Trypanosoma brucei Tb927.2.6100 is an essential protein associated with kinetoplast DNA

From a bimodal to a multi-stage view on trypanosomes' differential RNA editing

Significant variations in the abundance of mitochondrial RNA processing proteins and their target RNAs across trypanosome life stages present an opportunity to explore the regulatory mechanisms that drive these changes. Utilizing omics approaches can uncover unconventional targets, aiding our understanding of the parasites' adaptation and enabling targeted interventions for differentiation.

Mitochondrial genome maintenance-the kinetoplast story

Mitochondrial DNA replication is an essential process in most eukaryotes. Similar to the diversity in mitochondrial genome size and organization in the different eukaryotic supergroups, there is considerable diversity in the replication process of the mitochondrial DNA. In this review, we summarize the current knowledge of mitochondrial DNA replication and the associated factors in trypanosomes with a focus on Trypanosoma brucei, and provide a new model of minicircle replication for this protozoan parasite. The model assumes the mitochondrial DNA (kinetoplast DNA, kDNA) of T. brucei to be loosely diploid in nature and the replication of the genome to occur at two replication centers at the opposing ends of the kDNA disc (also known as antipodal sites, APS). The new model is consistent with the localization of most replication factors and in contrast to the current model, it does not require the assumption of an unknown sorting and transport complex moving freshly replicated DNA to the APS. In combination with the previously proposed sexual stages of the parasite in the insect vector, the new model provides a mechanism for maintenance of the mitochondrial genetic diversity.

Characterization of two novel proteins involved in mitochondrial DNA anchoring in Trypanosoma brucei

Trypanosoma brucei is a single celled eukaryotic parasite in the group of the Kinetoplastea. The parasite harbors a single mitochondrion with a singular mitochondrial genome that is known as the kinetoplast DNA (kDNA). The kDNA consists of a unique network of thousands of interlocked circular DNA molecules. To ensure proper inheritance of the kDNA to the daughter cells, the genome is physically linked to the basal body, the master organizer of the cell cycle in trypanosomes. The connection that spans, cytoplasm, mitochondrial membranes and the mitochondrial matrix is mediated by the Tripartite Attachment Complex (TAC). Using a combination of proteomics and RNAi we test the current model of hierarchical TAC assembly and identify TbmtHMG44 and TbKAP68 as novel candidates of a complex that connects the TAC to the kDNA. Depletion of TbmtHMG44 or TbKAP68 each leads to a strong kDNA loss but not missegregation phenotype as previously defined for TAC components. We demonstrate that the proteins rely on both the TAC and the kDNA for stable localization to the interface between these two structures. In vitro experiments suggest a direct interaction between TbmtHMG44 and TbKAP68 and that recombinant TbKAP68 is a DNA binding protein. We thus propose that TbmtHMG44 and TbKAP68 are part of a distinct complex connecting the kDNA to the TAC.

Genome-wide subcellular protein map for the flagellate parasite Trypanosoma brucei

Trypanosoma brucei is a model trypanosomatid, an important group of human, animal and plant unicellular parasites. Understanding their complex cell architecture and life cycle is challenging because, as with most eukaryotic microbes, ~50% of genome-encoded proteins have completely unknown functions. Here, using fluorescence microscopy and cell lines expressing endogenously tagged proteins, we mapped the subcellular localization of 89% of the T. brucei proteome, a resource we call TrypTag. We provide clues to function and define lineage-specific organelle adaptations for parasitism, mapping the ultraconserved cellular architecture of eukaryotes, including the first comprehensive 'cartographic' analysis of the eukaryotic flagellum, which is vital for morphogenesis and pathology. To demonstrate the power of this resource, we identify novel organelle subdomains and changes in molecular composition through the cell cycle. TrypTag is a transformative resource, important for hypothesis generation for both eukaryotic evolutionary molecular cell biology and fundamental parasite cell biology.

Expanded Proteomic Survey of the Human Parasite Leishmania major Focusing on Changes in Null Mutants of the Golgi GDP-Mannose/Fucose/Arabinopyranose Transporter LPG2 and of the Mitochondrial Fucosyltransferase FUT1

The trypanosomatid protozoan parasite Leishmania has a significant impact on human health globally. Understanding the pathways associated with virulence within this significant pathogen is critical for identifying novel vaccination and chemotherapy targets. Within this study we leverage an ultradeep proteomic approach to improve our understanding of two virulence-associated genes in Leishmania, encoding the Golgi mannose/arabinopyranose/fucose nucleotide-sugar transporter (LPG2) and the mitochondrial fucosyltransferase (FUT1). Using deep peptide fractionation followed by complementary fragmentation approaches with higher-energy collisional dissociation (HCD) and electron transfer dissociation (ETD) allowed the identification of over 6,500 proteins, nearly doubling the experimentally known Leishmania major proteome. This deep proteomic analysis revealed significant quantitative differences in both Δlpg2- and Δfut1s mutants with FUT1-dependent changes linked to marked alterations within mitochondrion-associated proteins, while LPG2-dependent changes impacted many pathways, including the secretory pathway. While the FUT1 enzyme has been shown to fucosylate peptides in vitro, no evidence for protein fucosylation was identified within our ultradeep analysis, nor did we observe fucosylated glycans within Leishmania glycopeptides isolated using hydrophilic interaction liquid chromatography (HILIC) enrichment. This work provides a critical resource for the community on the observable Leishmania proteome as well as highlighting phenotypic changes associated with LPG2 or FUT1, ablation of which may guide the development of future therapeutics. IMPORTANCE Leishmania is a widespread trypanosomatid protozoan parasite of humans, with ~12 million cases currently, ranging from mild to fatal, and hundreds of millions asymptomatically infected. This work advances knowledge of the experimental proteome by nearly 2-fold, to more than 6,500 proteins and thus provides a great resource to investigators seeking to decode how this parasite is transmitted and causes disease and to identify new targets for therapeutic intervention. The ultradeep proteomics approach identified potential proteins underlying the "persistence-without-pathology" phenotype of mutants with deletion of the Golgi nucleotide transporter LPG2, showing many alterations and several candidates. Studies of a rare mutant with deletion of the mitochondrial fucosyltransferase FUT1 revealed changes underlying its strong mitochondrial dysfunction but did not reveal examples of fucosylation of either peptides or N-glycans. This suggests that this vital protein's elusive target(s) may be more complex than the methods used could detect or that this target may not be a protein but perhaps another glycoconjugate or glycolipid.

Actin-related protein 4: An unconventional negative regulator of mitochondrial calcium in protozoan parasite Leishmania

Regulation of mitochondrial calcium import is less understood in evolutionarily distinct protozoan parasites, such as Leishmania, as some of the mitochondrial calcium uniporter complex proteins are either missing or functionally diverged. Here, we show that Actin-related protein4 (ARP4), localizes exclusively into the Leishmania mitochondrion and depletion of this protein causes cells to accumulate calcium in the mitochondrion. The ARP4 depleted cells show increased activation of pyruvate dehydrogenase and production of ATP. Overall, our results indicate that ARP4 negatively regulates calcium uptake in the Leishmania mitochondrion.

Basic Biology of Trypanosoma cruzi

The present review addresses basic aspects of the biology of the pathogenic protozoa Trypanosoma cruzi and some comparative information of Trypanosoma brucei. Like eukaryotic cells, their cellular organization is similar to that of mammalian hosts. However, these parasites present structural particularities. That is why the following topics are emphasized in this paper: developmental stages of the life cycle in the vertebrate and invertebrate hosts; the cytoskeleton of the protozoa, especially the sub-pellicular microtubules; the flagellum and its attachment to the protozoan body through specialized junctions; the kinetoplast-mitochondrion complex, including its structural organization and DNA replication; glycosome and its role in the metabolism of the cell; acidocalcisome, describing its morphology, biochemistry, and functional role; cytostome and the endocytic pathway; the organization of the endoplasmic reticulum and Golgi complex; the nucleus, describing its structural organization during interphase and division; and the process of interaction of the parasite with host cells. The unique characteristics of these structures also make them interesting chemotherapeutic targets. Therefore, further understanding of cell biology aspects contributes to the development of drugs for chemotherapy.

Importance of Angomonas deanei KAP4 for kDNA arrangement, cell division and maintenance of the host-bacterium relationship

Angomonas deanei coevolves in a mutualistic relationship with a symbiotic bacterium that divides in synchronicity with other host cell structures. Trypanosomatid mitochondrial DNA is contained in the kinetoplast and is composed of thousands of interlocked DNA circles (kDNA). The arrangement of kDNA is related to the presence of histone-like proteins, known as KAPs (kinetoplast-associated proteins), that neutralize the negatively charged kDNA, thereby affecting the activity of mitochondrial enzymes involved in replication, transcription and repair. In this study, CRISPR-Cas9 was used to delete both alleles of the A. deanei KAP4 gene. Gene-deficient mutants exhibited high compaction of the kDNA network and displayed atypical phenotypes, such as the appearance of a filamentous symbionts, cells containing two nuclei and one kinetoplast, and division blocks. Treatment with cisplatin and UV showed that Δkap4 null mutants were not more sensitive to DNA damage and repair than wild-type cells. Notably, lesions caused by these genotoxic agents in the mitochondrial DNA could be repaired, suggesting that the kDNA in the kinetoplast of trypanosomatids has unique repair mechanisms. Taken together, our data indicate that although KAP4 is not an essential protein, it plays important roles in kDNA arrangement and replication, as well as in the maintenance of symbiosis.

Diverse telomeres in trypanosomatids

Telomeres are the ends of linear eukaryotic chromosomes facilitating the resolution of the ‘end replication and protection’ problems, associated with linearity. At the nucleotide level, telomeres typically represent stretches of tandemly arranged telomeric repeats, which vary in length and sequence among different groups of organisms. Recently, a composition of the telomere-associated protein complex has been scrutinized in Trypanosoma brucei. In this work, we subjected proteins from that list to a more detailed bioinformatic analysis and delineated a core set of 20 conserved proteins putatively associated with telomeres in trypanosomatids. Out of these, two proteins (Ku70 and Ku80) are conspicuously missing in representatives of the genus Blastocrithidia, yet telomeres in these species do not appear to be affected. In this work, based on the analysis of a large set of trypanosomatids widely different in their phylogenetic position and life strategies, we demonstrated that telomeres of trypanosomatids are diverse in length, even within groups of closely related species. Our analysis showed that the expression of two proteins predicted to be associated with telomeres (those encoding telomerase and telomere-associated hypothetical protein orthologous to Tb927.6.4330) may directly affect and account for the differences in telomere length within the species of the Leishmania mexicana complex.

Potent Antitrypanosomal Activities of 3-Aminosteroids against African Trypanosomes: Investigation of Cellular Effects and of Cross-Resistance with Existing Drugs

Treatment of animal African trypanosomiasis (AAT) requires urgent need for safe, potent and affordable drugs and this has necessitated this study. We investigated the trypanocidal activities and mode of action of selected 3-aminosteroids against Trypanosoma brucei brucei. The in vitro activity of selected compounds of this series against T. congolense (Savannah-type, IL3000), T. b. brucei (bloodstream trypomastigote, Lister strain 427 wild-type (427WT)) and various multi-drug resistant cell lines was assessed using a resazurin-based cell viability assay. Studies on mode of antitrypanosomal activity of some selected 3-aminosteroids against Tbb 427WT were also carried out. The tested compounds mostly showed moderate-to-low in vitro activities and low selectivity to mammalian cells. Interestingly, a certain aminosteroid, holarrhetine (10, IC50 = 0.045 ± 0.03 µM), was 2 times more potent against T. congolense than the standard veterinary drug, diminazene aceturate, and 10 times more potent than the control trypanocide, pentamidine, and displayed an excellent in vitro selectivity index of 2130 over L6 myoblasts. All multi-drug resistant strains of T. b. brucei tested were not significantly cross-resistant with the purified compounds. The growth pattern of Tbb 427WT on long and limited exposure time revealed gradual but irrecoverable growth arrest at ≥ IC50 concentrations of 3-aminosteroids. Trypanocidal action was not associated with membrane permeabilization of trypanosome cells but instead with mitochondrial membrane depolarization, reduced adenosine triphosphate (ATP) levels and G₂/M cell cycle arrest which appear to be the result of mitochondrial accumulation of the aminosteroids. These findings provided insights for further development of this new and promising class of trypanocide against African trypanosomes.

Kinetoplast Division Factors in a Trypanosome

Inheritance of the single mitochondrial nucleoid (kinetoplast) in the trypanosome requires numerous proteins, many of whose precise roles are unclear. By considering kinetoplast DNA (kDNA) as a template for cleavage into two equal-size networks, we predicted sets of mutant kinetoplasts associated with defects in each of the five steps in the kinetoplast cycle. Comparison of these kinetoplasts with those obtained after gene knockdowns enabled assignment of proteins to five classes - kDNA synthesis, site of scission selection, scission, separation, and partitioning. These studies highlight how analysis of mutant kinetoplast phenotypes may be used to predict functional categories of proteins involved in the biogenesis of kinetoplasts.

Multi-target mode of action of a Clerodane-type diterpenoid from Polyalthia longifolia targeting African trypanosomes

Natural products have made remarkable contributions to drug discovery and therapy. In this work we exploited various biochemical approaches to investigate the mode of action of 16-α-hydroxy-cleroda-3,13 (14)-Z-dien-15,16-olide (HDK-20), which we recently isolated from Polyalthia longifolia, on Trypanosoma brucei bloodstream trypomastigotes. HDK20 at concentrations ≥ EC₅₀ (0.4 μg/ml) was trypanocidal, with its effect irreversible after only a brief exposure time (<1 h). Fluorescence microscopic assessment of DNA configuration revealed severe cell cycle defects after 8 h of incubation with the compound, the equivalent of a single generation time. This was accompanied by DNA fragmentation as shown by Terminal deoxynucleotidyl transferase dUTP Nick-End Labelling (TUNEL) assays. HDK-20 also induced a fast and profound depolarisation of the parasites’ mitochondrial membrane potential and depleted intracellular ATP levels of T. brucei. Overall, HDK20 showed a multi-target mechanism of action, which provides a biochemical explanation for the promising anti-trypanosomatid activity in our previous report.

Expanded repertoire of kinetoplast associated proteins and unique mitochondrial DNA arrangement of symbiont-bearing trypanosomatids

In trypanosomatids, the kinetoplast is the portion of the single mitochondrion that is connected to the basal body and contains the kDNA, a network composed by circular and interlocked DNA. The kDNA packing is conducted by Kinetoplast Associated Proteins (KAPs), which are similar to eukaryotic histone H1. In symbiont-harboring trypanosomatids (SHTs) such as Angomonas deanei and Strigomonas culicis, a ß-proteobacterium co-evolves with the host in a mutualistic relationship. The prokaryote confers nutritional benefits to the host and affects its cell structure. Atomic force microscopy showed that the topology of isolated kDNA networks is quite similar in the two SHT species. Ultrastructural analysis using high-resolution microscopy techniques revealed that the DNA fibrils are more compact in the kinetoplast region that faces the basal body and that the presence of the symbiotic bacterium does not interfere with kDNA topology. However, RT-PCR data revealed differences in the expression of KAPs in wild-type protozoa as compared to aposymbiotic cells. Immunolocalization showed that different KAPs present distinct distributions that are coincident in symbiont-bearing and in symbiont-free cells. Although KAP4 and KAP7 are shared by all trypanosomatid species, the expanded repertoire of KAPs in SHTs can be used as phylogenetic markers to distinguish different genera.

A leucine aminopeptidase is involved in kinetoplast DNA segregation in Trypanosoma brucei

The kinetoplast (k), the uniquely packaged mitochondrial DNA of trypanosomatid protists is formed by a catenated network of minicircles and maxicircles that divide and segregate once each cell cycle. Although many proteins involved in kDNA replication and segregation are now known, several key steps in the replication mechanism remain uncharacterized at the molecular level, one of which is the nabelschnur or umbilicus, a prominent structure which in the mammalian parasite Trypanosoma brucei connects the daughter kDNA networks prior to their segregation. Here we characterize an M17 family leucyl aminopeptidase metalloprotease, termed TbLAP1, which specifically localizes to the kDNA disk and the nabelschur and represents the first described protein found in this structure. We show that TbLAP1 is required for correct segregation of kDNA, with knockdown resulting in delayed cytokinesis and ectopic expression leading to kDNA loss and decreased cell proliferation. We propose that TbLAP1 is required for efficient kDNA division and specifically participates in the separation of daughter kDNA networks.

PNT1 Is a C11 Cysteine Peptidase Essential for Replication of the Trypanosome Kinetoplast

The structure of a C11 peptidase PmC11 from the gut bacterium, Parabacteroides merdae, has recently been determined, enabling the identification and characterization of a C11 orthologue, PNT1, in the parasitic protozoon Trypanosoma brucei. A phylogenetic analysis identified PmC11 orthologues in bacteria, archaea, Chromerids, Coccidia, and Kinetoplastida, the latter being the most divergent. A primary sequence alignment of PNT1 with clostripain and PmC11 revealed the position of the characteristic His-Cys catalytic dyad (His⁹⁹ and Cys¹³⁶), and an Asp (Asp¹³⁴) in the potential S₁ binding site. Immunofluorescence and cryoelectron microscopy revealed that PNT1 localizes to the kinetoplast, an organelle containing the mitochondrial genome of the parasite (kDNA), with an accumulation of the protein at or near the antipodal sites. Depletion of PNT1 by RNAi in the T. brucei bloodstream form was lethal both in in vitro culture and in vivo in mice and the induced population accumulated cells lacking a kinetoplast. In contrast, overexpression of PNT1 led to cells having mislocated kinetoplasts. RNAi depletion of PNT1 in a kDNA independent cell line resulted in kinetoplast loss but was viable, indicating that PNT1 is required exclusively for kinetoplast maintenance. Expression of a recoded wild-type PNT1 allele, but not of an active site mutant restored parasite viability after induction in vitro and in vivo confirming that the peptidase activity of PNT1 is essential for parasite survival. These data provide evidence that PNT1 is a cysteine peptidase that is required exclusively for maintenance of the trypanosome kinetoplast.

Trypanocidal action of bisphosphonium salts through a mitochondrial target in bloodstream form Trypanosoma brucei

Lipophilic bisphosphonium salts are among the most promising antiprotozoal leads currently under investigation. As part of their preclinical evaluation we here report on their mode of action against African trypanosomes, the etiological agents of sleeping sickness. The bisphosphonium compounds CD38 and AHI-9 exhibited rapid inhibition of Trypanosoma brucei growth, apparently the result of cell cycle arrest that blocked the replication of mitochondrial DNA, contained in the kinetoplast, thereby preventing the initiation of S-phase. Incubation with either compound led to a rapid reduction in mitochondrial membrane potential, and ATP levels decreased by approximately 50% within 1 h. Between 4 and 8 h, cellular calcium levels increased, consistent with release from the depolarized mitochondria. Within the mitochondria, the Succinate Dehydrogenase complex (SDH) was investigated as a target for bisphosphonium salts, but while its subunit 1 (SDH1) was present at low levels in the bloodstream form trypanosomes, the assembled complex was hardly detectable. RNAi knockdown of the SDH1 subunit produced no growth phenotype, either in bloodstream or in the procyclic (insect) forms and we conclude that in trypanosomes SDH is not the target for bisphosphonium salts. Instead, the compounds inhibited ATP production in intact mitochondria, as well as the purified F₁ ATPase, to a level that was similar to 1 mM azide. Co-incubation with azide and bisphosphonium compounds did not inhibit ATPase activity more than either product alone. The results show that, in T. brucei, bisphosphonium compounds do not principally act on succinate dehydrogenase but on the mitochondrial F_oF₁ ATPase.

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Bisphosphonium salts display highly promising antiprotozoal activity.

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It has been reported that, in Leishmania, they act on the mitochondrial SDH complex.

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We show that in Trypanosoma brucei SDH is not essential and not the drug target.

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Instead, we present strong evidence that the F1F0 ATPase is the target.

RNA interference screen reveals a high proportion of mitochondrial proteins essential for correct cell cycle progress in Trypanosoma brucei

Background

Trypanosomatid parasites possess a single mitochondrion which is classically involved in the energetic metabolism of the cell, but also, in a much more original way, through its single and complex DNA (termed kinetoplast), in the correct progress of cell division. In order to identify proteins potentially involved in the cell cycle, we performed RNAi knockdowns of 101 genes encoding mitochondrial proteins using procyclic cells of Trypanosoma brucei.

Results

A major cell growth reduction was observed in 10 cases and a moderate reduction in 29 other cases. These data are overall in agreement with those previously obtained by a case-by-case approach performed on chromosome 1 genes, and quantitatively with those obtained by “high-throughput phenotyping using parallel sequencing of RNA interference targets” (RIT-seq). Nevertheless, a detailed analysis revealed many qualitative discrepancies with the RIT-seq-based approach. Moreover, for 37 out of 39 mutants for which a moderate or severe growth defect was observed here, we noted abnormalities in the cell cycle progress, leading to increased proportions of abnormal cell cycle stages, such as cells containing more than 2 kinetoplasts (K) and/or more than 2 nuclei (N), and modified proportions of the normal phenotypes (1N1K, 1N2K and 2N2K).

Conclusions

These data, together with the observation of other abnormal phenotypes, show that all the corresponding mitochondrial proteins are involved, directly or indirectly, in the correct progress or, less likely, in the regulation, of the cell cycle in T. brucei. They also show how post-genomics analyses performed on a case-by-case basis may yield discrepancies with global approaches.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1505-5) contains supplementary material, which is available to authorized users.

Malleable mitochondrion of Trypanosoma brucei

The importance of mitochondria for a typical aerobic eukaryotic cell is undeniable, as the list of necessary mitochondrial processes is steadily growing. Here, we summarize the current knowledge of mitochondrial biology of an early-branching parasitic protist, Trypanosoma brucei, a causative agent of serious human and cattle diseases. We present a comprehensive survey of its mitochondrial pathways including kinetoplast DNA replication and maintenance, gene expression, protein and metabolite import, major metabolic pathways, Fe-S cluster synthesis, ion homeostasis, organellar dynamics, and other processes. As we describe in this chapter, the single mitochondrion of T. brucei is everything but simple and as such rivals mitochondria of multicellular organisms.

Genome and phylogenetic analyses of Trypanosoma evansi reveal extensive similarity to T. brucei and multiple independent origins for dyskinetoplasty

Two key biological features distinguish Trypanosoma evansi from the T. brucei group: independence from the tsetse fly as obligatory vector, and independence from the need for functional mitochondrial DNA (kinetoplast or kDNA). In an effort to better understand the molecular causes and consequences of these differences, we sequenced the genome of an akinetoplastic T. evansi strain from China and compared it to the T. b. brucei reference strain. The annotated T. evansi genome shows extensive similarity to the reference, with 94.9% of the predicted T. b. brucei coding sequences (CDS) having an ortholog in T. evansi, and 94.6% of the non-repetitive orthologs having a nucleotide identity of 95% or greater. Interestingly, several procyclin-associated genes (PAGs) were disrupted or not found in this T. evansi strain, suggesting a selective loss of function in the absence of the insect life-cycle stage. Surprisingly, orthologous sequences were found in T. evansi for all 978 nuclear CDS predicted to represent the mitochondrial proteome in T. brucei, although a small number of these may have lost functionality. Consistent with previous results, the F1FO-ATP synthase γ subunit was found to have an A281 deletion, which is involved in generation of a mitochondrial membrane potential in the absence of kDNA. Candidates for CDS that are absent from the reference genome were identified in supplementary de novo assemblies of T. evansi reads. Phylogenetic analyses show that the sequenced strain belongs to a dominant group of clonal T. evansi strains with worldwide distribution that also includes isolates classified as T. equiperdum. At least three other types of T. evansi or T. equiperdum have emerged independently. Overall, the elucidation of the T. evansi genome sequence reveals extensive similarity of T. brucei and supports the contention that T. evansi should be classified as a subspecies of T. brucei.

A dually located multi-HMG-box protein of Aspergillus nidulans has a crucial role in conidial and ascospore germination

Seven HMG-box proteins of Aspergillus nidulans have been identified in the genomic databases. Three of these have the characteristics of non-specific DNA-binding proteins. One of these, AN1267 (HmbB), comprises one canonical HMG-box in its C-terminus and upstream of the canonical box two structurally related boxes, to be called Shadow-HMG-boxes. This protein defines, together with the Podospora anserina mtHMG1, a clade of proteins present in the Pezizomycotina, with orthologues in some of the Taphrinomycotina. HmbB localizes primarily to the mitochondria but occasionally in nuclei. The deletion of the cognate gene results in a number of pleiotropic effects, including those on hyphal morphology, sensitivity to oxidative stress, absence of sterigmatocystin production and changes in the profile of conidial metabolites. The most striking phenotype of deletion strains is a dramatic decrease in conidial and ascospore viability. We show that this is most likely due to the protein being essential to maintain mitochondrial DNA in spores.