Three distinct editosomes, typified by mutually exclusive KREN1, KREN2, or KREN3 endonucleases, are essential for mitochondrial RNA editing in Trypanosoma brucei. The three editosomes differ in substrate endoribonucleolytic cleavage specificity, which may reflect the vast number of editing sites that need insertion or deletion of uridine nucleotides (Us). Each editosome requires the single RNase III domain in each endonuclease for catalysis. Studies reported here show that the editing endonucleases do not form homodimeric domains, and may therefore function as intermolecular heterodimers, perhaps with KREPB4 and/or KREPB5. Editosomes isolated via TAP tag fused to KREPB6, KREPB7, or KREPB8 have a common set of 12 proteins. In addition, KREN3 is only found in KREPB6 editosomes, KREN2 is only found in KREPB7 editosomes, and KREN1 is only found in KREPB8 editosomes. These are the same associations previously found in editosomes isolated via the TAP-tagged endonucleases KREN1, KREN2, or KREN3. Furthermore, TAP-tagged KREPB6, KREPB7, and KREPB8 complexes isolated from cells in which expression of their respective endonuclease were knocked down were disrupted and lacked the heterotrimeric insertion subcomplex (KRET2, KREPA1, and KREL2). These results and published data suggest that KREPB6, KREPB7, and KREPB8 associate with the deletion subcomplex, whereas the KREN1, KREN2, and KREN3 endonucleases associate with the insertion subcomplex
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