The MRB1 complex functions in kinetoplastid RNA processing

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Abstract
Mitochondrial (mt) gene expression in Trypanosoma brucei entails multiple types of RNA processing, including polycistronic transcript cleavage, mRNA editing, gRNA oligouridylation, and mRNA polyadenylation, which are catalyzed by various multiprotein complexes. We examined the novel mitochondrial RNA-binding 1 (MRB1) complex that has 16 associated proteins, four of which have motifs suggesting RNA interaction. RNase treatment or the lack of kDNA in mutants resulted in lower MRB1 complex sedimentation in gradients, indicating that MRB1 complex associates with kDNA transcripts. RNAi knockdowns of expression of the Tb10.406.0050 (TbRGGm, RGG motif), Tb927.6.1680 (C2H2 zinc finger), and Tb11.02.5390 (no known motif) MRB1 proteins each inhibited in vitro growth of procyclic form parasites and resulted in cells with abnormal numbers of nuclei. Knockdown of TbRGGm, but not the other two proteins, disrupted the MRB1 complex, indicating that it, but perhaps not the other two, is required for complex assembly and/or stability. The knockdowns resulted in similar but nonidentical patterns of altered in vivo abundances of edited, pre-edited, and preprocessed mt mRNAs, but did not appreciably affect the abundances of mRNAs that do not get edited. These results indicate that MRB1 complex is critical to the processing of mt RNAs, and although its specific function is unknown, it appears essential to parasite viability.