A ribosome-bound tRNA half stimulates mitochondrial translation during stress recovery in Trypanosoma brucei.

The protozoan parasite Trypanosoma brucei and its disease-causing relatives are among the few organisms that barely regulate the transcription of protein-coding genes. Yet, alterations in its gene expression are essential to survive in different host environments. Recently, tRNA-derived RNAs have been implicated as regulators of many cellular processes within and beyond translation. Previously, we identified the tRNAThr-3'-half (AGU) as a ribosome-associated non-coding RNA able to enhance global translation. Here we report that the tRNAThr-3'-half is generated upon starvation inside the mitochondria. The tRNAThr-3'-half associates with mitochondrial ribosomes and stimulates translation during stress recovery, positively affecting mitochondrial activity and, consequently, cellular energy production capacity. Our results describe an organelle ribosome-associated ncRNA involved in translation regulation to boost the central hub of energy metabolism as an immediate stress recovery response.

tRNA 3' shortening by LCCR4 as a response to stress in Trypanosoma brucei.

Sensing of environmental cues is crucial for cell survival. To adapt to changes in their surroundings cells need to tightly control the repertoire of genes expressed at any time. Regulation of translation is key, especially in organisms in which transcription is hardly controlled, like Trypanosoma brucei. In this study, we describe the shortening of the bulk of the cellular tRNAs during stress at the expense of the conserved 3' CCA-tail. This tRNA shortening is specific for nutritional stress and renders tRNAs unsuitable substrates for translation. We uncovered the nuclease LCCR4 (Tb927.4.2430), a homologue of the conserved deadenylase Ccr4, as being responsible for tRNA trimming. Once optimal growth conditions are restored tRNAs are rapidly repaired by the trypanosome tRNA nucleotidyltransferase thus rendering the recycled tRNAs amenable for translation. This mechanism represents a fast and efficient way to repress translation during stress, allowing quick reactivation with a low energy input.

tRNA Biology in Trypanosomes.

Besides their medical importance, the parasitic protozoan Trypanosoma brucei and its relatives are experimentally highly accessible model systems for many cell biological processes. Trypanosomes are phylogenetically essentially unrelated to the popular model eukaryotes, such as yeast and animals, and thus show several unique features, many of which are connected to RNA. Here we review the tRNA biology of trypanosomes. Even though tRNAs were already discovered 60 years ago, owing to current technological advances in the field, research on tRNA biology has seen a Renaissance in recent years. First we discuss the extensive mitochondrial tRNA import process and the consequences it has for the parasite. Next we focus on trypanosomal aminoacyl-tRNA synthetases, some of which may be exploited as drug targets. Furthermore, we summarize what is known about trypanosomal tRNA modifications in both the cytosol and the mitochondrion. Finally, we provide an overview on the emerging field of tRNA-derived fragments and their possible function as translation regulators.

A tRNA half modulates translation as stress response in Trypanosoma brucei.

In the absence of extensive transcription control mechanisms the pathogenic parasite Trypanosoma brucei crucially depends on translation regulation to orchestrate gene expression. However, molecular insight into regulating protein biosynthesis is sparse. Here we analyze the small non-coding RNA (ncRNA) interactome of ribosomes in T. brucei during different growth conditions and life stages. Ribosome-associated ncRNAs have recently been recognized as unprecedented regulators of ribosome functions. Our data show that the tRNAThr 3´half is produced during nutrient deprivation and becomes one of the most abundant tRNA-derived RNA fragments (tdRs). tRNAThr halves associate with ribosomes and polysomes and stimulate translation by facilitating mRNA loading during stress recovery once starvation conditions ceased. Blocking or depleting the endogenous tRNAThr halves mitigates this stimulatory effect both in vivo and in vitro. T. brucei and its close relatives lack the well-described mammalian enzymes for tRNA half processing, thus hinting at a unique tdR biogenesis in these parasites.