RNA editing in the free-living bodonid Bodo saltans

Abstract

In parasitic kinetoplastid protozoa, mitochondrial (mt) mRNAs are post-transcriptionally edited by insertion and deletion of uridylate residues, the information being provided by guide (g) RNAs. In order to further explore the role and evolutionary history of this process, we searched for editing in mt RNAs of the free-living bodonid Bodo saltans. We found extensive editing in the transcript for NADH dehydrogenase (ND) subunit 5, which is unedited in trypanosomatids. In contrast, B.saltans cytochrome c oxidase (cox) subunit 2 and maxicircle unidentified reading frame (MURF) 2 RNAs display limited editing in the same regions as their trypanosomatid counterparts. A putative intramolecular cox2 gRNA and the gene for gMURF2-I directing the insertion of only one U in the 5' editing domain of MURF2 RNA, are conserved in B.saltans. This lends (further) evolutionary support to the proposed role of these sequences as gRNAs. Phylogenetic analysis showed that B.saltans is more closely related to trypanosomatids than the cryptobiids Trypanoplasma borreli and Cryptobia helicis, in line with the trypanosomatid-like cox2 and MURF2 RNA editing patterns. Nevertheless, other features like the apparent absence of a catenated mtDNA network, are shared with bodonid and cryptobiid species. ND5 RNA editing may represent yet another example of editing 'on the way out' during kinetoplastid evolution, but in view of the fact that cox2 RNA is unedited in T. borreli and C.helicis, we infer that the editing of this RNA may have arisen relatively recently. Our results provide the first examples of RNA editing in a free-living kinetoplastid, indicating that there is no direct link between U-insertion/deletion editing and a parasitic lifestyle.