The involvement of gRNA-binding protein gBP21 in RNA editing-an in vitro and in vivo analysis.

RNA editing in the parasitic organism Trypanosoma brucei is characterised by the insertion and deletion of uridylate residues into otherwise incomplete primary transcripts. The processing reaction is a required pathway for the expression of most mitochondrial genes and proceeds by a cascade of enzyme-catalysed steps. RNA editing involves one or more macromolecular ribonucleoprotein complexes which are likely to interact with additional components as the reaction proceeds. Here we examined the involvement of the gRNA-binding polypeptide gBP21, a protein which has been demonstrated to be associated with active RNA editing complexes. We show that in vitro RNA editing can be suppressed by the addition of a gBP21-specific antibody or by immunodepletion of the protein. By creating a gBP21 knockout mutant we analysed the requirement for the protein in vivo. gBP21(-) trypanosomes are viable as bloodstream stage cells and contain edited mRNAs. However, the knockout mutant is not capable of differentiating from the bloodstream to the insect life cycle stage in vitro. Moreover, mutant cells are characterised by a low mitochondrial transcript abundance. Together, these data establish that gBP21 contributes a non-essential function to the RNA editing reaction and further suggest that the protein is involved in additional mitochondrial processes which impact a larger pool of mitochondrial transcripts.