CMTr mediated 2'-O-ribose methylation status of cap-adjacent nucleotides across animals

Trypanosome mRNA recapping is triggered by hypermethylation originating from cap 4

RNA methylation adjacent to the 5' cap plays a critical role in controlling mRNA stability and protein synthesis. In trypanosomes the 5'-terminus of mRNA is protected by hypermethylated cap 4. Trypanosomes encode a cytoplasmic recapping enzyme TbCe1 which possesses an RNA kinase and guanylyltransferase activities that can convert decapped 5'-monophosphate-terminated pRNA into GpppRNA. Here, we demonstrated that the RNA kinase activity is stimulated by two orders of magnitude on a hypermethylated pRNA derived from cap 4. The N6, N6-2'-O trimethyladenosine modification on the first nucleotide was primarily accountable for enhancing both the RNA kinase and the guanylyltransferase activity of TbCe1. In contrast, N6 methyladenosine severely inhibits the guanylyltransferase activity of the mammalian capping enzyme. Furthermore, we showed that TbCmt1 cap (guanine N7) methyltransferase was localized in the cytoplasm, and its activity was also stimulated by hypermethylation at 2'-O ribose, suggesting that TbCe1 and TbCmt1 act together as a recapping enzyme to regenerate translatable mRNA from decapped mRNA. Our result establishes the functional role of cap 4 hypermethylation in recruitment and activation of mRNA recapping pathway. Methylation status at the 5'-end of transcripts could serve as a chemical landmark to selectively regulate the level of functional mRNA by recapping enzymes.

RNA: De-silencing to the rescue

The fate of transcribed RNA dictates cellular function. A new study finds that mutations in specific RNA processing machinery genes result in de-silencing of a transcript encoding a subunit of the mitochondrial electron transport chain and rescue of a mitochondrial respiratory complex I defect.

Global expression pattern of genes containing positively selected sites in European anchovy (Engraulis encrasicolus L.) may shed light on teleost reproduction

European anchovy is a multiple-spawning and highly fecundate pelagic fish with high economic and ecological significance. Although fecundity is influenced by nutrition, temperature and weight of spawners, high reproductive capacity is related to molecular processes in the ovary. The ovary is an essential and complex reproductive organ composed of various somatic and germ cells, which interact to facilitate the development of the ovary and functional oocytes. Revealing the ovarian transcriptome profile of highly fecundate fishes provides insights into oocyte production in teleosts. Here we use a comprehensive tissue-specific RNA sequencing which yielded 102.3 billion clean bases to analyze the transcriptional profiles of the ovary compared with other organs (liver, kidney, ovary, testis, fin, cauda and gill) and juvenile tissues of European anchovy. We conducted a comparative transcriptome and positive selection analysis of seven teleost species with varying fecundity rates to identify genes potentially involved in oogenesis and oocyte development. Of the 2,272 single copies of orthologous genes found, up to 535 genes were under positive selection in European anchovy and these genes are associated with a wide spectrum of cellular and molecular functions, with enrichments such as RNA methylation and modification, ribosome biogenesis, DNA repair, cell cycle processing and peptide/amide biosynthesis. Of the 535 positively selected genes, 55 were upregulated, and 45 were downregulated in the ovary, most of which were related to RNA and DNA transferase, developmental transcription factors, protein kinases and replication factors. Overall, our analysis of the transcriptome level in the ovarian tissue of a teleost will provide further insights into molecular processes and deepen our genetic understanding of egg production in highly fecund fish.

The cap epitranscriptome: Early directions to a complex life as mRNA

Animal, protist and viral messenger RNAs (mRNAs) are most prominently modified at the beginning by methylation of cap-adjacent nucleotides at the 2'-O-position of the ribose (cOMe) by dedicated cap methyltransferases (CMTrs). If the first nucleotide of an mRNA is an adenosine, PCIF1 can methylate at the N⁶ -position (m⁶ A), while internally the Mettl3/14 writer complex can methylate. These modifications are introduced co-transcriptionally to affect many aspects of gene expression including localisation to synapses and local translation. Of particular interest, transcription start sites of many genes are heterogeneous leading to sequence diversity at the beginning of mRNAs, which together with cOMe and m⁶ Am could constitute an extensive novel layer of gene expression control. Given the role of cOMe and m⁶ A in local gene expression at synapses and higher brain functions including learning and memory, such code could be implemented at the transcriptional level for lasting memories through local gene expression at synapses.