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Pseudouridine (5-ribosyluracil, ψ) was the first of a host of modified nucleoside constituents detected in cellular RNA and it remains the most abundant, being broadly distributed in the RNA of archaebacteria, eubacteria and eukaryotes. Like some other modifications, ψ is particularly abundant in more complex organisms, reaching 2-3% of the total nucleoside constituents in tRNA, snRNA and rRNA of...
Most steps in the maturation of nuclear coded tRNAs occur in the nucleus in eukaryotic cells, but little is known as to the intranuclear location of this RNA maturation pathway. Indirect immunofluorescence experiments using antibody to N 2 ,N 2 dimethylguanosine-specific tRNA methyltransferase, a tRNA processing enzyme, and to Nup1p, a nuclear pore protein, show that both locate to...
Apolipoprotein (apo) B mRNA editing consists of a C -> U conversion of the first base of the codon CAA encoding glutamine 2153 in apoB mRNA to UAA, a stop codon. The cDNA for an apoB mRNA editing protein was recently cloned in rat and human. The human protein contains 236 amino acid residues and exists as a homodimer. The editing protein edits apoB mRNA in vitro only in the presence of tissue...
C to U transitions in plant mitochondrial mRNA (RNA editing) lead to amino acid changes as well as to the creation of new initiation or termination codons. We established an in vitro system to assay and to dissect the process of wheat mitochondrial mRNA editing. A deamination mechanism explains most easily the observed C to U transitions. Several fractions of organellar protein participate in the...
The study of modified nucleoside contributions to RNA chemistry, structure and function has been thwarted by the lack of a site-selected method of incorporation which is both versatile and adaptable to present synthetic technologies. A reproducible and versatile site-selected incorporation of nine differently modified nucleosides into hepta- and octadecamer RNAs has been achieved with automated phosphoramidite...
Two new enzymatic probes have been used for structural investigations of native and unmodified transcript tRNA molecules. Both probes were single-strand-specific nucleases isolated from higher plants. The results obtained after enzymatic hydrolysis of tRNAs support the earlier hypothesis that posttranscriptional modifications in tRNA help to stabilize its structure and make it more rigid.
Literature references dealing with the variations in the modification level of nucleosides in total eukaryotic tRNAs as a function of different physiological status and after drug administration as well as in sequenced cytoplasmic tRNAs between normal and tumor cells and in SV40-transformed cells are reviewed. In addition, special attention is given to guanine replacement of queuine in the first...
The cap-binding complex eIF-4F plays a major role in the control of translation initiation, and overexpression of its limiting subunit, eIF-4E, leads to the deregulation of cellular growth. The recent cloning of eIF-4E binding proteins (4E-BPs) has uncovered a previously unsuspected pathway for the regulation of eIF-4E activity, through sequestration of eIF-4E as a complex with 4E-BPs.
RNA editing alters genomically encoded cytidines to uridines posttranscriptionally in higher plant mitochondria. Most of these editing events occur in translated regions and consequently alter the amino acid sequence. In Oenothera berteriana more than 500 editing sites have been detected and the total number of editing sites exceeds 1000 sites in this mitochondrial genome. To identify the components...
Human ribosomes contain more than 200 modified nucleotides. These are made up as follows: more than 100 2'-O-methyl groups, 10 methylated bases, about 95 pseudouridines and at least one other modification. Other mammalian sources that have been examined, as well as the lower vertebrate Xenopus laevis, show very similar patterns of nucleotide modifications, especially as revealed by oligonucleotide...
In eukaryotic tRNA, guanosine at position 26 in the junction between the D-stem and the anticodon stem is mostly modified to N 2 ,N 2 -dimethylguanosine (m 22 G 26 ). Here we review the available information on the enzyme catalyzing the formation of this modified nucleoside, the SAM-dependent tRNA (m 22 G 26 )-methyltransferase,...
Enzymatic deamination of cytidine and adenosine bases in RNA have recently been shown to be mechanisms for changing the coding specificity of messenger and transfer RNAs. The structures of the enzymes that carry out deamination of the corresponding nucleosides have been analyzed by X-ray crystallography. They are quite different from one another in most respects, including quaternary and tertiary...
The ability of bovine mitochondrial tRNA Met with the anticodon f 5 CAU (where f 5 C is 5-formylcytidine) to decode AUG and AUA codons was examined in a codon-dependent ribosomal binding assay. The AUG codon stimulated the binding of Met-tRNA Met to mitochondrial ribosomes in the presence of EF-Tu/Ts mt . In contrast, the AUA codon...
The first position or 'wobble base' in the anticodon of tRNAs is frequently the site of post-transcriptional modification. InEscherichia coli , glutamine, glutamate, and lysine tRNAs contain 2-thiouridine derivatives in this position, and the significance of these modifications has been under investigation since their discovery. Here we describe the investigations to link 2-thiouridine derivatives...
In order to study the phylogenetic conservation of modified nucleotides in the spliceosomal U5 snRNA, we determined the nucleotide sequences of the U5 snRNAs from the slime mold Physarum polycephalum (EMBL data bank accession numbers: X74440 and X74441) and we identified the pseudouridine and 2'-O-methylated residues. From a comparison of all the U5 snRNAs studied at the level of nucleotide modifications,...
The sequence and structure of the peptidyl transferase region of large subunit ribosomal RNA is highly conserved and specific modified nucleotides could be important structural or functional elements in the catalytic center responsible for peptide bond formation. In fact, it has not been possible to reconstitute active E coli 50S subunits from in vitro transcripts of 23S rRNA and total 50S proteins...
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