Disruption of RNA editing in Leishmania tarentolae by the loss of minicircle-encoded guide RNA genes.

Abstract

RNA editing in kinetoplastids appears to be a labile genetic trait that is affected by prolonged cell culture. The transcripts of the G1‐G5 cryptogenes are pan‐edited in the recently isolated LEM125 strain of Leishmania tarentolae, but not in the UC strain which has been in culture for 55 years. At least 32 minicircle‐encoded guide RNAs (gRNAs) for the editing of G1‐G5 transcripts are present in LEM125 and absent in UC. We hypothesize that specific minicircle sequence classes encoding gRNAs for the editing of these transcripts were lost during the long culture history of the UC strain. The protein products, which include components of complex I of the respiratory chain, are probably not required during the culture stage of the Leishmania life cycle.

Characterization of the respiratory chain from cultured Crithidia fasciculata

“…Nevertheless, a complex I-type NADH dehydrogenase seems to be missing, which is in agreement with the absence of translatable mt mRNAs for NADH dehydrogenase subunits in cultured strains of C. fasciculata and Leishmania tarentolae [16,53,54]. Other lower eukaryotes, like the yeast Saccharomyces cerevisiae, do not have a complex I-type NADH dehydrogenase either, but in contrast to trypanosomes the yeast lacks mt genes encoding ND subunits altogether [55].…”

Is kinetoplastid pan-editing the result of an evolutionary balancing act?

“…Interestingly, their potential to edit transcripts encoding proteins which are not required during prolonged laboratory cultivation has been lost. When freshly isolated from the host, L. tarentolae (LEM 125 strain) does show extensive editing of six transcripts (14,15). Extensive editing has further been observed in Trypanosoma cruzi, Trypanosoma species E1-CP and Herpetomonas muscarum and the bodonid T. borreli (16).…”

Evolution of RNA editing in trypanosome mitochondria

“…Two of the minicircle-encoded gRNAs in the L. tarentolae UC strain, gLt19 (ϭgG4-III) and gB4 (ϭgND3-IX), represent nonessential gRNAs for these nonfunctional editing cascades. This was determined by analyzing the minicircleencoded gRNA complement of LEM125, a recently isolated strain of L. tarentolae (41). LEM125 has the same 15 maxicircle gRNA genes but also has an estimated 80 total minicircleencoded gRNAs, of which 30 have been cloned and sequenced and the remainder inferred to be present because of the existence of completely edited mRNA transcripts in this strain.…”

“…These additional gRNAs mediate the editing of three components of complex I of the respiratory chain, ND3, ND8, and ND9, and also two unidentified genes, which were termed G-rich regions 3 and 4 (G3 and G4). It was proposed that multiple gRNA-encoding minicircle sequence classes had been lost from the UC strain probably because of a lack of a requirement for complex I activity in culture (41,42). The presence of productively edited ND8, ND9, G3 (ϭCR3), G4 (ϭCR4), and ND3 mRNAs in T. brucei and the presence of productively edited G3 mRNA in P. serpens (D.A.M., unpublished results) implies that the corresponding minicircle-encoded gRNAs also exist in these species, and this provides phylogenetic evidence for our hypothesis that the ancestral cell had a complete complement of minicircle classes.…”