Directionally evolving genetic code: the UGA codon from stop to tryptophan in mitochondria

Xenomicrobiology: a roadmap for genetic code engineering

Biology is an analytical and informational science that is becoming increasingly dependent on chemical synthesis. One example is the high‐throughput and low‐cost synthesis of DNA, which is a foundation for the research field of synthetic biology (SB). The aim of SB is to provide biotechnological solutions to health, energy and environmental issues as well as unsustainable manufacturing processes in the frame of naturally existing chemical building blocks. Xenobiology (XB) goes a step further by implementing non‐natural building blocks in living cells. In this context, genetic code engineering respectively enables the re‐design of genes/genomes and proteins/proteomes with non‐canonical nucleic (XNAs) and amino (ncAAs) acids. Besides studying information flow and evolutionary innovation in living systems, XB allows the development of new‐to‐nature therapeutic proteins/peptides, new biocatalysts for potential applications in synthetic organic chemistry and biocontainment strategies for enhanced biosafety. In this perspective, we provide a brief history and evolution of the genetic code in the context of XB. We then discuss the latest efforts and challenges ahead for engineering the genetic code with focus on substitutions and additions of ncAAs as well as standard amino acid reductions. Finally, we present a roadmap for the directed evolution of artificial microbes for emancipating rare sense codons that could be used to introduce novel building blocks. The development of such xenomicroorganisms endowed with a ‘genetic firewall’ will also allow to study and understand the relation between code evolution and horizontal gene transfer.

An intronic open reading frame was released from one of group II introns in the mitochondrial genome of the haptophyte Chrysochromulina sp. NIES-1333

Mitochondrial (mt) genome sequences, which often bear introns, have been sampled from phylogenetically diverse eukaryotes. Thus, we can anticipate novel insights into intron evolution from previously unstudied mt genomes. We here investigated the origins and evolution of three introns in the mt genome of the haptophyte Chrysochromulina sp. NIES-1333, which was sequenced completely in this study. All the three introns were characterized as group II, on the basis of predicted secondary structure, and the conserved sequence motifs at the 5′ and 3′ termini. Our comparative studies on diverse mt genomes prompt us to propose that the Chrysochromulina mt genome laterally acquired the introns from mt genomes in distantly related eukaryotes. Many group II introns harbor intronic open reading frames for the proteins (intron-encoded proteins or IEPs), which likely facilitate the splicing of their host introns. However, we propose that a “free-standing,” IEP-like protein, which is not encoded within any introns in the Chrysochromulina mt genome, is involved in the splicing of the first cox1 intron that lacks any open reading frames.

The alternative translational profile that underlies the immune-evasive state of persistence in Chlamydiaceae exploits differential tryptophan contents of the protein repertoire

One form of immune evasion is a developmental state called "persistence" whereby chlamydial pathogens respond to the host-mediated withdrawal of L-tryptophan (Trp). A sophisticated survival mode of reversible quiescence is implemented. A mechanism has evolved which suppresses gene products necessary for rapid pathogen proliferation but allows expression of gene products that underlie the morphological and developmental characteristics of persistence. This switch from one translational profile to an alternative translational profile of newly synthesized proteins is proposed to be accomplished by maximizing the Trp content of some proteins needed for rapid proliferation (e.g., ADP/ATP translocase, hexose-phosphate transporter, phosphoenolpyruvate [PEP] carboxykinase, the Trp transporter, the Pmp protein superfamily for cell adhesion and antigenic variation, and components of the cell division pathway) while minimizing the Trp content of other proteins supporting the state of persistence. The Trp starvation mechanism is best understood in the human-Chlamydia trachomatis relationship, but the similarity of up-Trp and down-Trp proteomic profiles in all of the pathogenic Chlamydiaceae suggests that Trp availability is an underlying cue relied upon by this family of pathogens to trigger developmental transitions. The biochemically expensive pathogen strategy of selectively increased Trp usage to guide the translational profile can be leveraged significantly with minimal overall Trp usage by (i) regional concentration of Trp residue placements, (ii) amplified Trp content of a single protein that is required for expression or maturation of multiple proteins with low Trp content, and (iii) Achilles'-heel vulnerabilities of complex pathways to high Trp content of one or a few enzymes.

Separate origins of group I introns in two mitochondrial genes of the katablepharid Leucocryptos marina

Mitochondria are descendants of the endosymbiotic α-proteobacterium most likely engulfed by the ancestral eukaryotic cells, and the proto-mitochondrial genome should have been severely streamlined in terms of both genome size and gene repertoire. In addition, mitochondrial (mt) sequence data indicated that frequent intron gain/loss events contributed to shaping the modern mt genome organizations, resulting in the homologous introns being shared between two distantly related mt genomes. Unfortunately, the bulk of mt sequence data currently available are of phylogenetically restricted lineages, i.e., metazoans, fungi, and land plants, and are insufficient to elucidate the entire picture of intron evolution in mt genomes. In this work, we sequenced a 12 kbp-fragment of the mt genome of the katablepharid Leucocryptos marina. Among nine protein-coding genes included in the mt genome fragment, the genes encoding cytochrome b and cytochrome c oxidase subunit I (cob and cox1) were interrupted by group I introns. We further identified that the cob and cox1 introns host open reading frames for homing endonucleases (HEs) belonging to distantly related superfamilies. Phylogenetic analyses recovered an affinity between the HE in the Leucocryptos cob intron and two green algal HEs, and that between the HE in the Leucocryptos cox1 intron and a fungal HE, suggesting that the Leucocryptos cob and cox1 introns possess distinct evolutionary origins. Although the current intron (and intronic HE) data are insufficient to infer how the homologous introns were distributed to distantly related mt genomes, the results presented here successfully expanded the evolutionary dynamism of group I introns in mt genomes.

Probable presence of an ubiquitous cryptic mitochondrial gene on the antisense strand of the cytochrome oxidase I gene

BACKGROUND

Mitochondria mediate most of the energy production that occurs in the majority of eukaryotic organisms. These subcellular organelles contain a genome that differs from the nuclear genome and is referred to as mitochondrial DNA (mtDNA). Despite a disparity in gene content, all mtDNAs encode at least two components of the mitochondrial electron transport chain, including cytochrome c oxidase I (Cox1).

PRESENTATION OF THE HYPOTHESIS

A positionally conserved ORF has been found on the complementary strand of the cox1 genes of both eukaryotic mitochondria (protist, plant, fungal and animal) and alpha-proteobacteria. This putative gene has been named gau for gene antisense ubiquitous in mtDNAs. The length of the deduced protein is approximately 100 amino acids. In vertebrates, several stop codons have been found in the mt gau region, and potentially functional gau regions have been found in nuclear genomes. However, a recent bioinformatics study showed that several hypothetical overlapping mt genes could be predicted, including gau; this involves the possible import of the cytosolic AGR tRNA into the mitochondria and/or the expression of mt antisense tRNAs with anticodons recognizing AGR codons according to an alternative genetic code that is induced by the presence of suppressor tRNAs. Despite an evolutionary distance of at least 1.5 to 2.0 billion years, the deduced Gau proteins share some conserved amino acid signatures and structure, which suggests a possible conserved function. Moreover, BLAST analysis identified rare, sense-oriented ESTs with poly(A) tails that include the entire gau region. Immunohistochemical analyses using an anti-Gau monoclonal antibody revealed strict co-localization of Gau proteins and a mitochondrial marker.

TESTING THE HYPOTHESIS

This hypothesis could be tested by purifying the gau gene product and determining its sequence. Cell biological experiments are needed to determine the physiological role of this protein.

IMPLICATIONS OF THE HYPOTHESIS

Studies of the gau ORF will shed light on the origin of novel genes and their functions in organelles and could also have medical implications for human diseases that are caused by mitochondrial dysfunction. Moreover, this strengthens evidence for mitochondrial genes coded according to an overlapping genetic code.

Dinoflagellate mitochondrial genomes: stretching the rules of molecular biology

Mitochondrial genomes represent relict bacterial genomes derived from a progenitor alpha-proteobacterium that gave rise to all mitochondria through an ancient endosymbiosis. Evolution has massively reduced these genomes, yet despite relative simplicity their organization and expression has developed considerable novelty throughout eukaryotic evolution. Few organisms have reengineered their mitochondrial genomes as thoroughly as the protist lineage of dinoflagellates. Recent work reveals dinoflagellate mitochondrial genomes as likely the most gene-impoverished of any free-living eukaryote, encoding only two to three proteins. The organization and expression of these genomes, however, is far from the simplicity their gene content would suggest. Gene duplication, fragmentation, and scrambling have resulted in an inflated and complex genome organization. Extensive RNA editing then recodes gene transcripts, and trans-splicing is required to assemble full-length transcripts for at least one fragmented gene. Even after these processes, messenger RNAs (mRNAs) lack canonical start codons and most transcripts have abandoned stop codons altogether.

Evolving genetic code

In 1985, we reported that a bacterium, Mycoplasma capricolum, used a deviant genetic code, namely UGA, a "universal" stop codon, was read as tryptophan. This finding, together with the deviant nuclear genetic codes in not a few organisms and a number of mitochondria, shows that the genetic code is not universal, and is in a state of evolution. To account for the changes in codon meanings, we proposed the codon capture theory stating that all the code changes are non-disruptive without accompanied changes of amino acid sequences of proteins. Supporting evidence for the theory is presented in this review. A possible evolutionary process from the ancient to the present-day genetic code is also discussed.

Assessing the monophyly of chlorophyll-c containing plastids by multi-gene phylogenies under the unlinked model conditions

Recent multi-gene phylogenetic analyses of plastid-encoded genes have recovered a robust monophyly of chlorophyll-c containing plastids (Chl-c palstids) in cryptophytes, haptophytes, photosynthetic stramenopiles, and dinoflagellates. However, all the plastid multi-gene phylogenies published to date utilized the "linked" model, which ignores the heterogeneity of sequence evolution across genes in alignments. Both empirical and simulation studies show that, compared to the linked model, the "unlinked" model, which accounts for gene-specific evolution, can greatly improve multi-gene estimations. Here we newly sequenced 46 genes of Chl-c plastids, and examined the Chl-c plastid evolution by multi-gene analyses under the unlinked model. Unexpectedly, Chl-c plastid monophyly received only low to medium support in our analyses based on multi-gene data sets including up to 4829 alignment positions. Although we systematically surveyed and excluded the genes that could mislead estimation, the (inconclusive) support for Chl-c plastid monophyly was not significantly altered. We conclude that the estimates from the current plastid-encoded gene data are insufficient to resolve Chl-c plastid evolution with confidence, and are highly affected by genes subjected to the analyses, and methods for tree reconstruction applied. Thus, future data analyses of larger multi-gene data sets, preferentially under the unlinked model, are required to conclusively understand Chl-c plastid evolution.

The mechanisms of codon reassignments in mitochondrial genetic codes

Many cases of nonstandard genetic codes are known in mitochondrial genomes. We carry out analysis of phylogeny and codon usage of organisms for which the complete mitochondrial genome is available, and we determine the most likely mechanism for codon reassignment in each case. Reassignment events can be classified according to the gain-loss framework. The “gain” represents the appearance of a new tRNA for the reassigned codon or the change of an existing tRNA such that it gains the ability to pair with the codon. The “loss” represents the deletion of a tRNA or the change in a tRNA so that it no longer translates the codon. One possible mechanism is codon disappearance (CD), where the codon disappears from the genome prior to the gain and loss events. In the alternative mechanisms the codon does not disappear. In the unassigned codon mechanism, the loss occurs first, whereas in the ambiguous intermediate mechanism, the gain occurs first. Codon usage analysis gives clear evidence of cases where the codon disappeared at the point of the reassignment and also cases where it did not disappear. CD is the probable explanation for stop to sense reassignments and a small number of reassignments of sense codons. However, the majority of sense-to-sense reassignments cannot be explained by CD. In the latter cases, by analysis of the presence or absence of tRNAs in the genome and of the changes in tRNA sequences, it is sometimes possible to distinguish between the unassigned codon and the ambiguous intermediate mechanisms. We emphasize that not all reassignments follow the same scenario and that it is necessary to consider the details of each case carefully.

A comparative genomics analysis of codon reassignments reveals a link with mitochondrial proteome size and a mechanism of genetic code change via suppressor tRNAs

Using a comparative genomics approach we demonstrate a negative correlation between the number of codon reassignments undergone by 222 mitochondrial genomes and the mitochondrial genome size, the number of mitochondrial ORFs, and the sizes of the large and small subunit mitochondrial rRNAs. In addition, we show that the TGA-to-tryptophan codon reassignment, which has occurred 11 times in mitochondrial genomes, is found in mitochondrial genomes smaller than those which have not undergone the reassignment. We therefore propose that mitochondrial codon reassignments occur in a wide range of phyla, particularly in Metazoa, due to a reduced "proteomic constraint" on the mitochondrial genetic code, compared to the nuclear genetic code. The reduced proteomic constraint reflects the small size of the mitochondrial-encoded proteome and allows codon reassignments to occur with less likelihood of lethality. In addition, we demonstrate a striking link between nonsense codon reassignments and the decoding properties of naturally occurring nonsense suppressor tRNAs. This suggests that natural preexisting nonsense suppression facilitated nonsense codon reassignments and constitutes a novel mechanism of genetic code change. These findings explain for the first time the identity of the stop codons and amino acids reassigned in mitochondrial and nuclear genomes. Nonsense suppressor tRNAs provided the raw material for nonsense codon reassignments, implying that the properties of the tRNA anticodon have dictated the identity of nonsense codon reassignments.

Fragmentation of mitochondrial large subunit rRNA in the dinoflagellate Alexandrium catenella and the evolution of rRNA structure in alveolate mitochondria

Extensive investigations on apicomplexan mitochondria, such as those of Plasmodium falciparum, revealed that ribosomal RNAs (rRNAs) are fragmented into multiple short pieces. In this study, we isolated three mitochondrial large subunit rRNA (mtLSU rRNA) fragments from the dinoflagellate Alexandrium catenella. A piece of mtLSU rRNA that possesses high sequence similarity to the P. falciparum LSU rRNA E fragment was identified in a 1.7-kbp mitochondrial (mt) DNA clone. We further confirmed that the A. catenella "E-like" fragment is indeed transcriptionally active and that the transcript could form appropriate RNA secondary structures. In addition, we identified expression of two additional rRNA fragments with sequence similarities to P. falciparum F and G fragments. Notably, the 1.7-kbp mt DNA clone contains only one of the three rRNA fragments identified in this study, suggesting that the rRNA fragments are separately encoded in the A. catenella mt genome. Given the sister relationship between apicomplexa and dinoflagellates in eukaryote phylogeny, it is most parsimonious to assume that the mt rRNA fragmentation was established prior to the separation of the two protist groups. However, current sequence data on dinoflagellate mitochondria are insufficient to reject the alternative scenario, in which the rRNA fragmentation evolved independently in apicomplexan and dinoflagellate mitochondria.

Mitochondrial Genetic Codes Evolve to Match Amino Acid Requirements of Proteins

Mitochondria often use genetic codes different from the standard genetic code. Now that many mitochondrial genomes have been sequenced, these variant codes provide the first opportunity to examine empirically the processes that produce new genetic codes. The key question is: Are codon reassignments the sole result of mutation and genetic drift? Or are they the result of natural selection? Here we present an analysis of 24 phylogenetically independent codon reassignments in mitochondria. Although the mutation-drift hypothesis can explain reassignments from stop to an amino acid, we found that it cannot explain reassignments from one amino acid to another. In particular--and contrary to the predictions of the mutation-drift hypothesis--the codon involved in such a reassignment was not rare in the ancestral genome. Instead, such reassignments appear to take place while the codon is in use at an appreciable frequency. Moreover, the comparison of inferred amino acid usage in the ancestral genome with the neutral expectation shows that the amino acid gaining the codon was selectively favored over the amino acid losing the codon. These results are consistent with a simple model of weak selection on the amino acid composition of proteins in which codon reassignments are selected because they compensate for multiple slightly deleterious mutations throughout the mitochondrial genome. We propose that the selection pressure is for reduced protein synthesis cost: most reassignments give amino acids that are less expensive to synthesize. Taken together, our results strongly suggest that mitochondrial genetic codes evolve to match the amino acid requirements of proteins.

Barcoding animal life: cytochrome c oxidase subunit 1 divergences among closely related species

With millions of species and their life-stage transformations, the animal kingdom provides a challenging target for taxonomy. Recent work has suggested that a DNA-based identification system, founded on the mitochondrial gene, cytochrome c oxidase subunit 1 (COI), can aid the resolution of this diversity. While past work has validated the ability of COI sequences to diagnose species in certain taxonomic groups, the present study extends these analyses across the animal kingdom. The results indicate that sequence divergences at COI regularly enable the discrimination of closely allied species in all animal phyla except the Cnidaria. This success in species diagnosis reflects both the high rates of sequence change at COI in most animal groups and constraints on intraspecific mitochondrial DNA divergence arising, at least in part, through selective sweeps mediated via interactions with the nuclear genome.

Phylogenetic analyses of small subunit ribosomal RNA coding regions reveal a monophyletic lineage of euglyphid testate amoebae (Order Euglyphida)

The Testaceafilosia includes amoebae with filopodia and with a proteinaceous, agglutinated or siliceous test. To explore the deeper phylogeny of this group, we sequenced the small subunit ribosomal RNA coding region of 13 species, including the first sequence of an amoeba with an agglutinated test, Pseudodifflugia sp. Phylogenetic analyses using maximum parsimony and maximum likelihood methods as well as neighbor joining method yielded the following results: the order Euglyphida forms a monophyletic lineage with the sarcomonads as sister group. The next related taxa are the Chlorarachnea and the unidentified filose strain N-Por. In agreement with the previous studies the Phytomyxea branch off at the base of this lineage. The Monadofilosa (Testaceafilosia and Sarcomonadea) appear monophyletic. The Testaceafilosia are polyphyletic, because Pseudodifflugia sp. is positioned as the sister taxon to the sarcomonads. Within the order Euglyphida Paulinella branches off first, together with Cyphoderia followed by Tracheleuglypha. In maximum likelihood and neighbor joining analyses, the genus Euglypha is monophyletic. The branching pattern within the order Euglyphida reflects the evolution of shell morphology from simple to complex built test.

Rewiring the keyboard: evolvability of the genetic code

The genetic code evolved in two distinct phases. First, the 'canonical' code emerged before the last universal ancestor; subsequently, this code diverged in numerous nuclear and organelle lineages. Here, we examine the distribution and causes of these secondary deviations from the canonical genetic code. The majority of non-standard codes arise from alterations in the tRNA, with most occurring by post-transcriptional modifications, such as base modification or RNA editing, rather than by substitutions within tRNA anticodons.

Evolution of RNA editing in trypanosome mitochondria

Two different RNA editing systems have been described in the kinetoplast-mitochondrion of trypanosomatid protists. The first involves the precise insertion and deletion of U residues mostly within the coding regions of maxicircle-encoded mRNAs to produce open reading frames. This editing is mediated by short overlapping complementary guide RNAs encoded in both the maxicircle and the minicircle molecules and involves a series of enzymatic cleavage-ligation steps. The second editing system is a C(34) to U(34) modification in the anticodon of the imported tRNA(Trp), thereby permitting the decoding of the UGA stop codon as tryptophan. U-insertion editing probably originated in an ancestor of the kinetoplastid lineage and appears to have evolved in some cases by the replacement of the original pan-edited cryptogene with a partially edited cDNA. The driving force for the evolutionary fixation of these retroposition events was postulated to be the stochastic loss of entire minicircle sequence classes and their encoded guide RNAs upon segregation of the single kinetoplast DNA network into daughter cells at cell division. A large plasticity in the relative abundance of minicircle sequence classes has been observed during cell culture in the laboratory. Computer simulations provide theoretical evidence for this plasticity if a random distribution and segregation model of minicircles is assumed. The possible evolutionary relationship of the C to U and U-insertion editing systems is discussed.

Protein evolution drives the evolution of the genetic code and vice versa

A model for the developmental pathway of the genetic code, grounded on group theory and the thermodynamics of codon-anticodon interaction is presented. At variance with previous models, it takes into account not only the optimization with respect to amino acid attributes but, also physicochemical constraints and initial conditions. A 'simple-first' rule is introduced after ranking the amino acids with respect to two current measures of chemical complexity. It is shown that a primeval code of only seven amino acids is enough to build functional proteins. It is assumed that these proteins drive the further expansion of the code. The proposed primeval code is compared with surrogate codes randomly generated and with another proposal for primeval code found in the literature. The departures from the 'universal' code, observed in many organisms and cellular compartments, fit naturally in the proposed evolutionary scheme. A strong correlation is found between, on one side, the two classes of aminoacyl-tRNA synthetases, and on the other, the amino acids grouped by end-atom-type and by codon type. An inverse of Davydov's rules, to associate the amino acid end atoms (O/N and non-O/non-N) of 18 amino acids with codons containing a weak base (A/U), extended to the 20 amino acids, is derived.