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Abstract Unknown factors regulate mitochondrial U-insertion/deletion (U-indel) RNA editing in procyclic-form (PCF) and bloodstream-form (BSF) T. brucei. This editing, directed by anti-sense gRNAs, creates canonical protein-encoding mRNAs and may developmentally control respiration. Canonical editing by gRNAs that specify protein-encoding mRNA sequences occurs amid massive non-canonical editing of unclear sources and biological significance. We found PCF-specific repression at a major early checkpoint in mRNA ND7, involving helicase KREH2-dependent opposite modulation of canonical and non-canonical ‘terminator’ gRNA utilization. Terminator-programmed editing derails canonical editing and installs proposed repressive structure in 30% of the ND7 transcriptome. BSF-to-PCF differentiation in vitro recreated this negative control. Remarkably, KREH2-RNAi knockdown relieved repression and increased editing progression by reverting canonical/terminator gRNA utilization. ND7 transcripts lacking early terminator-directed editing in PCF exhibited similar negative editing control along the mRNA sequence, suggesting global modulation of gRNA utilization fidelity. The terminator is a ‘moonlighting’ gRNA also associated with mRNA COX3 canonical editing, so the gRNA transcriptome seems multifunctional. Thus, KREH2 is the first identified repressor in developmental editing control. This and our prior work support a model whereby KREH2 activates or represses editing in a stage and substrate-specific manner. KREH2’s novel dual role tunes mitochondrial gene expression in either direction during development. 

In vertebrate haemoglobin (Hb), the NH2-terminal residues of the α- and β-chain subunits are thought to play an important role in the allosteric binding of protons (Bohr effect), CO2 (as carbamino derivatives), chloride ions, and organic phosphates. Accordingly, acetylation of the α- and/or β-chain NH2-termini may have significant effects on the oxygenation properties of Hb. Here we investigate the effect of NH2-terminal acetylation by using a newly developed expression plasmid system that enables us to compare recombinantly expressed Hbs that are structurally identical except for the presence or absence of NH2-terminal acetyl groups. Experiments with native and recombinant Hbs of representative vertebrates reveal that NH2-terminal acetylation does not impair the Bohr effect, nor does it significantly diminish responsiveness to allosteric cofactors, such as chloride ions or organic phosphates. These results suggest that observed variation in the oxygenation properties of vertebrate Hbs is principally explained by amino acid divergence in the constituent globin chains rather than post-translational modifications of the globin chain NH2-termini. 

Multi-zinc finger proteins are an emerging class of cofactors in DEAH-RHA RNA helicases across highly divergent eukaryotic lineages. DEAH-RHA helicase•zinc finger cofactor partnerships predate the split of kinetoplastid protozoa, which include several human pathogens, from other eukaryotic lineages 100-400 Ma. Despite a long evolutionary history, the prototypical DEAH-RHA domains remain highly conserved. This short review focuses on a recently identified DEAH-RHA helicase•zinc finger cofactor system in kinetoplastid RNA editing, and its potential functional parallels with analogous systems in embryogenesis control in nematodes and antivirus protection in humans.