Codon usage bias controls mRNA and protein abundance in trypanosomatids

Regulation of gene expression in trypanosomatids: living with polycistronic transcription

In trypanosomes, RNA polymerase II transcription is polycistronic and individual mRNAs are excised by trans-splicing and polyadenylation. The lack of individual gene transcription control is compensated by control of mRNA processing, translation and degradation. Although the basic mechanisms of mRNA decay and translation are evolutionarily conserved, there are also unique aspects, such as the existence of six cap-binding translation initiation factor homologues, a novel decapping enzyme and an mRNA stabilizing complex that is recruited by RNA-binding proteins. High-throughput analyses have identified nearly a hundred regulatory mRNA-binding proteins, making trypanosomes valuable as a model system to investigate post-transcriptional regulation.

African trypanosomes

African trypanosomes cause human African trypanosomiasis and animal African trypanosomiasis. They are transmitted by tsetse flies in sub-Saharan Africa. Although most famous for their mechanisms of immune evasion by antigenic variation, there have been recent important studies that illuminate important aspects of the biology of these parasites both in their mammalian host and during passage through their tsetse fly vector. This Primer overviews current research themes focused on these parasites and discusses how these biological insights and the development of new technologies to interrogate gene function are being used in the search for new approaches to control the parasite. The new insights into the biology of trypanosomes in their host and vector highlight that we are in a ‘golden age’ of discovery for these fascinating parasites.

Translation affects mRNA stability in a codon-dependent manner in human cells

mRNA translation decodes nucleotide into amino acid sequences. However, translation has also been shown to affect mRNA stability depending on codon composition in model organisms, although universality of this mechanism remains unclear. Here, using three independent approaches to measure exogenous and endogenous mRNA decay, we define which codons are associated with stable or unstable mRNAs in human cells. We demonstrate that the regulatory information affecting mRNA stability is encoded in codons and not in nucleotides. Stabilizing codons tend to be associated with higher tRNA levels and higher charged/total tRNA ratios. While mRNAs enriched in destabilizing codons tend to possess shorter poly(A)-tails, the poly(A)-tail is not required for the codon-mediated mRNA stability. This mechanism depends on translation; however, the number of ribosome loads into a mRNA modulates the codon-mediated effects on gene expression. This work provides definitive evidence that translation strongly affects mRNA stability in a codon-dependent manner in human cells.

Proteins are made by joining together building blocks called amino acids into strings. The proteins are ‘translated’ from genetic sequences called mRNA molecules. These sequences can be thought of as series of ‘letters’, which are read in groups of three known as codons. Molecules called tRNAs recognize the codons and add the matching amino acids to the end of the protein. Each tRNA can recognize one or several codons, and the levels of different tRNAs inside the cell vary.

There are 61 codons that code for amino acids, but only 20 amino acids. This means that some codons produce the same amino acid. Despite this, there is evidence to suggest that not all of the codons that produce the same amino acid are exactly equivalent. In bacteria, yeast and zebrafish, some codons seem to make the mRNA molecule more stable, and others make it less stable. This might help the cell to control how many proteins it makes. It was not clear whether the same is true for humans.

To find out, Wu et al. used three separate methods to examine mRNA stability in four types of human cell. Overall, the results revealed that some codons help to stabilize the mRNA, while others make the mRNA molecule break down faster. The effect seems to depend on the supply of tRNAs that have a charged amino acid; mRNA molecules were more likely to self-destruct in cells that contained codons with low levels of the tRNA molecules.

Wu et al. also found that conditions in the cell can alter how strongly the codons affect mRNA stability. For example, a cell that has been infected by a virus reduces translation. Under these conditions, the identity of the codons in the mRNA has less effect on the stability of the mRNA molecule.

Changes to protein production happen in many diseases. Understanding what controls these changes could help to reveal more about our fundamental biology, and what happens when it goes wrong.

Mechanistic Similarities between Antigenic Variation and Antibody Diversification during Trypanosoma brucei Infection

Trypanosoma brucei, which causes African trypanosomiasis, avoids immunity by periodically switching its surface composition. The parasite is coated by 10 million identical, monoallelically expressed variant surface glycoprotein (VSG) molecules. Multiple distinct parasites (with respect to their VSG coat) coexist simultaneously during each wave of parasitemia. This substantial antigenic load is countered by B cells whose antigen receptors (antibodies or immunoglobulins) are also monoallelically expressed, and that diversify dynamically to counter each variant antigen. Here we examine parallels between the processes that generate VSGs and antibodies. We also discuss current insights into VSG mRNA regulation that may inform the emerging field of Ig mRNA biology. We conclude by extending the parallels between VSG and Ig to the protein level.

The Trypanosoma brucei RNA-Binding Protein TbRRM1 is Involved in the Transcription of a Subset of RNA Pol II-Dependent Genes

It has been long thought that RNA Polymerase (Pol) II transcriptional regulation does not operate in trypanosomes. However, recent reports have suggested that these organisms could regulate RNA Pol II transcription by epigenetic mechanisms. In this paper, we investigated the role of TbRRM1 in transcriptional regulation of RNA Pol II-dependent genes by focusing both in genes located in a particular polycistronic transcription unit (PTU) and in the monocistronic units of the SL-RNA genes. We showed that TbRRM1 is recruited throughout the PTU, with a higher presence on genes than intergenic regions. However, its depletion leads both to the decrease of nascent RNA and to chromatin compaction only of regions located distal to the main transcription start site. These findings suggest that TbRRM1 facilitates the RNA Pol II transcriptional elongation step by collaborating to maintain an open chromatin state in particular regions of the genome. Interestingly, the SL-RNA genes do not recruit TbRRM1 and, after TbRRM1 knockdown, nascent SL-RNAs accumulate while the chromatin state of these regions remains unchanged. Although it was previously suggested that TbRRM1 could regulate RNA Pol II-driven genes, we provide here the first experimental evidence which involves TbRRM1 to transcriptional regulation.

Structural and molecular mechanisms for the control of eukaryotic 5'-3' mRNA decay

5'-3' RNA decay pathways are critical for quality control and regulation of gene expression. Structural and biochemical studies have provided insights into the key nucleases that carry out deadenylation, decapping, and exonucleolysis during 5'-3' decay, but detailed understanding of how these activities are coordinated is only beginning to emerge. Here we review recent mechanistic insights into the control of 5'-3' RNA decay, including coupling between translation and decay, coordination between the complexes and activities that process 5' and 3' RNA termini, conformational control of enzymatic activity, liquid phase separation, and RNA modifications.

Evolution of Antigenic Variation in African Trypanosomes: Variant Surface Glycoprotein Expression, Structure, and Function

The process of antigenic variation in parasitic African trypanosomes is a remarkable mechanism for outwitting the immune system of the mammalian host, but it requires a delicate balancing act for the monoallelic expression, folding and transport of a single variant surface glycoprotein (VSG). Only one of hundreds of VSG genes is expressed at time, and this from just one of ≈15 dedicated expression sites. By switching expression of VSGs the parasite presents a continuously shifting antigenic facade leading to prolonged chronic infections lasting months to years. The basics of VSG structure and switching have been known for several decades, but recent studies have brought higher resolution to many aspects this process. New VSG structures, in silico modeling of infections, studies of VSG codon usage, and experimental ablation of VSG expression provide insights that inform how this remarkable system may have evolved.

Exploiting CRISPR-Cas9 technology to investigate individual histone modifications

Despite their importance for most DNA-templated processes, the function of individual histone modifications has remained largely unknown because in vivo mutational analyses are lacking. The reason for this is that histone genes are encoded by multigene families and that tools to simultaneously edit multiple genomic loci with high efficiency are only now becoming available. To overcome these challenges, we have taken advantage of the power of CRISPR-Cas9 for precise genome editing and of the fact that most DNA repair in the protozoan parasite Trypanosoma brucei occurs via homologous recombination. By establishing an episome-based CRISPR-Cas9 system for T. brucei, we have edited wild type cells without inserting selectable markers, inserted a GFP tag between an ORF and its 3'UTR, deleted both alleles of a gene in a single transfection, and performed precise editing of genes that exist in multicopy arrays, replacing histone H4K4 with H4R4 in the absence of detectable off-target effects. The newly established genome editing toolbox allows for the generation of precise mutants without needing to change other regions of the genome, opening up opportunities to study the role of individual histone modifications, catalytic sites of enzymes or the regulatory potential of UTRs in their endogenous environments.

Evolutionary Perspectives of Genotype-Phenotype Factors in Leishmania Metabolism

The sandfly midgut and the human macrophage phagolysosome provide antagonistic metabolic niches for the endoparasite Leishmania to survive and populate. Although these environments fluctuate across developmental stages, the relative changes in both these environments across parasite generations might remain gradual. Such environmental restrictions might endow parasite metabolism with a choice of specific genotypic and phenotypic factors that can constrain enzyme evolution for successful adaptation to the host. With respect to the available cellular information for Leishmania species, for the first time, we measure the relative contribution of eight inter-correlated predictors related to codon usage, GC content, gene expression, gene length, multi-functionality, and flux-coupling potential of an enzyme on the evolutionary rates of singleton metabolic genes and further compare their effects across three Leishmania species. Our analysis reveals that codon adaptation, multi-functionality, and flux-coupling potential of an enzyme are independent contributors of enzyme evolutionary rates, which can together explain a large variation in enzyme evolutionary rates across species. We also hypothesize that a species-specific occurrence of duplicated genes in novel subcellular locations can create new flux routes through certain singleton flux-coupled enzymes, thereby constraining their evolution. A cross-species comparison revealed both common and species-specific genes whose evolutionary divergence was constrained by multiple independent factors. Out of these, previously known pharmacological targets and virulence factors in Leishmania were identified, suggesting their evolutionary reasons for being important survival factors to the parasite. All these results provide a fundamental understanding of the factors underlying adaptive strategies of the parasite, which can be further targeted.

Codon Usage in Trypanosomatids: The Bias of Expression

Translation and RNA decay, two processes in which all mRNAs are engaged, are intimately related processes. Two new studies demonstrate that, in trypanosomatids, codon usage largely shapes mRNA abundance in a translation-dependent manner. The findings indicate that mRNA decay control by codon choice is an ancient and conserved mechanism.