An arginine-glycine-rich RNA binding protein impacts the abundance of specific mRNAs in the mitochondria of Trypanosoma brucei

In kinetoplastid parasites, regulation of mitochondrial gene expression occurs posttranscriptionally via RNA stability and RNA editing. In addition to the 20S editosome that contains the enzymes required for RNA editing, a dynamic complex called the mitochondrial RNA binding 1 (MRB1) complex is also essential for editing. Trypanosoma brucei RGG3 (TbRGG3) was originally identified through its interaction with the guide RNA-associated proteins 1 and 2 (GAP1/2), components of the MRB1 complex. Both the arginine-glycine-rich character of TbRGG3, which suggests a function in RNA binding, and its interaction with MRB1 implicate TbRGG3 in mitochondrial gene regulation. Here, we report an in vitro and in vivo characterization of TbRGG3 function in T. brucei mitochondria. We show that in vitro TbRGG3 binds RNA with broad sequence specificity and has the capacity to modulate RNA-RNA interactions. In vivo, inducible RNA interference (RNAi) studies demonstrate that TbRGG3 is essential for proliferation of insect vector stage T. brucei. TbRGG3 ablation does not cause a defect in RNA editing but, rather, specifically affects the abundance of two preedited transcripts as well as their edited counterparts. Protein-protein interaction studies show that TbRGG3 associates with GAP1/2 apart from the remainder of the MRB1 complex, as well as with several non-MRB1 proteins that are required for mitochondrial RNA editing and/or stability. Together, these studies demonstrate that TbRGG3 is an essential mitochondrial gene regulatory factor that impacts the stabilities of specific RNAs.