferritin in many different tissues (21)(22)(23)(24)(25)(26)(27)(28)(29)(30) Liver Polyribosomal and Post-Ribosomal RNA. Twenty pooled rat livers were homogenized in 2 volume of 0.1 M Tris-HCl, pH 7.6, 0.1 M NaCl, 0.1 mM EDTA, 5 mM MgCl2, 150 Ag/ml of heparin, 0.25 M sucrose, and centrifuged at 10,000 X g for 20 min. The resulting post-mitochondrial supernatant was filtered through glass wool and centrifuged at 42,000 rpm for 50-70 min in the Beckman Ti 50.1 rotor. The post-ribosomal supernatant was filtered through glass wool, diluted with 0.5 volume of 0.1 M Tris-HC1, pH 7.4, 0.1 M NaCl, 1 mM EDTA, and 50 ,ug/ml of heparin and made 1% in sodium dodecyl sulfate and 1 mM in EDTA. The pH was adjusted to 9.0 with 1 M NaOH prior to phenol extraction (33). Analysis on sucrose density gradients showed ribosome subunits to be nearly absent (18), and this was confirmed by sucrose gradient analysis of post-ribosomal RNA.Total polyribosomal RNA was isolated from the microsomal pellets essential...

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This paper describes a highly specific and sensitive radioimmunoassay for a-human atrial natriuretic factor (a-hANF), the C-terminal 28-amino-acid residue portion of human prepro-ANF in human plasma. A novel extraction and prepurification procedure allowed for detection of levels of immunoreactive-a-hANF as low as 0.5 fmol/ml. In normotensive subjects, levels in the range l-23 fmol/ml (mean = 8.9 fmol/ml) were found. Combined gel permeation and HPLC analysis demonstrated that this ir-a-hANF was comprised virtually exclusively of authentic 28-residue a-hANF. No evidence for occurrence of larger precursor forms in human plasma was acquired. A heterogenous group of hypertensive patients displayed considerably higher levels (mean = 62.2 fmol/ml), of interest in view of the hypotensive properties of ANF.

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