Reassigning cysteine in the genetic code of Escherichia coli

Cys-tRNAj as a Second Translation Initiator for Priming Proteins with Cysteine in Bacteria

We report the construction of an alternative protein priming system to recode genetic translation in Escherichia coli by designing, through trial and error, a chimeric initiator whose sequence identity points partly to elongator tRNACys and partly to initiator tRNAf Met. The elaboration of a selection based on the N-terminal cysteine imperative for the function of glucosamine-6-phosphate synthase, an essential enzyme in bacterial cell wall synthesis, was a crucial step to achieve the engineering of this Cys-tRNAj. Iterative improvement of successive versions of Cys-tRNAj was corroborated in vitro by using a biochemical luciferase assay and in vivo by selecting for translation priming of E. coli thymidylate synthase. Condensation assays using specific fluorescent reagent FITC-Gly-cyanobenzothiazole provided biochemical evidence of cysteine coding at the protein priming stage. We showed that translation can be initiated, by N-terminal incorporation of cysteine, at a codon other than UGC by expressing a tRNAj with the corresponding anticodon. The optimized tRNAj is now available to recode the priming of an arbitrary subset of proteins in the bacterial proteome with absolute control of their expression and to evolve the use of xenonucleotides and the emergence of a tXNAj in vivo.

Positions of cysteine residues reveal local clusters and hidden relationships to Sequons and Transmembrane domains in Human proteins

Membrane proteins often possess critical structural features, such as transmembrane domains (TMs), N-glycosylation, and disulfide bonds (SS bonds), which are essential to their structure and function. Here, we extend the study of the motifs carrying N-glycosylation, i.e. the sequons, and the Cys residues supporting the SS bonds, to the whole human proteome with a particular focus on the Cys positions in human proteins with respect to those of sequons and TMs. As the least abundant amino acid residue in protein sequences, the positions of Cys residues in proteins are not random but rather selected through evolution. We discovered that the frequency of Cys residues in proteins is length dependent, and the frequency of CC gaps formed between adjacent Cys residues can be used as a classifier to distinguish proteins with special structures and functions, such as keratin-associated proteins (KAPs), extracellular proteins with EGF-like domains, and nuclear proteins with zinc finger C2H2 domains. Most importantly, by comparing the positions of Cys residues to those of sequons and TMs, we discovered that these structural features can form dense clusters in highly repeated and mutually exclusive modalities in protein sequences. The evolutionary advantages of such complementarity among the three structural features are discussed, particularly in light of structural dynamics in proteins that are lacking from computational predictions. The discoveries made here highlight the sequence-structure-function axis in biological organisms that can be utilized in future protein engineering toward synthetic biology.

In vivo, in vitro and in silico: an open space for the development of microbe-based applications of synthetic biology

Living systems are studied using three complementary approaches: living cells, cell-free systems and computer-mediated modelling. Progresses in understanding, allowing researchers to create novel chassis and industrial processes rest on a cycle that combines in vivo, in vitro and in silico studies. This design-build-test-learn iteration loop cycle between experiments and analyses combines together physiology, genetics, biochemistry and bioinformatics in a way that keeps going forward. Because computer-aided approaches are not directly constrained by the material nature of the entities of interest, we illustrate here how this virtuous cycle allows researchers to explore chemistry which is foreign to that present in extant life, from whole chassis to novel metabolic cycles. Particular emphasis is placed on the importance of evolution.

Progress toward a reduced phage genetic code

All known living organisms use at least 20 amino acids as the basic building blocks of life. Efforts to reduce the number of building blocks in a replicating system to below the 20 canonical amino acids have not been successful to date. In this work, we use filamentous phage as a model system to investigate the feasibility of removing methionine (Met) from the proteome. We show that all 24 elongation Met sites in the M13 phage genome can be replaced by other canonical amino acids. Most of these changes involve substitution of methionine by leucine (Leu), but in some cases additional compensatory mutations are required. Combining Met substituted sites in the proteome generally led to lower viability/infectivity of the mutant phages, which remains the major challenge in eliminating all methionines from the phage proteome. To date a total of 15 (out of all 24) elongation Mets have been simultaneously deleted from the M13 proteome, providing a useful foundation for future efforts to minimize the genetic code.

Synthetic biology approaches to biological containment: pre-emptively tackling potential risks

Biocontainment comprises any strategy applied to ensure that harmful organisms are confined to controlled laboratory conditions and not allowed to escape into the environment. Genetically engineered microorganisms (GEMs), regardless of the nature of the modification and how it was established, have potential human or ecological impact if accidentally leaked or voluntarily released into a natural setting. Although all evidence to date is that GEMs are unable to compete in the environment, the power of synthetic biology to rewrite life requires a pre-emptive strategy to tackle possible unknown risks. Physical containment barriers have proven effective but a number of strategies have been developed to further strengthen biocontainment. Research on complex genetic circuits, lethal genes, alternative nucleic acids, genome recoding and synthetic auxotrophies aim to design more effective routes towards biocontainment. Here, we describe recent advances in synthetic biology that contribute to the ongoing efforts to develop new and improved genetic, semantic, metabolic and mechanistic plans for the containment of GEMs.

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.

Evaluating Sense Codon Reassignment with a Simple Fluorescence Screen

Understanding the interactions that drive the fidelity of the genetic code and the limits to which modifications can be made without breaking the translational system has practical implications for understanding the molecular mechanisms of evolution as well as expanding the set of encodable amino acids, particularly those with chemistries not provided by Nature. Because 61 sense codons encode 20 amino acids, reassigning the meaning of sense codons provides an avenue for biosynthetic modification of proteins, furthering both fundamental and applied biochemical research. We developed a simple screen that exploits the absolute requirement for fluorescence of an active site tyrosine in green fluorescent protein (GFP) to probe the pliability of the degeneracy of the genetic code. Our screen monitors the restoration of the fluorophore of GFP by incorporation of a tyrosine in response to a sense codon typically assigned another meaning in the genetic code. We evaluated sense codon reassignment at four of the 21 sense codons read through wobble interactions in Escherichia coli using the Methanocaldococcus jannaschii orthogonal tRNA/aminoacyl tRNA synthetase pair originally developed and commonly used for amber stop codon suppression. By changing only the anticodon of the orthogonal tRNA, we achieved sense codon reassignment efficiencies between 1% (Phe UUU) and 6% (Lys AAG). Each of the orthogonal tRNAs preferentially decoded the codon traditionally read via a wobble interaction in E. coli with the exception of the orthogonal tRNA with an AUG anticodon, which incorporated tyrosine in response to both the His CAU and His CAC codons with approximately equal frequencies. We applied our screen in a high-throughput manner to evaluate a 10(9)-member combined tRNA/aminoacyl tRNA synthetase library to identify improved sense codon reassigning variants for the Lys AAG codon. A single rapid screen with the ability to broadly evaluate reassignable codons will facilitate identification and improvement of the combinations of sense codons and orthogonal pairs that display efficient reassignment.

Sense codon emancipation for proteome-wide incorporation of noncanonical amino acids: rare isoleucine codon AUA as a target for genetic code expansion

One of the major challenges in contemporary synthetic biology is to find a route to engineer synthetic organisms with altered chemical constitution. In terms of core reaction types, nature uses an astonishingly limited repertoire of chemistries when compared with the exceptionally rich and diverse methods of organic chemistry. In this context, the most promising route to change and expand the fundamental chemistry of life is the inclusion of amino acid building blocks beyond the canonical 20 (i.e. expanding the genetic code). This strategy would allow the transfer of numerous chemical functionalities and reactions from the synthetic laboratory into the cellular environment. Due to limitations in terms of both efficiency and practical applicability, state-of-the-art nonsense suppression- or frameshift suppression-based methods are less suitable for such engineering. Consequently, we set out to achieve this goal by sense codon emancipation, that is, liberation from its natural decoding function – a prerequisite for the reassignment of degenerate sense codons to a new 21st amino acid. We have achieved this by redesigning of several features of the post-transcriptional modification machinery which are directly involved in the decoding process. In particular, we report first steps towards the reassignment of 5797 AUA isoleucine codons in Escherichia coli using efficient tools for tRNA nucleotide modification pathway engineering.

A metabolic prototype for eliminating tryptophan from the genetic code

We set out to reduce the chemical constitution of a living organism to 19 amino acids. A strain was constructed for reassigning the tryptophan codon UGG to histidine and eliminating tryptophan from Escherichia coli. Histidine codons in the gene for an essential enzyme were replaced with tryptophan codons and the restoration of catalytic activity by missense suppressor His-tRNA bearing a CCA anticodon was selected. We used automated cultivation to assess the stability of this genetic construct during evolution. Histidine to tryptophan mutation at codon 30 in the transketolase gene from yeast and its cognate suppressor tRNA were stably propagated in a tktAB deletant of E. coli over 2500 generations. The ratio of histidine misincorporation at tryptophan sites in the proteome increased from 0.0007 to 0.03 over 300 days of continuous culture. This result demonstrated that the genetic code can be forced to evolve by permanent metabolic selection.

50nm-scale localization of single unmodified, isotopically enriched, proteins in cells

Imaging single proteins within cells is challenging if the possibility of artefacts due to tagging or to recognition by antibodies is to be avoided. It is generally believed that the biological properties of proteins remain unaltered when ¹⁴N isotopes are replaced with ¹⁵N. ¹⁵N-enriched proteins can be localised by dynamic Secondary Ion Mass Spectrometry (D-SIMS). We describe here a novel imaging analysis algorithm to detect a few ¹⁵N-enriched proteins - and even a single protein - within a cell using D-SIMS. The algorithm distinguishes statistically between a low local increase in ¹⁵N isotopic fraction due to an enriched protein and a stochastic increase due to the background. To determine the number of enriched proteins responsible for the increase in the isotopic fraction, we use sequential D-SIMS images in which we compare the measured isotopic fractions to those expected if 1, 2 or more enriched proteins are present. The number of enriched proteins is the one that gives the best fit between the measured and the expected values. We used our method to localise ¹⁵N-enriched thymine DNA glycosylase (TDG) and retinoid X receptor α (RXRα) proteins delivered to COS-7 cells. We show that both a single TDG and a single RXRα can be detected. After 4 h incubation, both proteins were found mainly in the nucleus; RXRα as a monomer or dimer and TDG only as a monomer. After 7 h, RXRα was found in the nucleus as a monomer, dimer or tetramer, whilst TDG was no longer in the nucleus and instead formed clusters in the cytoplasm. After 24 h, RXRα formed clusters in the cytoplasm, and TDG was no longer detectable. In conclusion, single unmodified proteins in cells can be counted and localised with 50 nm resolution by combining D-SIMS with our method of analysis.

The farther, the safer: a manifesto for securely navigating synthetic species away from the old living world

Biotechnology has empirically established that it is easier to construct and evaluate variant genes and proteins than to account for the emergence and function of wild-type macromolecules. Systematizing this constructive approach, synthetic biology now promises to infer and assemble entirely novel genomes, cells and ecosystems. It is argued here that the theoretical and computational tools needed for this endeavor are missing altogether. However, such tools may not be required for diversifying organisms at the basic level of their chemical constitution by adding, substituting or removing elements and molecular components through directed evolution under selection. Most importantly, chemical diversification of life forms could be designed to block metabolic cross-feed and genetic cross-talk between synthetic and wild species and hence protect natural habitats and human health through novel types of containment.

Natural history and experimental evolution of the genetic code

The standard genetic code is a set of rules that relates the 20 canonical amino acids in proteins to groups of three bases in the mRNA. It evolved from a more primitive form and the attempts to reconstruct its natural history are based on its present-day features. Genetic code engineering as a new research field was developed independently in a few laboratories during the last 15 years. The main intention is to re-program protein synthesis by expanding the coding capacities of the genetic code via re-assignment of specific codons to un-natural amino acids. This article focuses on the question as to which extent hypothetical scenarios that led to codon re-assignments during the evolution of the genetic code are relevant for its further evolution in the laboratory. Current attempts to engineer the genetic code are reviewed with reference to theoretical works on its natural history. Integration of the theoretical considerations into experimental concepts will bring us closer to designer cells with target-engineered genetic codes that should open not only tremendous possibilities for the biotechnology of the twenty-first century but will also provide a basis for the design of novel life forms.

The Coevolution of Genes and Genetic Codes: Crick’s Frozen Accident Revisited

The standard genetic code is the nearly universal system for the translation of genes into proteins. The code exhibits two salient structural characteristics: it possesses a distinct organization that makes it extremely robust to errors in replication and translation, and it is highly redundant. The origin of these properties has intrigued researchers since the code was first discovered. One suggestion, which is the subject of this review, is that the code's organization is the outcome of the coevolution of genes and genetic codes. In 1968, Francis Crick explored the possible implications of coevolution at different stages of code evolution. Although he argues that coevolution was likely to influence the evolution of the code, he concludes that it falls short of explaining the organization of the code we see today. The recent application of mathematical modeling to study the effects of errors on the course of coevolution, suggests a different conclusion. It shows that coevolution readily generates genetic codes that are highly redundant and similar in their error-correcting organization to the standard code. We review this recent work and suggest that further affirmation of the role of coevolution can be attained by investigating the extent to which the outcome of coevolution is robust to other influences that were present during the evolution of the code.

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.

Prolegomena to Future Experimental Efforts on Genetic Code Engineering by Expanding Its Amino Acid Repertoire

Protein synthesis and its relation to the genetic code was for a long time a central issue in biology. Rapid experimental progress throughout the past decade, crowned with the recently elucidated ribosomal structures, provided an almost complete description of this process. In addition important experiments provided solid evidence that the natural protein translation machinery can be reprogrammed to encode genetically a vast number of non-coded (i.e. noncanonical) amino acids. Indeed, in the set of 20 canonical amino acids as prescribed by the universal genetic code, many desirable functionalities, such as halogeno, keto, cyano, azido, nitroso, nitro, and silyl groups, as well as C=C or C[triple bond]C bonds, are absent. The ability to encode genetically such chemical diversity will enable us to reprogram living cells, such as bacteria, to express tailor-made proteins exhibiting functional diversity. Accordingly, genetic code engineering has developed into an exciting emerging research field at the interface of biology, chemistry, and physics.

Incorporation of Nonnatural Amino Acids Into Proteins

The genetic code is established by the aminoacylation of transfer RNA, reactions in which each amino acid is linked to its cognate tRNA that, in turn, harbors the nucleotide triplet (anticodon) specific to the amino acid. The accuracy of aminoacylation is essential for building and maintaining the universal tree of life. The ability to manipulate and expand the code holds promise for the development of new methods to create novel proteins and to understand the origins of life. Recent efforts to manipulate the genetic code have fulfilled much of this potential. These efforts have led to incorporation of nonnatural amino acids into proteins for a variety of applications and have demonstrated the plausibility of specific proposals for early evolution of the code.

Replication of hexitol oligonucleotides as a prelude to the propagation of a third type of nucleic acid in vivo

No backbone motif other than phospho-ribose and phospho-deoxyribose has been found in natural nucleic acids, currently restricting the molecular types of replicable biopolymers to DNA and RNA. With the aim of propagating and expressing a third type of nucleic acid in vivo, we assessed the replicability of polynucleotides with a phospho-hexitol backbone (HNA) in vivo and in vitro. Faithful polymerisation of up to four deoxynucleotides templated by hexitol oligonucleotides was established in vitro using DNA polymerase from Escherichia coli (PolA Klenow exo-fragment) and Thermus aquaticus (Taq polymerase). Condensation of up to three successive hTTPs (hexitol thymidine triphosphate) in responses to a pentameric hexitol template (hA)5 could also be demonstrated in vitro. Such a marginal HNA-dependent HNA polymerase activity of natural polymerases may be evolved in the future to catalyse in vitro amplification of HNA. The transmission of a two-codon-long genetic message carried on a hexameric hexitol template was also established using a selection screen for restoring thymidylate synthase activity in E. coli. These results exemplify the potential that can be explored by converting artificial substrates with natural enzymes in the field of informational polymer synthesis.

No accident: genetic codes freeze in error-correcting patterns of the standard genetic code

The standard genetic code poses a challenge in understanding the evolution of information processing at a fundamental level of biological organization. Genetic codes are generally coadapted with, or 'frozen' by, the protein-coding genes that they translate, and so cannot easily change by natural selection. Yet the standard code has a significantly non-random pattern that corrects common errors in the transmission of information in protein-coding genes. Because of the freezing effect and for other reasons, this pattern has been proposed not to be due to selection but rather to be incidental to other evolutionary forces or even entirely accidental. We present results from a deterministic population genetic model of code-message coevolution. We explicitly represent the freezing effect of genes on genetic codes and the perturbative effect of changes in genetic codes on genes. We incorporate characteristic patterns of mutation and translational error, namely, transition bias and positional asymmetry, respectively. Repeated selection over small successive changes produces genetic codes that are substantially, but not optimally, error correcting. In particular, our model reproduces the error-correcting patterns of the standard genetic code. Aspects of our model and results may be applicable to the general problem of adaptation to error in other natural information-processing systems.

Cysteine activation is an inherent in vitro property of prolyl-tRNA synthetases

Aminoacyl-tRNA synthetases are well known for their remarkable precision in substrate selection during aminoacyl-tRNA formation. Some synthetases enhance the accuracy of this process by editing mechanisms that lead to hydrolysis of incorrectly activated and/or charged amino acids. Prolyl-tRNA synthetases (ProRSs) can be divided into two structurally divergent groups, archaeal-type and bacterial-type enzymes. A striking difference between these groups is the presence of an insertion domain (approximately 180 amino acids) in the bacterial-type ProRS. Because the archaeal-type ProRS enzymes have been shown to recognize cysteine, we tested selected ProRSs from all three domains of life to determine whether cysteine activation is a general property of ProRS. Here we show that cysteine is activated by recombinant ProRS enzymes from the archaea Methanocaldococcus jannaschii and Methanothermobacter thermautotrophicus, from the eukaryote Saccharomyces cerevisiae, and from the bacteria Aquifex aeolicus, Borrelia burgdorferi, Clostridium sticklandii, Cytophaga hutchinsonii, Deinococcus radiodurans, Escherichia coli, Magnetospirillum magnetotacticum, Novosphingobium aromaticivorans, Rhodopseudomonas palustris, and Thermus thermophilus. This non-cognate amino acid was efficiently acylated in vitro onto tRNA(Pro), and the misacylated Cys-tRNA(Pro) was not edited by ProRS. Therefore, ProRS exhibits a natural level of mischarging that is to date unequalled among the aminoacyl-tRNA synthetases.

Rapid mapping of protein structure, interactions, and ligand binding by misincorporation proton-alkyl exchange

Understanding protein conformation, interactions, and ligand binding is essential to all biological inquiry. We report a novel biochemical technique, called misincorporation proton-alkyl exchange (MPAX), that can be used to footprint protein structure at single amino acid resolution. MPAX exploits translational misincorporation of cysteine residues to generate probes for physical analysis. We apply MPAX to the triosephosphate isomerase (beta/alpha)(8) barrel, accurately determining its substrate-binding site, a protein-protein interaction surface, the solvent-accessible protein surface, and the stability of the barrel. Because MPAX requires only microgram quantities of material and is not limited by protein size, it is ideally suited for proteins not amenable to conventional structural methods, such as membrane proteins, partially folded or insoluble proteins, and large protein complexes.

The AmiE aliphatic amidase and AmiF formamidase of Helicobacter pylori: natural evolution of two enzyme paralogues

Aliphatic amidases (EC 3.5.1.4) are enzymes catalysing the hydrolysis of short-chain amides to produce ammonia and the corresponding organic acid. Such an amidase, AmiE, has been detected previously in Helicobacter pylori. Analysis of the complete H. pylori genome sequence revealed the existence of a duplicated amidase gene that we named amiF. The corresponding AmiF protein is 34% identical to its AmiE paralogue. Because gene duplication is widely considered to be a fundamental process in the acquisition of novel enzymatic functions, we decided to study and compare the functions of the paralogous amidases of H. pylori. AmiE and AmiF proteins were overproduced in Escherichia coli and purified by a two-step chromatographic procedure. The two H. pylori amidases could be distinguished by different biochemical characteristics such as optimum pH or temperature. AmiE hydrolysed propionamide, acetamide and acrylamide and had no activity with formamide. AmiF presented an unexpected substrate specificity: it only hydrolysed formamide. AmiF is thus the first formamidase (EC 3.5.1.49) related to aliphatic amidases to be described. Cys-165 in AmiE and Cys-166 in AmiF were identified as residues essential for catalysis of the corresponding enzymes. H. pylori strains carrying single and double mutations of amiE and amiF were constructed. The substrate specificities of these enzymes were confirmed in H. pylori. Production of AmiE and AmiF proteins is dependent on the activity of other enzymes involved in the nitrogen metabolism of H. pylori (urease and arginase respectively). Our results strongly suggest that (i) the H. pylori paralogous amidases have evolved to achieve enzymatic specialization after ancestral gene duplication; and (ii) the production of these enzymes is regulated to maintain intracellular nitrogen balance in H. pylori.

Enlarging the amino acid set of Escherichia coli by infiltration of the valine coding pathway

Aminoacyl transfer RNA (tRNA) synthetases establish the rules of the genetic code by catalyzing the aminoacylation of tRNAs. For some synthetases, accuracy depends critically on an editing function at a site distinct from the aminoacylation site. Mutants of Escherichia coli that incorrectly charge tRNA(Val) with cysteine were selected after random mutagenesis of the whole chromosome. All mutations obtained were located in the editing site of valyl-tRNA synthetase. More than 20% of the valine in cellular proteins from such an editing mutant organism could be replaced with the noncanonical aminobutyrate, sterically similar to cysteine. Thus, the editing function may have played a central role in restricting the genetic code to 20 amino acids. Disabling this editing function offers a powerful approach for diversifying the chemical composition of proteins and for emulating evolutionary stages of ambiguous translation.