Class I and class II lysyl-tRNA synthetase mutants and the genetic encoding of pyrrolysine in Methanosarcina spp

Construction of anti-codon table of the plant kingdom and evolution of tRNA selenocysteine (tRNASec)

Background

The tRNAs act as a bridge between the coding mRNA and incoming amino acids during protein translation. The anti-codon of tRNA recognizes the codon of the mRNA and deliver the amino acid into the protein translation chain. However, we did not know about the exact abundance of anti-codons in the genome and whether the frequency of abundance remains same across the plant lineage or not.

Results

Therefore, we analysed the tRNAnome of 128 plant species and reported an anti-codon table of the plant kingdom. We found that CAU anti-codon of tRNA^Met has highest (5.039%) whereas GCG anti-codon of tRNA^Arg has lowest (0.004%) abundance. However, when we compared the anti-codon frequencies according to the tRNA isotypes, we found tRNA^Leu (7.808%) has highest abundance followed by tRNA^Ser (7.668%) and tRNA^Gly (7.523%). Similarly, suppressor tRNA (0.036%) has lowest abundance followed by tRNA^Sec (0.066%) and tRNA^His (2.109). The genome of Ipomoea nil, Papaver somniferum, and Zea mays encoded the highest number of anti-codons (isoacceptor) at 59 each whereas the genome of Ostreococcus tauri was found to encode only 18 isoacceptors. The tRNA^Sec genes undergone losses more frequently than duplication and we found that tRNA^Sec showed anti-codon switch during the course of evolution.

Conclusion

The anti-codon table of the plant tRNA will enable us to understand the synonymous codon usage of the plant kingdom and can be very helpful to understand which codon is preferred over other during the translation.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-020-07216-3.

Natural pyrrolysine-biased translation of stop codons in mitochondrial peptides entirely coded by expanded codons

During the noncanonical deletion transcription, k nucleotides are systematically skipped/deleted after each transcribed trinucleotide producing deletion-RNAs (delRNAs). Peptides matching delRNAs either result from (a) canonical translation of delRNAs; or (b) noncanonical translation of regular transcripts along expanded codons. Only along frame "0" (start site) (a) and (b) produce identical peptides. Here, mitochondrial mass spectrometry data analyses assume expanded codon/del-transcription with 3 + k (k from 0 to 12) nucleotides. Detected peptides map preferentially on previously identified delRNAs. More peptides were detected for k (1-12) when del-transcriptional and expanded codon translations start sites coincide (i.e. the 0^th frame) than for frames +1 or +2. Hence, both (a) and (b) produced peptides identified here. Biases for frame 0 decrease for k > 2, reflecting codon/anticodon expansion limits. Further analyses find preferential pyrrolysine insertion at stop codons, suggesting Pyl-specific mitochondrial suppressor tRNAs loaded by Pyl-specific tRNA synthetases with unknown origins. Pyl biases at stops are stronger for regular than expanded codons suggesting that Pyl-tRNAs are less competitive with near-cognate tRNAs in expanded codon contexts. Statistical biases for these findings exclude that detected peptides are experimental and/or bioinformatic artefacts implying both del-transcription and expanded codons translation occur in human mitochondria.

Assessing methanotrophy and carbon fixation for biofuel production by Methanosarcina acetivorans

Background

Methanosarcina acetivorans is a model archaeon with renewed interest due to its unique reversible methane production pathways. However, the mechanism and relevant pathways implicated in (co)utilizing novel carbon substrates in this organism are still not fully understood. This paper provides a comprehensive inventory of thermodynamically feasible routes for anaerobic methane oxidation, co-reactant utilization, and maximum carbon yields of major biofuel candidates by M. acetivorans.

Results

Here, an updated genome-scale metabolic model of M. acetivorans is introduced (iMAC868 containing 868 genes, 845 reactions, and 718 metabolites) by integrating information from two previously reconstructed metabolic models (i.e., iVS941 and iMB745), modifying 17 reactions, adding 24 new reactions, and revising 64 gene-protein-reaction associations based on newly available information. The new model establishes improved predictions of growth yields on native substrates and is capable of correctly predicting the knockout outcomes for 27 out of 28 gene deletion mutants. By tracing a bifurcated electron flow mechanism, the iMAC868 model predicts thermodynamically feasible (co)utilization pathway of methane and bicarbonate using various terminal electron acceptors through the reversal of the aceticlastic pathway.

Conclusions

This effort paves the way in informing the search for thermodynamically feasible ways of (co)utilizing novel carbon substrates in the domain Archaea.

Electronic supplementary material

The online version of this article (doi:10.1186/s12934-015-0404-4) contains supplementary material, which is available to authorized users.

Structural diversity and protein engineering of the aminoacyl-tRNA synthetases

Aminoacyl-tRNA synthetases (aaRS) are the enzymes that ensure faithful transmission of genetic information in all living cells, and are central to the developing technologies for expanding the capacity of the translation apparatus to incorporate nonstandard amino acids into proteins in vivo. The 24 known aaRS families are divided into two classes that exhibit functional evolutionary convergence. Each class features an active site domain with a common fold that binds ATP, the amino acid, and the 3'-terminus of tRNA, embellished by idiosyncratic further domains that bind distal portions of the tRNA and enhance specificity. Fidelity in the expression of the genetic code requires that the aaRS be selective for both amino acids and tRNAs, a substantial challenge given the presence of structurally very similar noncognate substrates of both types. Here we comprehensively review central themes concerning the architectures of the protein structures and the remarkable dual-substrate selectivities, with a view toward discerning the most important issues that still substantially limit our capacity for rational protein engineering. A suggested general approach to rational design is presented, which should yield insight into the identities of the protein-RNA motifs at the heart of the genetic code, while also offering a basis for improving the catalytic properties of engineered tRNA synthetases emerging from genetic selections.

Genetic manipulation of Methanosarcina spp

The discovery of the third domain of life, the Archaea, is one of the most exciting findings of the last century. These remarkable prokaryotes are well known for their adaptations to extreme environments; however, Archaea have also conquered moderate environments. Many of the archaeal biochemical processes, such as methane production, are unique in nature and therefore of great scientific interest. Although formerly restricted to biochemical and physiological studies, sophisticated systems for genetic manipulation have been developed during the last two decades for methanogenic archaea, halophilic archaea and thermophilic, sulfur-metabolizing archaea. The availability of these tools has allowed for more complete studies of archaeal physiology and metabolism and most importantly provides the basis for the investigation of gene expression, regulation and function. In this review we provide an overview of methods for genetic manipulation of Methanosarcina spp., a group of methanogenic archaea that are key players in the global carbon cycle and which can be found in a variety of anaerobic environments.

Model organisms for genetics in the domain Archaea: methanogens, halophiles, Thermococcales and Sulfolobales

The tree of life is split into three main branches: eukaryotes, bacteria, and archaea. Our knowledge of eukaryotic and bacteria cell biology has been built on a foundation of studies in model organisms, using the complementary approaches of genetics and biochemistry. Archaea have led to some exciting discoveries in the field of biochemistry, but archaeal genetics has been slow to get off the ground, not least because these organisms inhabit some of the more inhospitable places on earth and are therefore believed to be difficult to culture. In fact, many species can be cultivated with relative ease and there has been tremendous progress in the development of genetic tools for both major archaeal phyla, the Euryarchaeota and the Crenarchaeota. There are several model organisms available for methanogens, halophiles, and thermophiles; in the latter group, there are genetic systems for Sulfolobales and Thermococcales. In this review, we present the advantages and disadvantages of working with each archaeal group, give an overview of their different genetic systems, and direct the neophyte archaeologist to the most appropriate model organism.

Metabolic reconstruction of the archaeon methanogen Methanosarcina Acetivorans

BACKGROUND

Methanogens are ancient organisms that are key players in the carbon cycle accounting for about one billion tones of biological methane produced annually. Methanosarcina acetivorans, with a genome size of ~5.7 mb, is the largest sequenced archaeon methanogen and unique amongst the methanogens in its biochemical characteristics. By following a systematic workflow we reconstruct a genome-scale metabolic model for M. acetivorans. This process relies on previously developed computational tools developed in our group to correct growth prediction inconsistencies with in vivo data sets and rectify topological inconsistencies in the model.

RESULTS

The generated model iVS941 accounts for 941 genes, 705 reactions and 708 metabolites. The model achieves 93.3% prediction agreement with in vivo growth data across different substrates and multiple gene deletions. The model also correctly recapitulates metabolic pathway usage patterns of M. acetivorans such as the indispensability of flux through methanogenesis for growth on acetate and methanol and the unique biochemical characteristics under growth on carbon monoxide.

CONCLUSIONS

Based on the size of the genome-scale metabolic reconstruction and extent of validated predictions this model represents the most comprehensive up-to-date effort to catalogue methanogenic metabolism. The reconstructed model is available in spreadsheet and SBML formats to enable dissemination.

Selenocysteine, pyrrolysine, and the unique energy metabolism of methanogenic archaea

Methanogenic archaea are a group of strictly anaerobic microorganisms characterized by their strict dependence on the process of methanogenesis for energy conservation. Among the archaea, they are also the only known group synthesizing proteins containing selenocysteine or pyrrolysine. All but one of the known archaeal pyrrolysine-containing and all but two of the confirmed archaeal selenocysteine-containing protein are involved in methanogenesis. Synthesis of these proteins proceeds through suppression of translational stop codons but otherwise the two systems are fundamentally different. This paper highlights these differences and summarizes the recent developments in selenocysteine- and pyrrolysine-related research on archaea and aims to put this knowledge into the context of their unique energy metabolism.

Top-down proteomics reveals novel protein forms expressed in Methanosarcina acetivorans

Using both automated nanospray and online liquid chromatography mass spectrometry LC-MS strategies, 99 proteins have been newly identified by top-down tandem mass spectrometry (MS/MS) in Methanosarcina acetivorans, the methanogen with the largest known genome [5.7 mega base pairs (Mb)] for an Archaeon. Because top-down MS/MS was used, 15 proteins were detected with mispredicted start sites along with an additional five from small open reading frames (SORFs). Beyond characterization of these more common discrepancies in genome annotation, one SORF resulted from a rare start codon (AUA) as the initiation site for translation of this protein. Also, a methylation on a 30S ribosomal protein (MA1259) was localized to Pro59-Val69, contrasting sharply from its homologue in Escherichia coli (rp S12) known to harbor an unusual beta-thiomethylated aspartic acid residue.

Specificity of pyrrolysyl-tRNA synthetase for pyrrolysine and pyrrolysine analogs

Pyrrolysine, the 22nd amino acid, is encoded by amber (TAG=UAG) codons in certain methanogenic archaea and bacteria. PylS, the pyrrolysyl-tRNA synthetase, ligates pyrrolysine to tRNA(Pyl) for amber decoding as pyrrolysine. PylS and tRNA(Pyl) have potential utility in making tailored recombinant proteins. Here, we probed interactions necessary for recognition of substrates by archaeal PylS via synthesis of close pyrrolysine analogs and testing their reactivity in amino acid activation assays. Replacement of the methylpyrroline ring of pyrrolysine with cyclopentane indicated that solely hydrophobic interactions with the ring-binding pocket of PylS are sufficient for substrate recognition. However, a 100-fold increase in the specificity constant of PylS was observed with an analog, 2-amino-6-((R)-tetrahydrofuran-2-carboxamido)hexanoic acid (2Thf-lys), in which tetrahydrofuran replaced the pyrrolysine methylpyrroline ring. Other analogs in which the electronegative atom was moved to different positions suggested PylS preference for a hydrogen-bond-accepting group at the imine nitrogen position in pyrrolysine. 2Thf-lys was a preferred substrate over a commonly employed pyrrolysine analog, but the specificity constant for 2Thf-lys was 10-fold lower than for pyrrolysine itself, largely due to the change in K(m). The in vivo activity of the analogs in supporting UAG suppression in Escherichia coli bearing genes for PylS and tRNA(Pyl) was similar to in vitro results, with L-pyrrolysine and 2Thf-lys supporting the highest amounts of UAG translation. Increasing concentrations of either PylS substrate resulted in a linear increase in UAG suppression, providing a facile method to assay bioactive pyrrolysine analogs. These results illustrate the relative importance of the H-bonding and hydrophobic interactions in the recognition of the methylpyrroline ring of pyrrolysine and provide a promising new series of easily synthesized pyrrolysine analogs that can serve as scaffolds for the introduction of novel functional groups into recombinant proteins.