A second eukaryotic group with mitochondrion-encoded tmRNA: in silico identification and experimental confirmation

Expansion of the tmRNA sequence database and new tools for search and visualization

Transfer-messenger RNA (tmRNA) contributes essential tRNA-like and mRNA-like functions during the process of trans-translation, a mechanism of quality control for the translating bacterial ribosome. Proper tmRNA identification benefits the study of trans-translation and also the study of genomic islands, which frequently use the tmRNA gene as an integration site. Automated tmRNA gene identification tools are available, but manual inspection is still important for eliminating false positives. We have increased our database of precisely mapped tmRNA sequences over 50-fold to 97 179 unique sequences. Group I introns had previously been found integrated within a single subsite within the TψC-loop; they have now been identified at four distinct subsites, suggesting multiple founding events of invasion of tmRNA genes by group I introns, all in the same vicinity. tmRNA genes were found in metagenomic archaeal genomes, perhaps a result of misbinning of bacterial sequences during genome assembly. With the expanded database, we have produced new covariance models for improved tmRNA sequence search and new secondary structure visualization tools.

The diverse structural modes of tRNA binding and recognition

tRNAs are short noncoding RNAs responsible for decoding mRNA codon triplets, delivering correct amino acids to the ribosome, and mediating polypeptide chain formation. Due to their key roles during translation, tRNAs have a highly conserved shape and large sets of tRNAs are present in all living organisms. Regardless of sequence variability, all tRNAs fold into a relatively rigid three-dimensional L-shaped structure. The conserved tertiary organization of canonical tRNA arises through the formation of two orthogonal helices, consisting of the acceptor and anticodon domains. Both elements fold independently to stabilize the overall structure of tRNAs through intramolecular interactions between the D- and T-arm. During tRNA maturation, different modifying enzymes posttranscriptionally attach chemical groups to specific nucleotides, which not only affect translation elongation rates but also restrict local folding processes and confer local flexibility when required. The characteristic structural features of tRNAs are also employed by various maturation factors and modification enzymes to assure the selection, recognition, and positioning of specific sites within the substrate tRNAs. The cellular functional repertoire of tRNAs continues to extend well beyond their role in translation, partly, due to the expanding pool of tRNA-derived fragments. Here, we aim to summarize the most recent developments in the field to understand how three-dimensional structure affects the canonical and noncanonical functions of tRNA.

Mitochondrial genome annotation with MFannot: a critical analysis of gene identification and gene model prediction

Compared to nuclear genomes, mitochondrial genomes (mitogenomes) are small and usually code for only a few dozen genes. Still, identifying genes and their structure can be challenging and time-consuming. Even automated tools for mitochondrial genome annotation often require manual analysis and curation by skilled experts. The most difficult steps are (i) the structural modelling of intron-containing genes; (ii) the identification and delineation of Group I and II introns; and (iii) the identification of moderately conserved, non-coding RNA (ncRNA) genes specifying 5S rRNAs, tmRNAs and RNase P RNAs. Additional challenges arise through genetic code evolution which can redefine the translational identity of both start and stop codons, thus obscuring protein-coding genes. Further, RNA editing can render gene identification difficult, if not impossible, without additional RNA sequence data. Current automated mito- and plastid-genome annotators are limited as they are typically tailored to specific eukaryotic groups. The MFannot annotator we developed is unique in its applicability to a broad taxonomic scope, its accuracy in gene model inference, and its capabilities in intron identification and classification. The pipeline leverages curated profile Hidden Markov Models (HMMs), covariance (CMs) and ERPIN models to better capture evolutionarily conserved signatures in the primary sequence (HMMs and CMs) as well as secondary structure (CMs and ERPIN). Here we formally describe MFannot, which has been available as a web-accessible service (https://megasun.bch.umontreal.ca/apps/mfannot/) to the research community for nearly 16 years. Further, we report its performance on particularly intron-rich mitogenomes and describe ongoing and future developments.

Microheliella maris possesses the most gene-rich mitochondrial genome in Diaphoretickes

The mitochondrial genomes are very diverse, but their evolutionary history is unclear due to the lack of efforts to sequence those of protists (unicellular eukaryotes), which cover a major part of the eukaryotic tree. Cryptista comprises cryptophytes, goniomonads, kathablepharids, and Palpitomonas bilix, and their mitochondrial genomes (mt-genomes) are characterized by various gene contents, particularly the presence/absence of an ancestral (bacterial) system for the cytochrome c maturation system. To shed light on mt-genome evolution in Cryptista, we report the complete mt-genome of Microheliella maris, which was recently revealed to branch at the root of Cryptista. The M. maris mt-genome was reconstructed as a circular mapping chromosome of 61.2 kbp with a pair of inverted repeats (12.9 kbp) and appeared to be the most gene-rich among the mt-genomes of the members of Diaphoretickes (a mega-scale eukaryotic assembly including Archaeplastida, Cryptista, Haptista, and SAR) studied so far, carrying 53 protein-coding genes. With this newly sequenced mt-genome, we inferred and discussed the evolution of the mt-genome in Cryptista and Diaphoretickes.

Ribosome-associated quality control and CAT tailing

Translation is the set of mechanisms by which ribosomes decode genetic messages as they synthesize polypeptides of a defined amino acid sequence. While the ribosome has been honed by evolution for high-fidelity translation, errors are inevitable. Aberrant mRNAs, mRNA structure, defective ribosomes, interactions between nascent proteins and the ribosomal exit tunnel, and insufficient cellular resources, including low tRNA levels, can lead to functionally irreversible stalls. Life thus depends on quality control mechanisms that detect, disassemble and recycle stalled translation intermediates. Ribosome-associated Quality Control (RQC) recognizes aberrant ribosome states and targets their potentially toxic polypeptides for degradation. Here we review recent advances in our understanding of RQC in bacteria, fungi, and metazoans. We focus in particular on an unusual modification made to the nascent chain known as a "CAT tail", or Carboxy-terminal Alanine and Threonine tail, and the mechanisms by which ancient RQC proteins catalyze CAT-tail synthesis.

Recent advances in understanding mitochondrial genome diversity

Ever since its discovery, the double-stranded DNA contained in the mitochondria of eukaryotes has fascinated researchers because of its bacterial endosymbiotic origin, crucial role in encoding subunits of the respiratory complexes, compact nature, and specific inheritance mechanisms. In the last few years, high-throughput sequencing techniques have accelerated the sequencing of mitochondrial genomes (mitogenomes) and uncovered the great diversity of organizations, gene contents, and modes of replication and transcription found in living eukaryotes. Some early divergent lineages of unicellular eukaryotes retain certain synteny and gene content resembling those observed in the genomes of alphaproteobacteria (the inferred closest living group of mitochondria), whereas others adapted to anaerobic environments have drastically reduced or even lost the mitogenome. In the three main multicellular lineages of eukaryotes, mitogenomes have pursued diverse evolutionary trajectories in which different types of molecules (circular versus linear and single versus multipartite), gene structures (with or without self-splicing introns), gene contents, gene orders, genetic codes, and transfer RNA editing mechanisms have been selected. Whereas animals have evolved a rather compact mitochondrial genome between 11 and 50 Kb in length with a highly conserved gene content in bilaterians, plants exhibit large mitochondrial genomes of 66 Kb to 11.3 Mb with large intergenic repetitions prone to recombination, and fungal mitogenomes have intermediate sizes of 12 to 236 Kb.

Co-evolution in the Jungle: From Leafcutter Ant Colonies to Chromosomal Ends

Biological entities are multicomponent systems where each part is directly or indirectly dependent on the others. In effect, a change in a single component might have a consequence on the functioning of its partners, thus affecting the fitness of the entire system. In this article, we provide a few examples of such complex biological systems, ranging from ant colonies to a population of amino acids within a single-polypeptide chain. Based on these examples, we discuss one of the central and still challenging questions in biology: how do such multicomponent consortia co-evolve? More specifically, we ask how telomeres, nucleo-protein complexes protecting the integrity of linear DNA chromosomes, originated from the ancestral organisms having circular genomes and thus not dealing with end-replication and end-protection problems. Using the examples of rapidly evolving topologies of mitochondrial genomes in eukaryotic microorganisms, we show what means of co-evolution were employed to accommodate various types of telomere-maintenance mechanisms in mitochondria. We also describe an unprecedented runaway evolution of telomeric repeats in nuclei of ascomycetous yeasts accompanied by co-evolution of telomere-associated proteins. We propose several scenarios derived from research on telomeres and supported by other studies from various fields of biology, while emphasizing that the relevant answers are still not in sight. It is this uncertainty and a lack of a detailed roadmap that makes the journey through the jungle of biological systems still exciting and worth undertaking.

The draft nuclear genome sequence and predicted mitochondrial proteome of Andalucia godoyi, a protist with the most gene-rich and bacteria-like mitochondrial genome

Background

Comparative analyses have indicated that the mitochondrion of the last eukaryotic common ancestor likely possessed all the key core structures and functions that are widely conserved throughout the domain Eucarya. To date, such studies have largely focused on animals, fungi, and land plants (primarily multicellular eukaryotes); relatively few mitochondrial proteomes from protists (primarily unicellular eukaryotic microbes) have been examined. To gauge the full extent of mitochondrial structural and functional complexity and to identify potential evolutionary trends in mitochondrial proteomes, more comprehensive explorations of phylogenetically diverse mitochondrial proteomes are required. In this regard, a key group is the jakobids, a clade of protists belonging to the eukaryotic supergroup Discoba, distinguished by having the most gene-rich and most bacteria-like mitochondrial genomes discovered to date.

Results

In this study, we assembled the draft nuclear genome sequence for the jakobid Andalucia godoyi and used a comprehensive in silico approach to infer the nucleus-encoded portion of the mitochondrial proteome of this protist, identifying 864 candidate mitochondrial proteins. The A. godoyi mitochondrial proteome has a complexity that parallels that of other eukaryotes, while exhibiting an unusually large number of ancestral features that have been lost particularly in opisthokont (animal and fungal) mitochondria. Notably, we find no evidence that the A. godoyi nuclear genome has or had a gene encoding a single-subunit, T3/T7 bacteriophage-like RNA polymerase, which functions as the mitochondrial transcriptase in all eukaryotes except the jakobids.

Conclusions

As genome and mitochondrial proteome data have become more widely available, a strikingly punctuate phylogenetic distribution of different mitochondrial components has been revealed, emphasizing that the pathways of mitochondrial proteome evolution are likely complex and lineage-specific. Unraveling this complexity will require comprehensive comparative analyses of mitochondrial proteomes from a phylogenetically broad range of eukaryotes, especially protists. The systematic in silico approach described here offers a valuable adjunct to direct proteomic analysis (e.g., via mass spectrometry), particularly in cases where the latter approach is constrained by sample limitation or other practical considerations.

The Origin and Evolution of Release Factors: Implications for Translation Termination, Ribosome Rescue, and Quality Control Pathways

The evolution of release factors catalyzing the hydrolysis of the final peptidyl-tRNA bond and the release of the polypeptide from the ribosome has been a longstanding paradox. While the components of the translation apparatus are generally well-conserved across extant life, structurally unrelated release factor peptidyl hydrolases (RF-PHs) emerged in the stems of the bacterial and archaeo-eukaryotic lineages. We analyze the diversification of RF-PH domains within the broader evolutionary framework of the translation apparatus. Thus, we reconstruct the possible state of translation termination in the Last Universal Common Ancestor with possible tRNA-like terminators. Further, evolutionary trajectories of the several auxiliary release factors in ribosome quality control (RQC) and rescue pathways point to multiple independent solutions to this problem and frequent transfers between superkingdoms including the recently characterized ArfT, which is more widely distributed across life than previously appreciated. The eukaryotic RQC system was pieced together from components with disparate provenance, which include the long-sought-after Vms1/ANKZF1 RF-PH of bacterial origin. We also uncover an under-appreciated evolutionary driver of innovation in rescue pathways: effectors deployed in biological conflicts that target the ribosome. At least three rescue pathways (centered on the prfH/RFH, baeRF-1, and C12orf65 RF-PH domains), were likely innovated in response to such conflicts.

Mitochondrial Genome of Palpitomonas bilix: Derived Genome Structure and Ancestral System for Cytochrome c Maturation

We here reported the mitochondrial (mt) genome of one of the heterotrophic microeukaryotes related to cryptophytes, Palpitomonas bilix. The P. bilix mt genome was found to be a linear molecule composed of “single copy region” (∼16 kb) and repeat regions (∼30 kb) arranged in an inverse manner at both ends of the genome. Linear mt genomes with large inverted repeats are known for three distantly related eukaryotes (including P. bilix), suggesting that this particular mt genome structure has emerged at least three times in the eukaryotic tree of life. The P. bilix mt genome contains 47 protein-coding genes including ccmA, ccmB, ccmC, and ccmF, which encode protein subunits involved in the system for cytochrome c maturation inherited from a bacterium (System I). We present data indicating that the phylogenetic relatives of P. bilix, namely, cryptophytes, goniomonads, and kathablepharids, utilize an alternative system for cytochrome c maturation, which has most likely emerged during the evolution of eukaryotes (System III). To explain the distribution of Systems I and III in P. bilix and its phylogenetic relatives, two scenarios are possible: (i) System I was replaced by System III on the branch leading to the common ancestor of cryptophytes, goniomonads, and kathablepharids, and (ii) the two systems co-existed in their common ancestor, and lost differentially among the four descendants.

Origins of tmRNA: the missing link in the birth of protein synthesis?

The RNA world hypothesis refers to the early period on earth in which RNA was central in assuring both genetic continuity and catalysis. The end of this era coincided with the development of the genetic code and protein synthesis, symbolized by the apparition of the first non-random messenger RNA (mRNA). Modern transfer-messenger RNA (tmRNA) is a unique hybrid molecule which has the properties of both mRNA and transfer RNA (tRNA). It acts as a key molecule during trans-translation, a major quality control pathway of modern bacterial protein synthesis. tmRNA shares many common characteristics with ancestral RNA. Here, we present a model in which proto-tmRNAs were the first molecules on earth to support non-random protein synthesis, explaining the emergence of early genetic code. In this way, proto-tmRNA could be the missing link between the first mRNA and tRNA molecules and modern ribosome-mediated protein synthesis.

A Comparative Analysis of Mitochondrial Genomes in Eustigmatophyte Algae

Eustigmatophyceae (Ochrophyta, Stramenopiles) is a small algal group with species of the genus Nannochloropsis being its best studied representatives. Nuclear and organellar genomes have been recently sequenced for several Nannochloropsis spp., but phylogenetically wider genomic studies are missing for eustigmatophytes. We sequenced mitochondrial genomes (mitogenomes) of three species representing most major eustigmatophyte lineages, Monodopsis sp. MarTras21, Vischeria sp. CAUP Q 202 and Trachydiscus minutus, and carried out their comparative analysis in the context of available data from Nannochloropsis and other stramenopiles, revealing a number of noticeable findings. First, mitogenomes of most eustigmatophytes are highly collinear and similar in the gene content, but extensive rearrangements and loss of three otherwise ubiquitous genes happened in the Vischeria lineage; this correlates with an accelerated evolution of mitochondrial gene sequences in this lineage. Second, eustigmatophytes appear to be the only ochrophyte group with the Atp1 protein encoded by the mitogenome. Third, eustigmatophyte mitogenomes uniquely share a truncated nad11 gene encoding only the C-terminal part of the Nad11 protein, while the N-terminal part is encoded by a separate gene in the nuclear genome. Fourth, UGA as a termination codon and the cognate release factor mRF2 were lost from mitochondria independently by the Nannochloropsis and T. minutus lineages. Finally, the rps3 gene in the mitogenome of Vischeria sp. is interrupted by the UAG codon, but the genome includes a gene for an unusual tRNA with an extended anticodon loop that we speculate may serve as a suppressor tRNA to properly decode the rps3 gene.

Widespread occurrence of organelle genome-encoded 5S rRNAs including permuted molecules

5S Ribosomal RNA (5S rRNA) is a universal component of ribosomes, and the corresponding gene is easily identified in archaeal, bacterial and nuclear genome sequences. However, organelle gene homologs (rrn5) appear to be absent from most mitochondrial and several chloroplast genomes. Here, we re-examine the distribution of organelle rrn5 by building mitochondrion- and plastid-specific covariance models (CMs) with which we screened organelle genome sequences. We not only recover all organelle rrn5 genes annotated in GenBank records, but also identify more than 50 previously unrecognized homologs in mitochondrial genomes of various stramenopiles, red algae, cryptomonads, malawimonads and apusozoans, and surprisingly, in the apicoplast (highly derived plastid) genomes of the coccidian pathogens Toxoplasma gondii and Eimeria tenella. Comparative modeling of RNA secondary structure reveals that mitochondrial 5S rRNAs from brown algae adopt a permuted triskelion shape that has not been seen elsewhere. Expression of the newly predicted rrn5 genes is confirmed experimentally in 10 instances, based on our own and published RNA-Seq data. This study establishes that particularly mitochondrial 5S rRNA has a much broader taxonomic distribution and a much larger structural variability than previously thought. The newly developed CMs will be made available via the Rfam database and the MFannot organelle genome annotator.

Ends of the line for tmRNA-SmpB

Genes for the RNA tmRNA and protein SmpB, partners in the trans-translation process that rescues stalled ribosomes, have previously been found in all bacteria and some organelles. During a major update of The tmRNA Website (relocated to http://bioinformatics.sandia.gov/tmrna), including addition of an SmpB sequence database, we found some bacteria that lack functionally significant regions of SmpB. Three groups with reduced genomes have lost the central loop of SmpB, which is thought to improve alanylation and EF-Tu activation: Carsonella, Hodgkinia, and the hemoplasmas (hemotropic Mycoplasma). Carsonella has also lost the SmpB C-terminal tail, thought to stimulate the decoding center of the ribosome. We validate recent identification of tmRNA homologs in oomycete mitochondria by finding partner genes from oomycete nuclei that target SmpB to the mitochondrion. We have moreover identified through exhaustive search a small number of complete, but often highly derived, bacterial genomes that appear to lack a functional copy of either the tmRNA or SmpB gene (but not both). One Carsonella isolate exhibits complete degradation of the tmRNA gene sequence yet its smpB shows no evidence for relaxed selective constraint, relative to other genes in the genome. After loss of the SmpB central loop in the hemoplasmas, one subclade apparently lost tmRNA. Carsonella also exhibits gene overlap such that tmRNA maturation should produce a non-stop smpB mRNA. At least some of the tmRNA/SmpB-deficient strains appear to further lack the ArfA and ArfB backup systems for ribosome rescue. The most frequent neighbors of smpB are the tmRNA gene, a ratA/rnfH unit, and the gene for RNaseR, a known physical and functional partner of tmRNA-SmpB.