Separating the Wheat from the Chaff: RNA Editing and Selection of Translatable mRNA in Trypanosome Mitochondria

Abstract

In the mitochondria of trypanosomes and related kinetoplastid protists, most mRNAs undergo a long and sophisticated maturation pathway before they can be productively translated by mitochondrial ribosomes. Some of the aspects of this pathway (identity of the promotors, transcription initiation, and termination signals) remain obscure, and some (post-transcriptional modification by U-insertion/deletion, RNA editing, 3′-end maturation) have been illuminated by research during the last decades. The RNA editing creates an open reading frame for a productive translation, but the fully edited mRNA often represents a minor fraction in the pool of pre-edited and partially edited precursors. Therefore, it has been expected that the final stages of the mRNA processing generate molecular hallmarks, which allow for the efficient and selective recognition of translation-competent templates. The general contours and several important details of this process have become known only recently and represent the subject of this review.

Parallel monitoring of RNA abundance, localization and compactness with correlative single molecule FISH on LR White embedded samples

“…Trypanosomes edit some of their mitochondrion-encoded mRNAs by additions and deletions of uridines ( 17 ). In extreme cases mRNAs are pan-edited, i.e.…”

“…We found no significant enrichment of either edited or unedited RNA at a specific location within the mitochondrion, in particularly not at the kinetoplast. In the related parasite Leishmania , it was previously observed by immunofluorescence that components of RNA editing complexes as well as mitoribosomes localize to one or two antipodal nodes adjacent to the disk of the kDNA, indicating that transcription, editing and translation may be linked ( 34 , 35 , 58 ). A link between transcription and editing would require the unedited RNA to localize close to the kinetoplast, which is the site of transcription; and if translation really takes place at the inner-mitochondrial membrane close to the kinetoplast, the translated fraction of the edited RNA is expected to localize there too.…”

“…Trypanosomes are also famous for a particular type of RNA editing: most mitochondrially encoded mRNAs are changed by a complex RNA editing machinery that adds or removes uridylyls. This process is directed by guide RNA molecules (gRNAs) that are encoded by the massive DNA network of the single mitochondrion (the kinetoplast); in extreme cases more than 50% of the RNA sequence is changed post-transcriptionally ( 17 ).…”

Parallel monitoring of mRNA abundance, localisation and compactness with correlative single molecule FISH on LR White embedded samples

“…We found no significant enrichment of either edited or unedited RNA at a specific location within the mitochondrion, in particularly not at the kinetoplast. In the related parasite Leishmania, it was previously observed by immunofluorescence that components of RNA editing complexes as well as mitoribosomes localise to one or two antipodal nodes adjacent to the disk of the kDNA, indicating that transcription, editing and translation may be linked 34,35,57 . A link between transcription and editing would require the unedited RNA to localise close to the kinetoplast, which is the site of transcription; and if translation really takes place at the inner-mitochondrial membrane close to the kinetoplast, the translated fraction of the edited RNA is expected to localise there too.…”

The mitochondrial genome segregation system ofT. brucei: Single p197 molecules connect the basal body with the outer membrane

“…The maxicircle encodes 18 protein coding genes, two rRNAs and corresponds to the mitochondrial DNA found in other species (5, 6). However, many of these genes are cryptogenes meaning that their transcripts need to be edited by multiple uridine insertions and/or deletions to become functional mRNAs (7–9). Maxicircle and minicircle are topologically highly interlocked forming the disc-shaped kDNA network that is localized to a specific region within the mitochondrion opposite of the basal body (BB) of the flagellum.…”