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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).

We report the characterization of a Trypanosoma brucei 75-kDa protein of the RGG (Arg-Gly-Gly) type, termed TBRGG1. Dicistronic and monocistronic transcripts of the TBRGG1 gene were produced by both alternative splicing and polyadenylation. TBRGG1 was found in two or three forms that differ in their electrophoretic mobility on SDS-polyacrylamide gel electrophoresis gels, one of which was more abundant in the procyclic form of the parasite. TBRGG1 was localized to the mitochondrion and appeared to be more abundant in bloodstream intermediate and stumpy forms in which the mitochondrion reactivates and during the procyclic stage, which possesses a fully functional mitochondrion. This protein was characterized to display oligo(U) binding characteristics and was found to co-localize with an in vitro RNA editing activity in a sedimentation analysis. TBRGG1 most likely corresponds to the 83-kDa oligo(U)-binding protein previously identified by UV crosslinking of guide RNA to mitochondrial lysates (Leegwater, P., Speijer, D., and Benne, R. (1995) Eur. J. Biochem. 227, 780 -786).Trypanosomes are primitive eukaryotes whose parasitic life cycle involves the differentiation into several successive adaptive forms in different hosts and environments. The main developmental stages are the bloodstream form in the mammalian host and the procyclic form in the tsetse fly vector. Each of these stages is characterized by a major surface protein, the variant surface glycoprotein in the bloodstream form and procyclin in the procyclic form (1, 2). Another major difference between these forms is the energy metabolism. Bloodstream forms possess an inactive mitochondrion and respire through the catabolism of glucose in specialized organelles termed glycosomes, whereas procyclic forms utilize a fully functional mitochondrion for oxidative phosphorylation, and amino acids probably serve as the major carbon source in vivo (3). In the bloodstream, the mitochondrion is reactivated when the trypanosomes differentiate from the proliferative slender form into the quiescent stumpy form through several intermediate stages.The nuclear genome of these organisms appears to be organized in long polycistronic transcription units and probably contains only a few promoters (4 -7). The expression of many genes analyzed so far appears to be stage-specific and reflects the developmental stage of the parasite. Interestingly, genes belonging to the same transcription unit are often differentially stage-regulated, indicating that post-transcriptional processes operating at the levels of RNA maturation, stability, and translation are primarily responsible for controlling cellular differentiation (4 -7). The primary polycistronic transcripts of trypanosomes are rapidly processed into mature mRNAs by trans-splicing and polyadenylation. These processing events appear to be coupled, the choice of a polyadenylation site being apparently dictated by the position of the downstream splice site, probably through the scanning of transcripts by a multifactorial complex enc...