The site of action of 5-fluoroorotic acid on the maturation of mouse liver ribonucleic acids

The effect of liver macrophages on in vitro cytolytic activity of 5FU and FUdR on colon carcinoma cells: evidence of macrophage activation

While investigating the effects of 5-fluorouracil (5FU) and 5-fluoro-2'-deoxyuridine (FUdR) on the tumoricidal state of rat liver macrophages activated in vitro by means of liposome-encapsulated muramyl dipeptide (MDP), we observed that 5FU in combination with macrophages produced substantially higher extents of cytolytic activity on tumor cells than 5FU alone. In contrast, FUdR failed to produce this effect; rather, at relatively low FUdR concentrations, lytic activity in the presence of macrophages was even significantly diminished as compared with FUdR in the absence of macrophages. Both 5FU and FUdR were able to enhance the cytolytic activity of macrophages activated by liposome-encapsulated MDP. This finding indicates that, rather than inhibiting the activation of macrophages by liposomal MDP, 5FU can act as a stimulator of macrophage activation by itself. This is further supported by the observations that (i) in combination with 5FU, the secretion of TNF induced by liposomal MDP was synergistically enhanced and (ii) that a second treatment of macrophages with the drug, 24 h after the first, fails to produce increased macrophage cytotoxicity. Our results also show that neither 5FU nor FUdR are likely to unfavorably influence the induction of cytotoxic activity of the macrophages. Rather, combinations of 5FU or FUdR and liposomal MDP may result in an additive or synergistic tumoricidal effect.

Localization and structure of endonuclease cleavage sites involved in the processing of the rat 32S precursor to ribosomal RNA

The initial endonuclease cleavage site in 32 S pre-rRNA (precursor to rRNA) is located within the rate rDNA sequence by S1-nuclease protection mapping of purified nucleolar 28 S rRNA and 12 S pre-rRNA. The heterogeneous 5'- and 3'-termini of these rRNA abut and map within two CTC motifs in tSi2 (internal transcribed spacer 2) located at 50-65 and 4-20 base-pairs upstream from the homogeneous 5'-end of the 28 S rRNA gene. These results show that multiple endonuclease cleavages occur at CUC sites in tSi2 to generate 28 S rRNA and 12 S pre-rRNA with heterogeneous 5'- and 3'-termini, respectively. These molecules have to be processed further to yield mature 28 S and 5.8 S rRNA. Thermal-denaturation studies revealed that the base-pairing association in the 12 S pre-rRNA:28 S rRNA complex is markedly stronger than that in the 5.8 S:28 S rRNA complex. The sequence of about one-quarter (1322 base-pairs) of the 5'-part of the rat 28 S rDNA was determined. A computer search reveals the possibility that the cleavage sites in the CUC motifs are single-stranded, flanked by strongly base-paired GC tracts, involving tSi2 and 28 S rRNA sequences. The subsequent nuclease cleavages, generating the termini of mature rRNA, seem to be directed by secondary-structure interactions between 5.8 S and 28 S rRNA segments in pre-rRNA. An analysis for base-pairing among evolutionarily conserved sequences in 32 S pre-rRNA suggests that the cleavages yielding mature 5.8 S and 28 S rRNA are directed by base-pairing between (i) the 3'-terminus of 5.8 S rRNA and the 5'-terminus of 28 S rRNA and (ii) the 5'-terminus of 5.8 S rRNA and internal sequences in domain I of 28 S rRNA. A general model for primary- and secondary-structure interactions in pre-rRNA processing is proposed, and its implications for ribosome biogenesis in eukaryotes are briefly discussed.

The effect of 5-fluoroorotic acid on the early labelling of nucleotides and RNA in whole liver and of RNA in subnuclear fractions in rats given 3H-orotic acid

Male white rats were given either orotic acid or 5-fluoroorotic acid 60 min, and 3H-orotic acid 3 1/2 min before sacrifice. Liver nucleotides were analyzed by isotachophoresis. The (F)UTP pool increased to the same extent in both groups and showed the same specific labelling. The RNA synthesis, as measured by nmol of UTP entering RNA-UMP/g wet liver tissue was significantly lower in fluoroorotic acid treated rats. In the nucleolar fraction there was an increase in the RNA/DNA ratio, a decrease in the specific RNA labelling and an essentially unaltered specific labelling of RNA/microgram/DNA. The effect on the specific labelling of RNA/microgram DNA was the same in the nucleolar fraction and the nucleolar-free pellet, suggesting that the transcription of RNA in these two fractions proceeded in a similar way. There was a slight increase in the specific RNA labelling of the supernatant subnuclear fraction, containing low-molecular weight RNA. Incorporation into UDP-hexoses and UDP-N-acetylhexosamine was unchanged.

A 12 S precursor to 5.8 S rRNA associated with rat liver nucleolar 28 S rRNA

The pre-rRNA and rRNA components of rat and mouse liver nucleolar RNA were analysed. It was shown that upon denaturation, part of the 32 S pre-rRNA is converted into 28 S rRNA and 12 S RNA. The 12 S RNA from mouse (Mr, 0.36 X 10(6)) is larger than the one from rat (Mr, 0.32 X 10(6). The 12 S RNA chain is intact and resists denaturation treatment. The non-covalent binding of this RNA with nucleolar 28 S rRNA is stronger than that of 5.8 S rRNA with 28 S rRNA. Hybridization with a rat internal-transcribed spacer rDNA fragment identifies 12 S RNA as corresponding to the 5'-end non-conserved segment of 32 S pre-rRNA, including 5.8 S rRNA. The significance of the formation of a 12 S precursor to 5.8 S rRNA in the biogenesis of ribosomes in mammalian cells is discussed.

Intermolecular renaturation of hairpin loops leads to stable and specific aggregates of 28-S rRNA and precursor rRNAs from Xenopus laevis

Under certain extraction conditions, 28-S rRNA of Xenopus laevis can form specific dimers by intermolecular renaturation of a particular hairpin loop near the B end of the molecules. This was demonstrated by electron microscopy of isolated aggregates under conditions where rRNA shows a reproducible pattern of secondary structure. In 50% formamide, 15 mM sodium chloride, 1.5 mM trisodium citrate, pH 7.0, the aggregates dissociate into their monomers at a temperature (61 degrees C) at which the most stable hairpin loops of the 28-S rRNA are melting. Precursor rRNAs containing the 28-S rRNA region also form aggregates with 28-S rRNA by the same linkage mechanism. Since such artificial aggregates yield molecular weights higher than expected for true precursors of Xenopus rRNA, they may interfere in precursor rRNA studies. Analysis of whole-cell RNA pulse-labeled with (Me-3H)methionine, or nucleolar RNA pulse-labeled with (3H)uridine in the presence of tubercidin (inhibitor of rRNA processing), as well as electron microscopy of high-molecular-weight RNA fractions confirm that the 40-S precursor rRNA (2.8 X 10(6)) is the primary transcript of the ribosomal genes in Xenopus laevis.