Polynucleotide fragments from the 28S ribosomal RNA of insects

Fragments derived from non-coding RNAs: how complex is genome regulation?

The human genome is highly dynamic and only a small fraction of it codes for proteins, but most of the genome is transcribed, highlighting the importance of non-coding RNAs on cellular functions. In addition, it is now known the generation of non-coding RNA fragments under particular cellular conditions and their functions have revealed unexpected mechanisms of action, converging, in some cases, with the biogenic pathways and action machineries of microRNAs or Piwi-interacting RNAs. This led us to the question why the cell produces so many apparently redundant molecules to exert similar functions and regulate apparently convergent processes? However, non-coding RNAs fragments can also function similarly to aptamers, with secondary and tertiary conformations determining their functions. In the present work, it was reviewed and analyzed the current information about the non-coding RNAs fragments, describing their structure and biogenic pathways, with special emphasis on their cellular functions.

Heterologous rRNA gene expression: internal fragmentation of Sciara coprophila 28S rRNA within microinjected Xenopus laevis oocytes

Species-specific pre-rRNA processing variations may result in fragmented 18S, 5.8S and 28S rRNAs. Some insect 5.8S and 28S rRNAs are further cleaved, creating within a 'hidden break' or 'gap'. We investigated the specificity of the processing mechanism by microinjecting Sciara coprophila (fungus fly) rDNA into Xenopus laevis oocytes to examine insect rRNA maturation within a cell where endogenous rRNAs are not cleaved at homologous sites. Results confirm insect rDNA transcription and pre-28S rRNA fragmentation, demonstrating that fly-specific processing machinery is not required. Instead, oocytes may provide required accessory factors, suggesting that the insect gap processing mechanism is served by an evolutionarily conserved apparatus. Alternatively, these results may suggest that processing in some lineages is an autocatalytic property of the rRNA.

Ribosomal RNA structure in the diploid and phylogenetically polyploid amphibian species Hyla and Odontophrynus

Ribosomal RNA of the diploid amphibian species Hyla chrysoscelis and Odontophrynus americanus is structurally modified by hidden breaks. Phylogenetically polyploid related species like the tetraploid Hyla versicolor, the tetraploid Odontophrynus americanus and the octoploid Ceratophrys ornata do not show hidden breaks in ribosomal RNA. Structural modifications of rRNA molecules in diploid amphibians has no detectable effect on the ribosomal activity in vitro.

Processing of the ribonucleic acid in the large ribosomal subunits of Urechis caupo

Ribosomal subunits were isolated from eggs or embryos of Urechis caupo, and the ribonucleic acid (RNA) was characterized by electrophoresis under denaturing conditions. The small ribosomal subunit contains a single 17S RNA sequence with a molecular weight of 6.20 X 10(5). The large ribosomal subunit contains four polynucleotide sequences. The 5S RNA has a molecular weight of 4.09 X 10(4). The 26S RNA complex isolated under nondenaturing conditions dissociates in the presence of formamide to yield a 5.8S RNA, molecular weight 5.46 X 10(4), and two approximately 17S and 17.5S RNA sequences with molecular weights of 6.04 X 10(5) and 6.61 X 10(5). The 17S and 17.5S RNAs of the large ribosomal subunits are formed in vivo from a 26S RNA precursor after assembly of the large ribosomal subunit. Large ribosomal subunits are transferred from the nucleus to the cytoplasm with the 26S RNA precursor intact. The hidden break to form the 17S and 17.5S RNAs is introduced in the cytoplasm. No intact 26S RNA could be detected in polysomes; this indicates that the conversion of the 26S RNA to the 17S and 17.5S RNAs may be required to produce large ribosomal subunits capable of participating in protein synthesis.

6.5 S RNA; preliminary characterisation of unusual small RNAs in Trypanosoma brucei

In this paper we report the preliminary characterisation of some unusual small rRNAs isolated from Trypanosoma brucei. These molecules correspond closely in size and properties to the 5 S and 5.8 S rRNAs of other eukaryotes and in addition there are two further small RNAs which we designate 6.5 S RNA. These have been characterised by sucrose gradient centrifugation and electrophoresis in denaturing gels. The two 6.5 S RNAs may be distinguished from each other on the basis of their size and the different conditions required for their dissociation from the ribosomes. They are present in the same abundance as 5.8 S RNA suggesting that they occur once per ribosome.

Introduction of hidden breaks during rRNA maturation and ageing in Tetrahymena pyriformis

The stability of Tetrahymena pyriformis cytoplasmic rRNAs and nuclear rRNA precursors has been studied by polyacrylamide gel electrophoresis under partly and completely denaturing conditions. Cytoplasmic 17-S rRNA (Mr = 0.66 X 10(6) consists of a continuous polynucleotide chain throughout its lifetime, whereas the bulk of 26-S rRNA (Mr = 1.2m X 10(6) dissociates upon denaturation. Two large fragments (F1, F2) of somewhat different molecular weights (Mr 0.63 X 10(6) and 0.58 X 10(6) and the small 5.8-S rRNA fragment (Mr about 50 000) are regularly observed. Some additional distinct minor fragments (F3-F6) are noted under certain preparative conditions, suggestive of artifactual origin. The following conclusions were made from the data obtained . (a) Newly synthesized 26-S rRNA molecules do not contain the 'central' hidden break (separating F1 and F2) until about 15 min after their appearance in the cytoplasm; however, they release during denaturation the 5.8-S and/or a short-lived 7-S fragment (Mr about 75 000) which might represent a direct precursor to the 5.8-S rRNA. (b) The immediate nuclear precursor to the 26-S rRNA (Mr 1.39 X 10(6) releases a small fragment of similar size (7 S). (c) The largest stable transcription product of the rDNA (pre-rRNA) does not contain any hidden break.

Evolution of ribosomal RNA

1. The G + C content of ribosomal RNA of animals seems correlated with the length of periods required for maturation of those organisms. 2. In Protostomes of the animal kingdom, the size of the 28S rRNA molecule does not seem to correlate with the evolutionary stage of the organism. 3. Aphids and water-fleas as well as some protozoa have the 18S rRNA with mol. wt of 0.9 x 10(6) against an overwhelming pressure of evolution to conserve the rRNA molecule of 0.7 x 10(6) daltons. 4. All the Deuterostomes examined were distinguished from Protostomes by having the 28S rRNA's void of the hidden break at the central point. 5. Aphids and nematodes are exceptional Protostomes in that they have the 28S rRNA's without the hidden break. This was discussed in the light of the evolutionary stage of these organisms. 6. Molecular properties of chloroplast rRNA seem to evidence for endosymbiotic origin of this organelle. Mitochondrial rRNA differs considerably from prokaryotic rRNA with respect to molecular size and base composition.

Arthropod ribosomes. Integrity of ribosomal ribonucleic acids from aphids and water fleas

The 28 S aphid rRNA differs from those of the other insects in two points: (1) it lacks the primary nick; (2) it is larger by 0.2 - 106 daltons. The 28 S rRNA from water fleas also, like that from crayfish, is larger than those from comon insects by 0.2 - 106 daltons. These crustacean 28-S rRNAs were shown to contain the primary nick, which is probably not located in the central point. The 18-S rRNAs from aphids and water fleas had molecular weights significantly larger than 0.7 - 106, the common vaalue for the eukaryotic 18 S rRNA. It was was suggested that the 3'-terminal base sequences of these RNAs are different from the common sequences of these RNAs are different from the common sequence proposed for the 18 S rRNAA of eukaryotes. These exceptional characteristics of the rRNAs from the parthenogenetic animals may provide a probe for general functions of the rRNA in the eukaryotic ribosomes.