Steps in the biosynthesis of ribosomal RNA in cultured Aedes aegypti cells

The effect of cytosine arabinoside on the synthesis of rapidly labeled RNA during DNA replicating and non-DNA replicating periods of the cell cycle

The effects of various concentrations of cytosine arabinoside (Ara-C) on the rates of DNA and RNA synthesis were investigated during the peak of DNA synthesis, using a naturally synchronized culture of Aedes aegypti (mosquito) cells. During this stage of the cell cycle, the synthesis of both DNA and RNA was found to be progressively inhibited with increasing concentrations of Ara-C. When the same concentrations of Ara-C were added to the culture at a time period when no DNA was being synthesized, it was found that the synthesis of RNA was not inhibited. Rapidly labeled polysomal RNA species were isolated from cultured cells in which DNA was being synthesized. The synthesis of these RNA species is inhibited completely in the presence of a concentration of Ara-C which inhibits 90% of DNA synthesis. It is suggested that during periods of DNA replication, Ara-C is a selective inhibitor of the synthesis of rapidly labeled RNA species or of the processing of these RNA species from the nucleus to the polysomes.

Conservation of the 5'-terminal nucleotide sequences of ribosomal 18-S RNA in eukaryotes. Differential evolution of large and small ribosomal RNA

The 5'-terminal nucleotide sequences of ribosomal 18-S and 28-S RNA of seven species of eukaryotes including three mammals, one bird, one amphibian, one echinoderm and one slime mold, were analyzed either by means of terminal phosphorylation of RNA with polynucleotide kinase of by fingerprint analysis of uniformly labeled RNA. The following conclusions were obtained. 1. The 5'-terminal sequences of the 18-S RNA of the mouse, chicken and Dictyostelium discoideum were pUpApCp(Cp,Up)Gp---, suggesting strongly that all the eukaryotes had this same sequence at the 5'-terminus. Preliminary analysis of the 5'-termini of the 18-S RNA from human, rat, Xenopus and sea urchin cells revealed the same pUp(Np)Gp--- type 5'-terminal structure, supporting the above hypothesis. 2. The 5'-terminal sequences of the 28-S RNA of the human, rat, mouse and chicken cells were all pCpGp---, whereas those of the lower animals such as Xenopus, sea urchin and Dictyostelium were different.

Properties and synthesis of ribosomal RNA in the free-living nematode Panagrellus silusiae

The normal S values of ribosomes and ribosomal subunits extracted from the free-living nematode. Panagrellus silusiae are similar to those of other eukaryotic organisms. Upon heating (70 degrees C, 15 min) a 5.5-S RNA is released from the larger ribosomal RNA (27 S; 1.40-10(6) daltons). Unlike other protostomes, however, the 27-S rRNA does not dissociate in an orderly manner to yield 18-S components. The electrophoretic mobility of the small ribosomal RNA species (18 S; 0.72-10(6) daltons) is unaltered by heat treatment. The base composition of both Panagrellus rRNA species has a 51% G+C content. Labelling worms with either [Me-3H] methionine or [14C] uridine at various stages of postembryonic development and subsequent analysis of the extracted RNA revealed eight radioactive fractions, two of which were the mature ribosomal RNAs. From both the time sequence of appearance of the eight RNA species and the relative degree of methylation of the various RNA fractions two maturation pathways for ribosomal RNA processing were deduced. A 37-S (3.6-10(6) daltons) RNA is the precursor to both 27-S and 18-S RNA. During processing about two-fifths of the original transcriptional unit is lost. In this organism the "transcribed spacer" regions may be methylated although alternate interpretations have not been precluded.

Studies on the 3'-terminal sequences of the large ribosomal ribonucleic acid of different eukaryotes and those associated with "hidden" breaks in heart-dissociable insects 26S ribonucleic acid

The 3'-terminal sequences associated with the large rRNA complex from a range of eukaryotes were determined after pancreatic or T(1)-ribonuclease digestion of RNA terminally labelled with [(3)H]isoniazid. In all higher eukaryotes examined except Drosophila melanogaster, the 3'-terminal sequences Y-G-U(OH) and G-C-U(OH) were demonstrated for the large RNA component(s) and for 6S RNA respectively. The 3'-terminal sequence of Saccharomyces cerevisiae 26S RNA was Y-G-U(OH) and that of 6S RNA Y-A-U-U-U(OH). Three 3'-terminal sequences were found in equimolar amounts in the heat-dissociable 26S rRNA characteristic of insect ribosomes. These were Y-G-U-G-U(OH), Y-C-G-U(OH) and G-C-U(OH) for cultured Antheraea eucalypti cells, Y-G-U(OH), Y-G-U(OH) and G-C-U(OH) for Galleria mellonella larvae and Y-C-G-A(OH), Y-G-U-A(OH) and G-Y-U-G(OH) for Drosophila melanogaster flies. Thus the introduction of the central scission in insect 26S rRNA results in the generation of a unique 3'-terminus and does not arise from random cleavage of the polynucleotide chain.

[RNA-Synthesis in telotrophic meroistic ovarioles of dysdercus intermedius DIST. (heteroptera, pyrrhoc.)]

RNA in insect ovaries was investigated by polyacrylamide gel electrophoresis to study site of synthesis, transportation and incorporation in mature eggs. Telotrophic-meroistic ovarioles ofDysdercus intermedius were selected for this work since they could be dissected in distinct portions: the apical one with polyploid nurse cells and the vitellarium with oocytes covered by a follicular epithelium. RNA was labeled by injecting radioactive precursors or incubating isolated ovarioles in vitro.In nurse cells labeled rRNA, tRNA and non-ribosomal RNA were found and evidence was presented for the processing of a 39s rRNA precursor molecule in 2 steps (36s and 32s) into 28s rRNA while the 18s rRNA was directly derived from the 36s molecule. A low concentration (0.5 Μg/ml) of α-Amanitin inhibited synthesis of non-ribosomal RNA thereby revealing a more distinct processing pattern for rRNA, while a high concentration (5 Μg/ml) blocked processing at the 36s molecule and led to its accumulation. Appropriate concentrations of Actinomycin D inhibited rRNA synthesis while distinct peaks of non-ribosomal RNA became apparent.RNA synthesized in the follicular cells was similar to the RNA made in the nurse cells only in the high molecular weight RNA. Some material, tentatively named "nucleotide fraction", with a mobility higher than tRNA was detected and could be localized in yolk granules. It was separable from RNA by chromatography on Sephadex G 100 and had an absorption maximum at 255 run. The "nucleotide fraction" could be rapidly labeled. It seemed to be utilized in RNA synthesis during embryogenesis. Mature eggs contained a relatively high amount of the "nucleotide fraction". If a single injection with precursor was done 4 days before analysing the RNA of mature eggs most of the label was found in the "nucleotide fraction". Eggs collected 5-8 days after injection contained only high molecular labeled RNA. This observation correlated well with the timing of egg maturation. It takes 4 days for a group of oocytes to reach the distal part of the vitellarium where yolk production and maturation go on for another 4 days. It is apparent, therefore, that the follicular epithelium contributes the "nucleotide fraction" to the oocyte, while most of the other RNA, including a stable polydisperse class of RNA, is made in the nurse cells and is transported into the oocyte.