Mitochondrial RNA-binding proteins MRP1 and MRP2 occur in a heteromeric complex that appears to play a role in U-insertion/deletion editing in trypanosomes. Reduction in the levels of MRP1 (gBP21) and/or MRP2 (gBP25) mRNA by RNA interference in procyclic Trypanosoma brucei resulted in severe growth inhibition. It also resulted in the loss of both proteins, even when only one of the MRP mRNAs was reduced, indicating a mutual dependence for stability. Elimination of the MRPs gave rise to substantially reduced levels of edited CyB and RPS12 mRNAs but little or no reduction of the level of edited Cox2, Cox3, and A6 mRNAs as measured by poisoned primer extension analyses. In contrast, edited NADH-dehydrogenase (ND) subunit 7 mRNA was increased 5-fold in MRP1؉2 double knockdown cells. Furthermore, MRP elimination resulted in reduced levels of Cox1, ND4, and ND5 mRNAs, which are never edited, whereas mitoribosomal 12 S rRNA levels were not affected. These data indicate that MRP1 and MRP2 are not essential for RNA editing per se but, rather, play a regulatory role in the editing of specific transcripts and other RNA processing activities.Kinetoplastida are early diverged flagellates that differ from other eukaryotes by a number of features. They contain a remarkable single mitochondrion, within which is a large mass of circular DNA molecules that are intercatenated in a unique arrangement (1). Moreover, their mitochondrial RNA processing is also highly unusual. The majority of mitochondrial mRNAs are extensively changed by RNA editing, which is the extensive insertion and less frequent deletion of uridines (Us) at multiple sites. Small guide RNA (gRNA) 1 molecules direct the pattern of U insertions and deletions by base pairing between the pre-edited mRNA and gRNA. The editing process occurs via a series of "cut-and-paste" steps, and several of the enzymes that catalyze this process, including RNA ligases and terminal uridylyl transferases, have now been identified (for recent reviews see Refs. 2-5).

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In trypanosomes, MRP1 and MRP2 (previously called gBPx and gBPy, in which x and y indicate the MW of these proteins in a particular species) are guide (g)RNA binding proteins that are part of a large heteromeric complex that may play a role in U‐insertion/deletion editing of mitochondrial mRNAs. In order to shed more light on the function of these proteins, we generated procyclic Trypanosoma brucei cell lines in which the levels of MRP 1 and/or MRP2 mRNA were downregulated by RNA interference (RNAi). Here we report that the RNAi‐mediated knockdown of MRP1 and/or MRP2 resulted in severe growth inhibition and loss of both proteins. This loss occurred even in cells in which only one of the MRPs was targeted by RNAi, indicating a mutual dependence for stability of these proteins. The elimination of the MRPs substantially reduced the levels of edited cytB and RPS12 mRNAs, but resulted in little or no reduction of edited cox2, cox3 and A6 mRNAs, as measured in poisoned primer extension analyses. Surprisingly, we found a five‐fold increase in ND7 mRNA editing in MRP1+2 double knockdown cells. In addition, the knockdowns also resulted in reduction in the amounts of mRNAs that do not undergo RNA editing (cox1, ND4 and ND5 mRNAs), but little change was observed for mitoribosomal 12S rRNA. Together, the results indicate that in procyclic T. brucei, MRP1 and MRP2 play a role in transcript‐specific editing and other RNA processing activities.