Genetic code-guided protein synthesis and folding in Escherichia coli

Translation Rates and Protein Folding

The mRNA coding sequence defines not only the amino acid sequence of the protein, but also the speed at which the ribosomes move along the mRNA while making the protein. The non-uniform local kinetics - denoted as translational rhythm - is similar among mRNAs coding for related protein folds. Deviations from this conserved rhythm can result in protein misfolding. In this review we summarize the experimental evidence demonstrating how local translation rates affect cotranslational protein folding, with the focus on the synonymous codons and patches of charged residues in the nascent peptide as best-studied examples. Alterations in nascent protein conformations due to disturbed translational rhythm can persist off the ribosome, as demonstrated by the effects of synonymous codon variants of several disease-related proteins. Charged amino acid patches in nascent chains also modulate translation and cotranslational protein folding, and can abrogate translation when placed at the N-terminus of the nascent peptide. During cotranslational folding, incomplete nascent chains navigate through a unique conformational landscape in which earlier intermediate states become inaccessible as the nascent peptide grows. Precisely tuned local translation rates, as well as interactions with the ribosome, guide the folding pathway towards the native structure, whereas deviations from the natural translation rhythm may favor pathways leading to trapped misfolded states. Deciphering the 'folding code' of the mRNA will contribute to understanding the diseases caused by protein misfolding and to rational protein design.

How soluble misfolded proteins bypass chaperones at the molecular level

Subpopulations of soluble, misfolded proteins can bypass chaperones within cells. The extent of this phenomenon and how it happens at the molecular level are unknown. Through a meta-analysis of the experimental literature we find that in all quantitative protein refolding studies there is always a subpopulation of soluble but misfolded protein that does not fold in the presence of one or more chaperones, and can take days or longer to do so. Thus, some misfolded subpopulations commonly bypass chaperones. Using multi-scale simulation models we observe that the misfolded structures that bypass various chaperones can do so because their structures are highly native like, leading to a situation where chaperones do not distinguish between the folded and near-native-misfolded states. More broadly, these results provide a mechanism by which long-time scale changes in protein structure and function can persist in cells because some misfolded states can bypass components of the proteostasis machinery.

A BioBricks Metabolic Engineering Platform for the Biosynthesis of Anthracyclinones in Streptomyces coelicolor

Actinomycetes produce a variety of clinically indispensable molecules, such as antineoplastic anthracyclines. However, the actinomycetes are hindered in their further development as genetically engineered hosts for the synthesis of new anthracycline analogues due to their slow growth kinetics associated with their mycelial life cycle and the lack of a comprehensive genetic toolbox for combinatorial biosynthesis. In this report, we tackled both issues via the development of the BIOPOLYMER (BIOBricks POLYketide Metabolic EngineeRing) toolbox: a comprehensive synthetic biology toolbox consisting of engineered strains, promoters, vectors, and biosynthetic genes for the synthesis of anthracyclinones. An improved derivative of the production host Streptomyces coelicolor M1152 was created by deleting the matAB gene cluster that specifies extracellular poly-β-1,6-N-acetylglucosamine (PNAG). This resulted in a loss of mycelial aggregation, with improved biomass accumulation and anthracyclinone production. We then leveraged BIOPOLYMER to engineer four distinct anthracyclinone pathways, identifying optimal combinations of promoters, genes, and vectors to produce aklavinone, 9-epi-aklavinone, auramycinone, and nogalamycinone at titers between 15-20 mg/L. Optimization of nogalamycinone production strains resulted in titers of 103 mg/L. We structurally characterized six anthracyclinone products from fermentations, including new compounds 9,10-seco-7-deoxy-nogalamycinone and 4-O-β-d-glucosyl-nogalamycinone. Lastly, we tested the antiproliferative activity of the anthracyclinones in a mammalian cancer cell viability assay, in which nogalamycinone, auramycinone, and aklavinone exhibited moderate cytotoxicity against several cancer cell lines. We envision that BIOPOLYMER will serve as a foundational platform technology for the synthesis of designer anthracycline analogues.

Oxidative stress strongly restricts the effect of codon choice on the efficiency of protein synthesis in Escherichia coli

Introduction

The response of enterobacteria to oxidative stress is usually considered to be regulated by transcription factors such as OxyR and SoxR. Nevertheless, several reports have shown that under oxidative stress the levels, modification and aminoacylation of tRNAs may be altered suggesting a role of codon bias in regulation of gene expression under this condition.

Methods

In order to characterize the effects of oxidative stress on translation elongation we constructed a library of 61 plasmids, each coding for the green fluorescent protein (GFP) translationally fused to a different set of four identical codons.

Results

Using these reporters, we observed that GFP production levels vary widely (~15 fold) when Escherichia coli K-12 is cultured in minimal media as a consequence of codon choice variations. When bacteria are cultured under oxidative stress caused by paraquat the levels of GFP produced by most clones is reduced and, in contrast to control conditions, the range of GFP levels is restricted to a ~2 fold range. Restricting elongation of particular sequences does not increase the range of GFP production under oxidative stress, but altering translation initiation rates leads to an increase in this range.

Discussion

Altogether, our results suggest that under normal conditions the speed of translation elongation is in the range of the speed of initiation and, consequently, codon choice impacts the speed of protein synthesis. In contrast, under oxidative stress translation initiation becomes much slower than elongation, limiting the speed of translation such that codon choice has at most only subtle effects on the overall output of translation.

Application of Statistical Learning to Identify Omicron Mutations in SARS-CoV-2 Viral Genome Sequence Data From Populations in Africa and the United States

IMPORTANCE

With timely collection of SARS-CoV-2 viral genome sequences, it is important to apply efficient data analytics to detect emerging variants at the earliest time.

OBJECTIVE

To evaluate the application of a statistical learning strategy (SLS) to improve early detection of novel SARS-CoV-2 variants using viral sequence data from global surveillance.

DESIGN, SETTING, AND PARTICIPANTS

This case series applied an SLS to viral genomic sequence data collected from 63 686 individuals in Africa and 531 827 individuals in the United States with SARS-CoV-2. Data were collected from January 1, 2020, to December 28, 2021.

MAIN OUTCOMES AND MEASURES

The outcome was an indicator of Omicron variant derived from viral sequences. Centering on a temporally collected outcome, the SLS used the generalized additive model to estimate locally averaged Omicron caseload percentages (OCPs) over time to characterize Omicron expansion and to estimate when OCP exceeded 10%, 25%, 50%, and 75% of the caseload. Additionally, an unsupervised learning technique was applied to visualize Omicron expansions, and temporal and spatial distributions of Omicron cases were investigated.

RESULTS

In total, there were 2698 cases of Omicron in Africa and 12 141 in the United States. The SLS found that Omicron was detectable in South Africa as early as December 31, 2020. With 10% OCP as a threshold, it may have been possible to declare Omicron a variant of concern as early as November 4, 2021, in South Africa. In the United States, the application of SLS suggested that the first case was detectable on November 21, 2021.

CONCLUSIONS AND RELEVANCE

The application of SLS demonstrates how the Omicron variant may have emerged and expanded in Africa and the United States. Earlier detection could help the global effort in disease prevention and control. To optimize early detection, efficient data analytics, such as SLS, could assist in the rapid identification of new variants as soon as they emerge, with or without lineages designated, using viral sequence data from global surveillance.

Mutations in viral nucleocapsid protein and endoRNase are discovered to associate with COVID19 hospitalization risk

SARS-CoV-2 is spreading worldwide with continuously evolving variants, some of which occur in the Spike protein and appear to increase viral transmissibility. However, variants that cause severe COVID-19 or lead to other breakthroughs have not been well characterized. To discover such viral variants, we assembled a cohort of 683 COVID-19 patients; 388 inpatients ("cases") and 295 outpatients ("controls") from April to August 2020 using electronically captured COVID test request forms and sequenced their viral genomes. To improve the analytical power, we accessed 7137 viral sequences in Washington State to filter out viral single nucleotide variants (SNVs) that did not have significant expansions over the collection period. Applying this filter led to the identification of 53 SNVs that were statistically significant, of which 13 SNVs each had 3 or more variant copies in the discovery cohort. Correlating these selected SNVs with case/control status, eight SNVs were found to significantly associate with inpatient status (q-values < 0.01). Using temporal synchrony, we identified a four SNV-haplotype (t19839-g28881-g28882-g28883) that was significantly associated with case/control status (Fisher's exact p = 2.84 × 10-11). This haplotype appeared in April 2020, peaked in June, and persisted into January 2021. The association was replicated (OR = 5.46, p-value = 4.71 × 10-12) in an independent cohort of 964 COVID-19 patients (June 1, 2020 to March 31, 2021). The haplotype included a synonymous change N73N in endoRNase, and three non-synonymous changes coding residues R203K, R203S and G204R in the nucleocapsid protein. This discovery points to the potential functional role of the nucleocapsid protein in triggering "cytokine storms" and severe COVID-19 that led to hospitalization. The study further emphasizes a need for tracking and analyzing viral sequences in correlations with clinical status.

A Code Within a Code: How Codons Fine-Tune Protein Folding in the Cell

The genetic code sets the correspondence between the sequence of a given nucleotide triplet in an mRNA molecule, called a codon, and the amino acid that is added to the growing polypeptide chain during protein synthesis. With four bases (A, G, U, and C), there are 64 possible triplet codons: 61 sense codons (encoding amino acids) and 3 nonsense codons (so-called, stop codons that define termination of translation). In most organisms, there are 20 common/standard amino acids used in protein synthesis; thus, the genetic code is redundant with most amino acids (with the exception of Met and Trp) are being encoded by more than one (synonymous) codon. Synonymous codons were initially presumed to have entirely equivalent functions, however, the finding that synonymous codons are not present at equal frequencies in mRNA suggested that the specific codon choice might have functional implications beyond coding for amino acid. Observation of nonequivalent use of codons in mRNAs implied a possibility of the existence of auxiliary information in the genetic code. Indeed, it has been found that genetic code contains several layers of such additional information and that synonymous codons are strategically placed within mRNAs to ensure a particular translation kinetics facilitating and fine-tuning co-translational protein folding in the cell via step-wise/sequential structuring of distinct regions of the polypeptide chain emerging from the ribosome at different points in time. This review summarizes key findings in the field that have identified the role of synonymous codons and their usage in protein folding in the cell.

Combination of the mutations for improving activity of TEV protease in inclusion bodies

Tobacco etch virus protease (TEVp) is an enzymatic reagent to remove fusion tag, but additional purification steps are required for removing the TEVp after cleavage reaction is finished. Use of carrier-free and dependent TEVp immobilizates can eliminate protease contamination. In this work, we identified that, among the four constructed missense variants, the insoluble variant with the highest activity was correspondent with the soluble one tested formerly. The activities of the insoluble 15 codon variants were assayed and the variant with highest activity was selected. The K45F and/or E106G mutations have been reported on slightly improving protein stability of the wild-type TEVp, but only E106G mutation enhanced soluble production and activity of the selected TEVp variant, and it increased soluble amounts of two codon variants with the impaired folding. The decreased activity and use efficiency of the optimized TEVp variant in inclusion bodies was balanced by the determined high level production, lower leaking amounts of the protein, the enhanced resistance to the limited proteolysis mediated by protease K and trypsin, and the increased inhibition of auto-cleavage, as comparison to those of the immobilized soluble one. Thus, the TEVp construct is a potential alternate for simplifying protein purification protocols after tag-removal.

[Synonymous Codon Usage-a Guide for Co-Translational Protein Folding in the Cell]

In the cell, protein folding begins during protein synthesis/translation and thus is a co-translational process. Co-translational protein folding is tightly linked to translation elongation, which is not a uniform process. While there are many reasons for translation non-uniformity, it is generally believed that non-uniform synonymous codon usage is one of the key factors modulating translation elongation rates. Frequent/optimal codons as a rule are translated more rapidly than infrequently used ones and vice versa. Over 30 years ago, it was hypothesized that changes in synonymous codon usage affecting translation elongation rates could impinge on co-translation protein folding and that many synonymous codons are strategically placed within mRNA to ensure a particular translation kinetics facilitating productive step-by-step co-translational folding of proteins. It was suggested that this particular translation kinetics (and, specifically, translation pause sites) may define the window of opportunity for the protein parts to fold locally, particularly at the critical points where folding is far from equilibrium. It was thus hypothesized that synonymous codons may provide a secondary code for protein folding in the cell. Although, mostly accepted now, this hypothesis appeared to be difficult to prove and many convincing results were obtained only relatively recently. Here, I review the progress in the field and explain, why this simple idea appeared to be so challenging to prove.

Plasmodium apicoplast tyrosyl-tRNA synthetase recognizes an unusual, simplified identity set in cognate tRNATyr

The life cycle of Plasmodium falciparum, the agent responsible for malaria, depends on both cytosolic and apicoplast translation fidelity. Apicoplast aminoacyl-tRNA synthetases (aaRS) are bacterial-like enzymes devoted to organellar tRNA aminoacylation. They are all encoded by the nuclear genome and are translocated into the apicoplast only after cytosolic biosynthesis. Apicoplast aaRSs contain numerous idiosyncratic sequence insertions: An understanding of the roles of these insertions has remained elusive and they hinder efforts to heterologously overexpress these proteins. Moreover, the A/T rich content of the Plasmodium genome leads to A/U rich apicoplast tRNA substrates that display structural plasticity. Here, we focus on the P. falciparum apicoplast tyrosyl-tRNA synthetase (Pf-apiTyrRS) and its cognate tRNATyr substrate (Pf-apitRNATyr). Cloning and expression strategies used to obtain an active and functional recombinant Pf-apiTyrRS are reported. Functional analyses established that only three weak identity elements in the apitRNATyr promote specific recognition by the cognate Pf-apiTyrRS and that positive identity elements usually found in the tRNATyr acceptor stem are excluded from this set. This finding brings to light an unusual behavior for a tRNATyr aminoacylation system and suggests that Pf-apiTyrRS uses primarily negative recognition elements to direct tyrosylation specificity.

Disulfide bridge formation influences ligand recognition by the ATAD2 bromodomain

The ATPase family, AAA domain-containing protein 2 (ATAD2) has a C-terminal bromodomain, which functions as a chromatin reader domain recognizing acetylated lysine on the histone tails within the nucleosome. ATAD2 is overexpressed in many cancers and its expression is correlated with poor patient outcomes, making it an attractive therapeutic target and potential biomarker. We solved the crystal structure of the ATAD2 bromodomain and found that it contains a disulfide bridge near the base of the acetyllysine binding pocket (Cys1057-Cys1079). Site-directed mutagenesis revealed that removal of a free C-terminal cysteine (C1101) residue greatly improved the solubility of the ATAD2 bromodomain in vitro. Isothermal titration calorimetry experiments in combination with the Ellman's assay demonstrated that formation of an intramolecular disulfide bridge negatively impacts the ligand binding affinities and alters the thermodynamic parameters of the ATAD2 bromodomain interaction with a histone H4K5ac peptide as well as a small molecule bromodomain ligand. Molecular dynamics simulations indicate that the formation of the disulfide bridge in the ATAD2 bromodomain does not alter the structure of the folded state or flexibility of the acetyllysine binding pocket. However, consideration of this unique structural feature should be taken into account when examining ligand-binding affinity, or in the design of new bromodomain inhibitor compounds that interact with this acetyllysine reader module.

Codon usage revisited: Lack of correlation between codon usage and the number of tRNA genes in enterobacteria

It is widely believed that if a high number of genes are found for any tRNA in a rapidly replicating bacteria, then the cytoplasmic levels of that tRNA will be high and an open reading frame containing a higher frequency of the complementary codon will be translated faster. This idea is based on correlations between the number of tRNA genes, tRNA concentration and the frequency of codon usage observed in a limited number of strains as well as from the fact that artificially changing the number of tRNA genes alters translation efficiency and consequently the amount of properly folded protein synthesized. tRNA gene number may greatly vary in a genome due to duplications, deletions and lateral transfer which in turn would alter the levels and functionality of many proteins. Such changes are potentially deleterious for fitness and as a result it is expected that changes in tRNA gene numbers should be accompanied by a modification of the frequency of codon usage. In contrast to this model, when comparing the number of tRNA genes and the frequency of codon usage of several Salmonella enterica and Escherichia coli strains we found that changes in the number of tRNA genes are not correlated to changes in codon usage. Furthermore, these changes are not correlated with a change in the efficiency of codon translation. These results suggest that once a genome gains or loses tRNA genes, it responds by modulating the concentrations of tRNAs rather than modifying its frequency of codon usage.

Non-equilibrium coupling of protein structure and function to translation-elongation kinetics

Protein folding research has been dominated by the assumption that thermodynamics determines protein structure and function. And that when the folding process is compromised in vivo the proteostasis machinery-chaperones, deaggregases, the proteasome-work to restore proteins to their soluble, functional form or degrade them to maintain the cellular pool of proteins in a quasi-equilibrium state. During the past decade, however, more and more proteins have been identified for which altering only their speed of synthesis alters their structure and function, the efficiency of the down-stream processes they take part in, and cellular phenotype. Indeed, evidence has emerged that evolutionary selection pressures have encoded translation-rate information into mRNA molecules to coordinate diverse co-translational processes. Thus, non-equilibrium physics can play a fundamental role in influencing nascent protein behavior, mRNA sequence evolution, and disease. Here, we discuss how our understanding of this phenomenon is being advanced by the application of theoretical tools from the physical sciences.

Optimizing assembly and production of native bispecific antibodies by codon de-optimization

When production of bispecific antibodies requires the co-expression and assembly of three or four polypeptide chains, low expression of one chain can significantly limit assembly and yield. κλ bodies, fully human bispecific antibodies with native IgG structure, are composed of a common heavy chain and two different light chains, one kappa and one lambda. No engineering is applied to force pairing of the chains, thus both monospecific and bispecific antibodies are secreted in the supernatant. In this context, stoichiometric expression of the two light chains allows for maximal assembly of the bispecific antibody. In this study, we selected a κλ body with suboptimal characteristics due to low kappa chain expression. Codon optimization to increase expression of the kappa chain did not improve bispecific yield. Surprisingly, progressive introduction of non-optimal codons into the sequence of the lambda chain resulted in lowering its expression for an optimal tuning of the relative distribution of monospecific and bispecific antibodies. This codon de-optimization led to doubling of the κλ body yield. These results indicate that assembly of different proteins into a recombinant complex is an interconnected process and that reducing the expression of one polypeptide can actually increase the overall yield.

Predicting synonymous codon usage and optimizing the heterologous gene for expression in E. coli

Of the 20 common amino acids, 18 are encoded by multiple synonymous codons. These synonymous codons are not redundant; in fact, all of codons contribute substantially to protein expression, structure and function. In this study, the codon usage pattern of genes in the E. coli was learned from the sequenced genomes of E. coli. A machine learning based method, Presyncodon was proposed to predict synonymous codon selection in E. coli based on the learned codon usage patterns of the residue in the context of the specific fragment. The predicting results indicate that Presycoden could be used to predict synonymous codon selection of the gene in the E. coli with the high accuracy. Two reporter genes (egfp and mApple) were designed with a combination of low- and high-frequency-usage codons by the method. The fluorescence intensity of eGFP and mApple expressed by the (egfp and mApple) designed by this method was about 2.3- or 1.7- folds greater than that from the genes with only high-frequency-usage codons in E. coli. Therefore, both low- and high-frequency-usage codons make positive contributions to the functional expression of the heterologous proteins. This method could be used to design synthetic genes for heterologous gene expression in biotechnology.

Fast Protein Translation Can Promote Co- and Posttranslational Folding of Misfolding-Prone Proteins

Chemical kinetic modeling has previously been used to predict that fast-translating codons can enhance cotranslational protein folding by helping to avoid misfolded intermediates. Consistent with this prediction, protein aggregation in yeast and worms was observed to increase when translation was globally slowed down, possibly due to increased cotranslational misfolding. Observation of similar behavior in molecular simulations would confirm predictions from the simpler chemical kinetic model and provide a molecular perspective on cotranslational folding, misfolding, and the impact of translation speed on these processes. All-atom simulations cannot reach the timescales relevant to protein synthesis, and most conventional structure-based coarse-grained models do not allow for nonnative structure formation. Here, we introduce a protocol to incorporate misfolding using the functional forms of publicly available force fields. With this model we create two artificial proteins that are capable of undergoing structural transitions between a native and a misfolded conformation and simulate their synthesis by the ribosome. Consistent with the chemical kinetic predictions, we find that rapid synthesis of misfolding-prone nascent-chain segments increases the fraction of folded proteins by kinetically partitioning more molecules through on-pathway intermediates, decreasing the likelihood of sampling misfolded conformations. Novel to this study, to our knowledge, we observe that differences in protein dynamics, arising from different translation-elongation schedules, can persist long after the nascent protein has been released from the ribosome, and that a sufficient level of energetic frustration is needed for fast-translating codons to be beneficial for folding. These results provide further evidence that fast-translating codons can be as biologically important as pause sites in coordinating cotranslational folding.

Increasing Protein Production Rates Can Decrease the Rate at Which Functional Protein Is Produced and Their Steady-State Levels

The rate at which soluble, functional protein is produced by the ribosome has recently been found to vary in complex and unexplained ways as various translation-associated rates are altered through synonymous codon substitutions. To understand this phenomenon, here, we combine a well-established ribosome-traffic model with a master-equation model of cotranslational domain folding to explore the scenarios that are possible for the protein production rate, J, and the functional-nascent protein production rate, F, as the rates of various translation processes are altered for five different E. coli proteins. We find that while J monotonically increases as the rates of translation-initiation, -elongation, and -termination increase, F can either increase or decrease. We show that F's nonmonotonic behavior arises within the model from two opposing trends: the tendency for increased translation rates to produce more total protein but less cotranslationally folded protein. We further demonstrate that under certain conditions these nonmonotonic changes in F can result in nonmonotonic variations in post-translational, steady-state levels of functional protein. These results provide a potential explanation for recent experimental observations in which the specific activity of enzymatic proteins decreased with increased synthesis rates. Additionally our model has the potential to be used to rationally design transcripts to maximize the production of functional nascent protein by simultaneously optimizing translation initiation, elongation, and termination rates.

Effect of DNA sequence of Fab fragment on yield characteristics and cell growth of E. coli

Codon usage is one of the factors influencing recombinant protein expression. We were interested in the codon usage of an antibody Fab fragment gene exhibiting extreme toxicity in the E. coli host. The toxic synthetic human Fab gene contained domains optimized by the “one amino acid-one codon” method. We redesigned five segments of the Fab gene with a “codon harmonization” method described by Angov et al. and studied the effects of these changes on cell viability, Fab yield and display on filamentous phage using different vectors and bacterial strains. The harmonization considerably reduced toxicity, increased Fab expression from negligible levels to 10 mg/l, and restored the display on phage. Testing the impact of the individual redesigned segments revealed that the most significant effects were conferred by changes in the constant domain of the light chain. For some of the Fab gene variants, we also observed striking differences in protein yields when cloned from a chloramphenicol resistant vector into an identical vector, except with ampicillin resistance. In conclusion, our results show that the expression of a heterodimeric secretory protein can be improved by harmonizing selected DNA segments by synonymous codons and reveal additional complexity involved in heterologous protein expression.

High yields and soluble expression of superoxide dismutases in Escherichia coli due to the HIV-1 Tat peptide via increases in mRNA transcription

This study aimed to validate the high yield and soluble expression of proteins carrying the transactivator of transcription (Tat) peptide tag, and further explored the potential mechanism by which the Tat tag increases expression. Escherichia coli superoxide dismutase (SOD) proteins, including SodA, SodB and SodC, were selected for analysis. As expected, the yields and the solubility of Tat-tagged proteins were higher than those of Tat-free proteins, and similar results were observed for the total SOD enzyme activity. Bacterial cells that overexpressed Tat-tagged proteins exhibited increased anti-paraquat activity compared with those expressing Tat-free proteins that manifested as SodA>SodC>SodB. When compared with an MG1655 wild-type strain, the growth of a ΔSodA mutant strain was found to be inhibited after paraquat treatment; the growth of ΔSodB and ΔSodC mutant strains was also slightly inhibited. The mRNA transcript level of genes encoding Tat-tagged proteins was higher than that of genes encoding Tat-free proteins. Furthermore, the α-helix and turn of Tat-tagged proteins were higher than those of Tat-free proteins, but the β-sheet and random coil content was lower. These results indicated that the incorporation of the Tat core peptide as a significant basic membrane transduction peptide in fusion proteins could increase mRNA transcripts and promote the high yield and soluble expression of heterologous proteins in E. coli.

Synonymous Codons Direct Cotranslational Folding toward Different Protein Conformations

In all genomes, most amino acids are encoded by more than one codon. Synonymous codons can modulate protein production and folding, but the mechanism connecting codon usage to protein homeostasis is not known. Here we show that synonymous codon variants in the gene encoding gamma-B crystallin, a mammalian eye-lens protein, modulate the rates of translation and cotranslational folding of protein domains monitored in real time by Förster resonance energy transfer and fluorescence-intensity changes. Gamma-B crystallins produced from mRNAs with changed codon bias have the same amino acid sequence but attain different conformations, as indicated by altered in vivo stability and in vitro protease resistance. 2D NMR spectroscopic data suggest that structural differences are associated with different cysteine oxidation states of the purified proteins, providing a link between translation, folding, and the structures of isolated proteins. Thus, synonymous codons provide a secondary code for protein folding in the cell.

The Yin and Yang of codon usage

The genetic code is degenerate. With the exception of two amino acids (Met and Trp), all other amino acid residues are each encoded by multiple, so-called synonymous codons. Synonymous codons were initially presumed to have entirely equivalent functions, however, the finding that synonymous codons are not present at equal frequencies in genes/genomes suggested that codon choice might have functional implications beyond amino acid coding. The pattern of non-uniform codon use (known as codon usage bias) varies between organisms and represents a unique feature of an organism. Organism-specific codon choice is related to organism-specific differences in populations of cognate tRNAs. This implies that, in a given organism, frequently used codons will be translated more rapidly than infrequently used ones and vice versa A theory of codon-tRNA co-evolution (necessary to balance accurate and efficient protein production) was put forward to explain the existence of codon usage bias. This model suggests that selection favours preferred (frequent) over un-preferred (rare) codons in order to sustain efficient protein production in cells and that a given un-preferred codon will have the same effect on an organism's fitness regardless of its position within an mRNA's open reading frame. However, many recent studies refute this prediction. Un-preferred codons have been found to have important functional roles and their effects appeared to be position-dependent. Synonymous codon usage affects the efficiency/stringency of mRNA decoding, mRNA biogenesis/stability, and protein secretion and folding. This review summarizes recent developments in the field that have identified novel functions of synonymous codons and their usage.

The ribosome in action: Tuning of translational efficiency and protein folding

The cellular proteome is shaped by the combined activities of the gene expression and quality control machineries. While transcription plays an undoubtedly important role, in recent years also translation emerged as a key step that defines the composition and quality of the proteome and the functional activity of proteins in the cell. Among the different post-transcriptional control mechanisms, translation initiation and elongation provide multiple checkpoints that can affect translational efficiency. A multitude of specific signals in mRNAs can determine the frequency of translation initiation, choice of the open reading frame, global and local elongation velocities, and the folding of the emerging protein. In addition to specific signatures in the mRNAs, also variations in the global pools of translation components, including ribosomes, tRNAs, mRNAs, and translation factors can alter translational efficiencies. The cellular outcomes of phenomena such as mRNA codon bias are sometimes difficult to understand due to the staggering complexity of covariates that affect codon usage, translation, and protein folding. Here we summarize the experimental evidence on how the ribosome-together with the other components of the translational machinery-can alter translational efficiencies of mRNA at the initiation and elongation stages and how translation velocity affects protein folding. We seek to explain these findings in the context of mechanistic work on the ribosome. The results argue in favour of a new understanding of translation control as a hub that links mRNA homeostasis to production and quality control of proteins in the cell.

Insights into Cotranslational Nascent Protein Behavior from Computer Simulations

Regulation of protein stability and function in vivo begins during protein synthesis, when the ribosome translates a messenger RNA into a nascent polypeptide. Cotranslational processes involving a nascent protein include folding, binding to other macromolecules, enzymatic modification, and secretion through membranes. Experiments have shown that the rate at which the ribosome adds amino acids to the elongating nascent chain influences the efficiency of these processes, with alterations to these rates possibly contributing to diseases, including some types of cancer. In this review, we discuss recent insights into cotranslational processes gained from molecular simulations, how different computational approaches have been combined to understand cotranslational processes at multiple scales, and the new scenarios illuminated by these simulations. We conclude by suggesting interesting questions that computational approaches in this research area can address over the next few years.

Physical Origins of Codon Positions That Strongly Influence Cotranslational Folding: A Framework for Controlling Nascent-Protein Folding

An emerging paradigm in the field of in vivo protein biophysics is that nascent-protein behavior is a type of nonequilibrium phenomenon, where translation-elongation kinetics can be more important in determining nascent-protein behavior than the thermodynamic properties of the protein. Synonymous codon substitutions, which change the translation rate at select codon positions along a transcript, have been shown to alter cotranslational protein folding, suggesting that evolution may have shaped synonymous codon usage in the genomes of organisms in part to increase the amount of folded and functional nascent protein. Here, we develop a Monte Carlo-master-equation method that allows for the control of nascent-chain folding during translation through the rational design of mRNA sequences to guide the cotranslational folding process. We test this framework using coarse-grained molecular dynamics simulations and find it provides optimal mRNA sequences to control the simulated, cotranslational folding of a protein in a user-prescribed manner. With this approach we discover that some codon positions in a transcript can have a much greater impact on nascent-protein folding than others because they tend to be positions where the nascent chain populates states that are far from equilibrium, as well as being dependent on a complex ratio of time scales. As a consequence, different cotranslational profiles of the same protein can have different critical codon positions and different numbers of synonymous mRNA sequences that encode for them. These findings explain that there is a fundamental connection between the nonequilibrium nature of cotranslational processes, nascent-protein behavior, and synonymous codon usage.

High-level production of creatine amidinohydrolase from Arthrobacter nicotianae 23710 in Escherichia coli

In the present study, the gene encoding creatinase was amplified from Arthrobacter nicotianae 23710 (CICC) and functionally overexpressed in Escherichia coli. By applying a two-stage temperature control strategy, the production of creatinase was increased up to 61.3 U/mL in 3-L fermentor with a high productivity of 6.1 U/mL/h. The recombinant creatinase shows excellent resistance to the chelating agent EDTA, the surfactants (Tween 20, Tween 80, and Triton X-100) and the common preservative NaN3 (20 mM). High-level expression of the recombinant creatinase will contribute to its application in clinical diagnosis of renal function.

Synonymous rare arginine codons and tRNA abundance affect protein production and quality of TEV protease variant

It has been identified that a TEV protease (TEVp) variant, TEVp^5M, displays improved solubility. Here, we constructed fifteen TEVp^5M variants with one or more of six rare arginine codons in the coding sequence replaced with abundant E. coli arginine codons. These codon variants expressed in either E. coli BL21 (DE3) or Rossetta (DE3) showed different solubility and activity. Supply of rare tRNAs increased the tendency of certain codon variants to form insoluble aggregates at early induction stage, as determined by the fused S-tag. About 32% increase in soluble protein production of M5 variant with four synonymously mutated arginine codons was identified in Rossetta (DE3) cells using GFP fusion reporter, comparable to that of TEVp^5M. After purification, two other codon variants from both E. coli strains exhibited less activity than TEVp^5M on cleaving the native or modified recognition sequence incorporated between GST and E. coli diaminopropionate ammonialyase by enzyme-coupled assay, whereas purified M5 variant showed activity similar to the TEVp^5M. Supply of rare tRNAs caused the decrease of activity of TEVp^5M and M5 by about 21%. Our results revealed that engineering of highly soluble TEVp variants can be achieved by the combined mutations of amino acid residues and optimization of specific rare codons, whereas simple augment of rare tRNAs abundance resulted in partial loss of activity.

A critical analysis of codon optimization in human therapeutics

Codon optimization describes gene engineering approaches that use synonymous codon changes to increase protein production. Applications for codon optimization include recombinant protein drugs and nucleic acid therapies, including gene therapy, mRNA therapy, and DNA/RNA vaccines. However, recent reports indicate that codon optimization can affect protein conformation and function, increase immunogenicity, and reduce efficacy. We critically review this subject, identifying additional potential hazards including some unique to nucleic acid therapies. This analysis highlights the evolved complexity of codon usage and challenges the scientific bases for codon optimization. Consequently, codon optimization may not provide the optimal strategy for increasing protein production and may decrease the safety and efficacy of biotech therapeutics. We suggest that the use of this approach is reconsidered, particularly for in vivo applications.