Identification of human tRNA:m(5)C methyltransferase catalysing intron-dependent m(5)C formation in the first position of the anticodon of the [Formula: see text]

We identified a human orthologue of tRNA:m(5)C methyltransferase from Saccharomyces cerevisiae, which has been previously shown to catalyse the specific modification of C(34) in the intron-containing yeast [Formula: see text]. Using transcripts of intron-less and intron-containing human [Formula: see text] genes as substrates, we have shown that m(5)C(34) is introduced only in the intron-containing tRNA precursors when the substrates were incubated in the HeLa extract. m(5)C(34) formation depends on the nucleotide sequence surrounding the wobble cytidine and on the structure of the prolongated anticodon stem. Expression of the human Trm4 (hTrm4) cDNA in yeast partially complements the lack of the endogenous Trm4p enzyme. The yeast extract prepared from the strain deprived of the endogenous TRM4 gene and transformed with hTrm4 cDNA exhibits the same activity and substrate specificity toward human pre-tRNA(Leu) transcripts as the HeLa extract. The hTrm4 MTase has a much narrower specificity against the yeast substrates than its yeast orthologue: human enzyme is not able to form m(5)C at positions 48 and 49 of human and yeast tRNA precursors. To our knowledge, this is the first report showing intron-dependent methylation of human [Formula: see text] and identification of human gene encoding tRNA methylase responsible for this reaction

MicroRNAs are intensively regulated during induction of somatic embryogenesis in Arabidopsis

Several genes encoding transcription factors (TFs) were indicated to have a key role in the induction of somatic embryogenesis (SE), which is triggered in the somatic cells of plants. In order to further explore the genetic regulatory network that is involved in the embryogenic transition induced in plant somatic cells, micro-RNA (miRNAs) molecules, the products of MIRNA (MIR) genes and the common regulators of TF transcripts, were analyzed in an embryogenic culture of Arabidopsis thaliana. In total, the expression of 190 genes of the 114 MIRNA families was monitored during SE induction and the levels of the primary (pri-miRNAs) transcripts vs. the mature miRNAs were investigated. The results revealed that the majority (98%) of the MIR genes were active and that most of them (64%) were differentially expressed during SE. A distinct attribute of the MIR expression in SE was the strong repression of MIR transcripts at the early stage of SE followed by their significant up-regulation in the advanced stage of SE. Comparison of the mature miRNAs vs. pri-miRNAs suggested that the extensive post-transcriptional regulation of miRNA is associated with SE induction. Candidate miRNA molecules of the assumed function in the embryogenic response were identified among the mature miRNAs that had a differential expression in SE, including miR156, miR157, miR159, miR160, miR164, miR166, miR169, miR319, miR390, miR393, miR396, and miR398. Consistent with the central role of phytohormones and stress factors in SE induction, the functions of the candidate miRNAs were annotated to phytohormone and stress responses. To confirm the functions of the candidate miRNAs in SE, the expression patterns of the mature miRNAs and their presumed targets were compared and regulatory relation during SE was indicated for most of the analyzed miRNA-target pairs. The results of the study contribute to the refinement of the miRNA-controlled regulatory pathways that operate during embryogenic induction in plants and provide a valuable platform for the identification of the genes that are targeted by the candidate miRNAs in SE induction

The Old and New RNA World

Among the numerous hypotheses offering a scenario for the origin of life on Earth, the one called “The RNA World” has gained the most attention. According to this hypothesis RNA acted as a genetic information storage material, as a catalyst of all metabolic reactions, and as a regulator of all processes in the primordial world. Various experiments show that RNA molecules could have been synthesized abiotically, with the potential to mediate a whole repertoire of metabolic reactions. Ribozymes carrying out aminoacyl-tRNA reactions have been found in SELEX (systematic evolution of ligands by exponential enrichment) approaches and the development of a ribosome from a RNA-built protoribosome is easy to imagine. Transfer RNA aminoacylation, protoribosome origin, and the availability of amino acids on early Earth allowed the genetic code to evolve. Encoded proteins most likely stabilized RNA molecules and were able to create channels across membranes. In the modern cell, DNA replaced RNA as the main depositor of genetic information and proteins carry out almost all metabolic reactions. However, RNA is still playing versatile, crucial roles in the cell. Apart from its classical functions in the cell, a huge small RNA world is controlling gene expression, chromatin condensation, response to environmental cues, and protecting the cell against the invasion of various nucleic acids forms. Long non-coding RNAs act as crucial gene expression regulators. Riboswitches act at the level of transcription, splicing or translation and mediate feedback regulation on biosynthesis and transport of the ligand they sense. Alternative splicing generates genetic variability and increases the protein repertoire in response to developmental or environmental changes. All these regulatory functions are essential in shaping cell plasticity in the changing milieu. Recent discoveries of new, unexpected and important functions of RNA molecules support the hypothesis that we live in a New RNA World

RNA interference and its role in the regulation of eucaryotic gene expression.

Several years ago it was discovered that plant transformation with a transcribed sense transgene could shut down the expression of a homologous endogenous gene. Moreover, it was shown that the introduction into the cell of dsRNA (double-stranded RNA) containing nucleotide sequence complementary to an mRNA sequence causes selective degradation of the latter and thus silencing of a specific gene. This phenomenon, called RNA interference (RNAi) was demonstrated to be present in almost all eukaryotic organisms. RNAi is also capable of silencing transposons in germ line cells and fighting RNA virus infection. Enzymes involved in this process exhibit high homology across species. Some of these enzymes are involved in other cellular processes, for instance developmental timing, suggesting strong interconnections between RNAi and other metabolic pathways. RNAi is probably an ancient mechanism that evolved to protect eukaryotic cells against invasive forms of nucleic acids

MicroRNA biogenesis: Epigenetic modifications as another layer of complexity in the microRNA expression regulation

Since their discovery, microRNAs have led to a huge shift in our understanding of the regulation of key biological processes. The discovery of epigenetic modifications that affect microRNA expression has added another layer of complexity to the already tightly controlled regulatory machinery. Modifications like uridylation, adenylation and RNA editing have been shown to have variable effects on miRNA biogenesis and action. Methylation of the N6 adenosine has been studied extensively in mRNA. Presence of the N6-methyl-adenosine (m6A) mark and its critical importance in miRNA biogenesis in animals adds to our understanding of the regulatory mechanisms, while its effect on miRNA biogenesis in plants is yet to be understood