Abstract
Mitochondrial transcript maturation in African trypanosomes requires a U-nucleotide specific RNA editing reaction. In its most extreme form hundreds of U’s are inserted into and deleted from primary transcripts to generate functional mRNAs. Unfortunately, both origin and biological role of the process have remained enigmatic. Here we report a so far unrecognized structural feature of pre-edited mRNAs. We demonstrate that the cryptic pre-mRNAs contain numerous clustered G-nt, which fold into G-quadruplex (GQ) structures. We identified 27 GQ’s in the different pre-mRNAs and demonstrate a positive correlation between the steady state abundance of guide (g)RNAs and the sequence position of GQ-elements. We postulate that the driving force for selecting G-rich sequences lies in the formation of DNA/RNA hybrid G-quadruplex (HQ) structures between the pre-edited transcripts and the non-template strands of mitochondrial DNA. HQ’s are transcription termination/replication initiation sites and thus guarantee an unperturbed replication of the mt-genome. This is of special importance in the insect-stage of the parasite. In the transcription-on state, the identified GQ’s require editing as a GQ-resolving activity indicating a link between replication, transcription and RNA editing. We propose that the different processes have coevolved and suggest the parasite life-cycle and the single mitochondrion as evolutionary driving forces.
Highlights
African trypanosomes are single cell blood-parasites and as such they are of medical importance[1]
By analyzing the nucleotide propensity of all pre-edited mitochondrial mRNAs in Trypanosoma brucei we identified an extreme purine nucleotide bias in all pan-edited pre-mRNAs and we demonstrate that G-nucleotides are arranged in tracts of G’s
We further demonstrate that RNA editing progressively resolves the different GQ-elements and that the positions of the various GQ’s correlate with the steady state abundance of gRNAs
Summary
African trypanosomes are single cell blood-parasites and as such they are of medical importance[1]. Each maxicircle has a molecular size of 23 kbp It carries the genetic information for two ribosomal RNAs (9S, 12S), one ribosomal protein (S12) and seventeen additional protein-coding sequences (Fig. 1c). 12 of the 18 open reading frames (ORF) require RNA editing in order to be converted into translatable mRNAs4–6. For nine of these ORF’s this involves the site-specific insertion and deletion of literally hundreds of U-nucleotides, a phenomenon that has been dubbed pan-editing[7]. In analogy to other biological systems we hypothesize that the GQ-elements function to separate mitochondrial transcription from mitochondrial DNA-synthesis, which provides a new conceptual framework for the evolutionary driving force(s) of RNA editing in African trypanosomes and other kinetoplastid organisms
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