The Fitness Effects of Codon Composition of the Horizontally Transferred Antibiotic Resistance Genes Intensify at Sub-lethal Antibiotic Levels.

The rampant variability in codon bias existing between bacterial genomes is expected to interfere with horizontal gene transfer (HGT), a phenomenon that drives bacterial adaptation. However, delineating the constraints imposed by codon bias on functional integration of the transferred genes is complicated by multiple genomic and functional barriers controlling HGT, and by the dependence of the evolutionary outcomes of HGT on the host's environment. Here, we designed an experimental system in which codon composition of the transferred genes is the only variable triggering fitness change of the host. We replaced Escherichia coli's chromosomal folA gene encoding dihydrofolate reductase, an essential enzyme that constitutes a target for trimethoprim, with combinatorial libraries of synonymous codons of folA genes from trimethoprim-sensitive Listeria grayi and trimethoprim-resistant Neisseria sicca. The resulting populations underwent selection at a range of trimethoprim concentrations, and the ensuing changes in variant frequencies were used to infer the fitness effects of the individual combinations of codons. We found that when HGT causes overstabilization of the 5'-end mRNA, the fitness contribution of mRNA folding stability dominates over that of codon optimality. The 5'-end overstabilization can also lead to mRNA accumulation outside of the polysome, thus preventing the decay of the foreign transcripts despite the codon composition-driven reduction in translation efficiency. Importantly, the fitness effects of mRNA stability or codon optimality become apparent only at sub-lethal levels of trimethoprim individually tailored for each library, emphasizing the central role of the host's environment in shaping the codon bias compatibility of horizontally transferred genes.

Congenital analbuminemia associated with compound heterozygous novel nucleotide variations in a young adult with coronary thrombosis.

Congenital analbuminemia (CAA) is a very rare genetic disorder characterized by a significant reduced or even complete absence of human serum albumin. Our data describe the clinical features and laboratory results of a case confirmed by mutation analysis of the albumin gene in a 35-year-old man presenting recurrent acute coronary syndrome. To the best of our knowledge, only two cases of coronary artery disease have been reported worldwide without recurrence. Our findings contribute to shed light on the clinical characteristics and biochemical parameters of this disease and confirm that cardiovascular complications must be taken seriously in this pathology. Mutational screening disclosed two novel compound heterozygous nucleotide variations located in intron 12 and in 3'UTR. The prediction of the functional and structural impact of the reported variations using different bioinformatics tools demonstrates that these genetics variations affect RNA transcription and mRNA folding.

Modeling the effect of rRNA-mRNA interactions and mRNA folding on mRNA translation in chloroplasts.

The process of translation initiation in prokaryotes is mediated by the hybridization of the 16S rRNA of the small ribosomal subunit with the mRNA in a short region called the ribosomal binding site. However, translation initiation in chloroplasts, which have evolved from an ancestral bacterium, is not well understood. Some studies suggest that in many cases it differs from translation initiation in bacteria and involves various novel interactions of the mRNA structures with intracellular factors; however currently, there is no generic quantitative model related to these aspects in chloroplasts. We developed a novel computational pipeline and models that can be used for understanding and modeling translation regulation in chloroplasts. We demonstrate that local folding and co-folding energy of the rRNA and the mRNA correlates with codon usage estimators of expression levels (r = -0.63) and infer predictive models that connect these energies and codon usage to protein levels (with correlation up to 0.71). In addition, we demonstrate that the ends of the transcripts in chloroplasts are populated with various structural elements that may be functional. Furthermore, we report a database of 166 novel structures in the chloroplast transcripts that are predicted to be functional. We believe that the models reported here improve existing understandings of genomic evolution and the biophysics of translation in chloroplasts; as such, they can aid gene expression engineering in chloroplasts for various biotechnological objectives.

Implication of rare genetic variants of NODAL and ACVR1B in congenital heart disease patients from Indian population.

NODAL signaling plays an essential role in vertebrate embryonic patterning and heart development. Accumulating evidences suggest that genetic mutations in TGF-ß/NODAL signaling pathway can cause congenital heart disease in humans. To investigate the implication of NODAL signaling in isolated cardiovascular malformation, we have screened 300 non-syndromic CHD cases and 200 controls for NODAL and ACVR1B by Sanger sequencing and identified two rare missense (c.152C > T; p.P51L and c.981 T > A; p.D327E) variants in NODAL and a novel missense variant c.1035G > A; p.M345I in ACVR1B. All these variants are absent in 200 controls. Three-dimensional protein-modelling demonstrates that both p.P51L and p.D327E variations of NODAL and p.M345I mutation of ACVR1B, affect the tertiary structure of respective proteins. Variants of NODAL (p.P51L and p.D327E) and ACVR1B (p.M345I), significantly reduce the transactivation of AR3-Luc, (CAGA)12-Luc and (SBE)4-Luc promoters. Moreover, qRT-PCR results have also deciphered a reduction in the expression of cardiac-enriched transcription factors namely Gata4, Nkx2-5, and Tbx5 in both the mutants of NODAL. Decreased expression of, Gata4, Nkx2-5, Tbx5, and lefty is observed in p.M345I mutant of ACVR1B as well. Additionally, reduced phosphorylation of SMAD2/3 in response to these variants, suggests impaired NODAL signaling and possibly responsible for defective cell fate decision and differentiation of cardiomyocytes leading to CHD phenotype.

A gain-of-function mutation in CITED2 is associated with congenital heart disease.

CITED2 is a transcription co-activator that interacts with TFAP2 and CBP/ P300 transcription factors to regulate the proliferation and differentiation of the cardiac progenitor cells. It acts upstream to NODAL-PITX2 pathways and regulates the left-right asymmetry. Both human genetic and model organism studies have shown that altered expression of CITED2 causes various forms of congenital heart disease. Therefore, we sought to screen the coding region of CITED2 to identify rare genetic variants and assess their impact on the structure and function of the protein. Here, we have screened 271 non-syndromic, sporadic CHD cases by Sanger's sequencing method and detected a non-synonymous variant (c.301C>T, p.P101S) and two synonymous variants (c.21C>A, p.A7A; c.627C>G, p.P209P). The non-synonymous variant c.301C>T (rs201639244) is a rare variant with a minor allele frequency of 0.00011 in the gnomAD browser and 0.0018 in the present study. in vitro analysis has demonstrated that p.P101S mutation upregulates the expression of downstream target genes Gata4, Mef2c, Nfatc1&2, Nodal, Pitx2, and Tbx5 in P19 cells. Luciferase reporter assay also demonstrates enhanced activation of downstream target promoters. Further, in silico analyses implicate that increased activity of mutant CITED2 is possibly due to phosphorylation of Serine residue by proline-directed kinases. Homology modeling and alignment analysis have also depicted differences in hydrogen bonding and tertiary structures of wild-type versus mutant protein. The impact of synonymous variations on the mRNA structure of CITED2has been analyzed by Mfold and relative codon bias calculations. Mfold results have revealed that both the synonymous variants can alter the mRNA structure and stability. Relative codon usage analysis has suggested that the rate of translation is attenuated due to these variations. Altogether, our results from genetic screening as well as in vitro and in silico studies support a possible role of nonsynonymous and synonymous mutations in CITED2contributing to pathogenesis of CHD.

Improving the Expression of Human Granulocyte Colony Stimulating Factor in Escherichia coli by Reducing the GC-content and Increasing mRNA Folding Free Energy at 5'-Terminal End.

Purpose: Strategy for improving the production of biopharmaceutical protein continues to develop due to increasing market demand. Human granulocyte colony stimulating factor (hG-CSF) is one of biopharmaceutical proteins that has many applications, and easily produced in Escherichia coli expression system. Previous studies reported that codon usage, rare codon, mRNA folding and GC-content at 5'-terminal end were crucial for protein production in E. coli. In the present study, the effect of reducing the GC-content and increasing the mRNA folding free energy at the 5'-terminal end on the expression level of hG-CSF proteins was investigated. Methods: Synonymous codon substitutions were performed to generate mutant variants of open reading frame (ORF) with lower GC-content at 5'-terminal ends. Oligoanalyzer tool was used to calculate the GC content of eight codons sequence after ATG. Whereas, mRNA folding free energy was predicted using KineFold and RNAfold tools. The template DNA was amplified using three variant forward primers and one same reverse primer. Those DNA fragments were individually cloned into pJexpress414 expression vector and were confirmed using restriction and DNA sequencing analyses. The confirmed constructs were transformed into E. coli NiCo21(DE3) host cells and the recombinant protein was expressed using IPTG-induction. Total protein obtained were characterized using SDS-PAGE, Western blot and ImageJ software analyses. Results: The result showed that the mutant variant with lower GC-content and higher mRNA folding free energy near the translation initiation region (TIR) could produce a higher amount of hG-CSF proteins compared to the original gene sequence. Conclusion: This study emphasized the important role of the nucleotide composition immediately downstream the start codon to achieve high-yield protein product on heterologous expression in E. coli.

Translational regulation of environmental adaptation in bacteria.

Bacteria must rapidly respond to both intracellular and environmental changes to survive. One critical mechanism to rapidly detect and adapt to changes in environmental conditions is control of gene expression at the level of protein synthesis. At each of the three major steps of translation-initiation, elongation, and termination-cells use stimuli to tune translation rate and cellular protein concentrations. For example, changes in nutrient concentrations in the cell can lead to translational responses involving mechanisms such as dynamic folding of riboswitches during translation initiation or the synthesis of alarmones, which drastically alter cell physiology. Moreover, the cell can fine-tune the levels of specific protein products using programmed ribosome pausing or inducing frameshifting. Recent studies have improved understanding and revealed greater complexity regarding long-standing paradigms describing key regulatory steps of translation such as start-site selection and the coupling of transcription and translation. In this review, we describe how bacteria regulate their gene expression at the three translational steps and discuss how translation is used to detect and respond to changes in the cellular environment. Finally, we appraise the costs and benefits of regulation at the translational level in bacteria.

A genetic selection reveals functional metastable structures embedded in a toxin-encoding mRNA.

Post-transcriptional regulation plays important roles to fine-tune gene expression in bacteria. In particular, regulation of type I toxin-antitoxin (TA) systems is achieved through sophisticated mechanisms involving toxin mRNA folding. Here, we set up a genetic approach to decipher the molecular underpinnings behind the regulation of a type I TA in Helicobacter pylori. We used the lethality induced by chromosomal inactivation of the antitoxin to select mutations that suppress toxicity. We found that single point mutations are sufficient to allow cell survival. Mutations located either in the 5' untranslated region or within the open reading frame of the toxin hamper its translation by stabilizing stem-loop structures that sequester the Shine-Dalgarno sequence. We propose that these short hairpins correspond to metastable structures that are transiently formed during transcription to avoid premature toxin expression. This work uncovers the co-transcriptional inhibition of translation as an additional layer of TA regulation in bacteria.

Large-Effect Beneficial Synonymous Mutations Mediate Rapid and Parallel Adaptation in a Bacterium.

Contrary to previous understanding, recent evidence indicates that synonymous codon changes may sometimes face strong selection. However, it remains difficult to generalize the nature, strength, and mechanism(s) of such selection. Previously, we showed that synonymous variants of a key enzyme-coding gene (fae) of Methylobacterium extorquens AM1 decreased enzyme production and reduced fitness dramatically. We now show that during laboratory evolution, these variants rapidly regained fitness via parallel yet variant-specific, highly beneficial point mutations in the N-terminal region of fae These mutations (including four synonymous mutations) had weak but consistently positive impacts on transcript levels, enzyme production, or enzyme activity. However, none of the proposed mechanisms (including internal ribosome pause sites or mRNA structure) predicted the fitness impact of evolved or additional, engineered point mutations. This study shows that synonymous mutations can be fixed through strong positive selection, but the mechanism for their benefit varies depending on the local sequence context.

Codon bias and the folding dynamics of the cystic fibrosis transmembrane conductance regulator.

Synonymous or silent mutations are often overlooked in genetic analyses for disease-causing mutations unless they are directly associated with potential splicing defects. More recent studies, however, indicate that some synonymous single polynucleotide polymorphisms (sSNPs) are associated with changes in protein expression, and in some cases, protein folding and function. The impact of codon usage and mRNA structural changes on protein translation rates and how they can affect protein structure and function is just beginning to be appreciated. Examples are given here that demonstrate how synonymous mutations alter the translational kinetics and protein folding and/or function. The mechanism for how this occurs is based on a model in which codon usage modulates the translational rate by introducing pauses caused by nonoptimal or rare codons or by introducing changes in the mRNA structure, and this in turn influences co-translational folding. Two examples of this include the multidrug resistance protein (p-glycoprotein) and the cystic fibrosis transmembrane conductance regulator gene (CFTR). CFTR is also used here as a model to illustrate how synonymous mutations can be examined using in silico predictive methods to identify which sSNPs have the potential to change protein structure. The methodology described here can be used to help identify "non-silent" synonymous mutations in other genes.

Nucleotide sequence composition adjacent to intronic splice sites improves splicing efficiency via its effect on pre-mRNA local folding in fungi.

RNA splicing is the central process of intron removal in eukaryotes known to regulate various cellular functions such as growth, development, and response to external signals. The canonical sequences indicating the splicing sites needed for intronic boundary recognition are well known. However, the roles and evolution of the local folding of intronic and exonic sequence features adjacent to splice sites has yet to be thoroughly studied. Here, focusing on four fungi (Saccharomyces cerevisiae, Schizosaccharomyces pombe, Aspergillus nidulans, and Candida albicans), we performed for the first time a comprehensive high-resolution study aimed at characterizing the encoding of intronic splicing efficiency in pre-mRNA transcripts and its effect on intron evolution. Our analysis supports the conjecture that pre-mRNA local folding strength at intronic boundaries is under selective pressure, as it significantly affects splicing efficiency. Specifically, we show that in the immediate region of 12-30 nucleotides (nt) surrounding the intronic donor site there is a preference for weak pre-mRNA folding; similarly, in the region of 15-33 nt surrounding the acceptor and branch sites there is a preference for weak pre-mRNA folding. We also show that in most cases there is a preference for strong pre-mRNA folding further away from intronic splice sites. In addition, we demonstrate that these signals are not associated with gene-specific functions, and they correlate with splicing efficiency measurements (r = 0.77, P = 2.98 × 10(-21)) and with expression levels of the corresponding genes (P = 1.24 × 10(-19)). We suggest that pre-mRNA folding strength in the above-mentioned regions has a direct effect on splicing efficiency by improving the recognition of intronic boundaries. These new discoveries are contributory steps toward a broader understanding of splicing regulation and intronic/transcript evolution.

Experimental Characterization of Hb Flurlingen (HBA2: c.177 C > G, p.His > Gln) and Hb Boghé (HBA2: c.177 C > A, p.His > Gln) Reveals Contradictory HBA2 Expression and Translation Patterns Despite Identical Amino Acid Substitutions.

In this study, we describe the clinical features and provide experimental analyses of Hb Flurlingen (HBA2: c.177 C > G, p.His > Gln) that contrasted with Hb Boghé (HBA2: c.177 C > A, p.His > Gln). Despite the identical amino acid substitution in both variants, Hb Flurlingen shows the phenotype of α-thalassemia (α-thal), whereas Hb Boghé has no impact on α2-globin (HBA2) production. For in vitro transcription analysis, HBA2 expression constructs carrying the HBA2-WT (wild type), Hb Flurlingen and Hb Boghé sequences were generated and expressed in human bladder carcinoma 5637 cells for downstream analyses by quantitative real time-polymerase chain reaction (qReTi-PCR) and immunofluorochemistry (IFC). In silico analysis of secondary folding structures of the HBA2-WT, Hb Flurlingen and Hb Boghé mRNA sequences was performed using Mfold software. The gene transcription and translation analyses revealed that cells transfected with the Hb Flurlingen construct had significantly lower HBA2 transcription (-55.4%, p ≤ 0.01) and reduced protein synthesis when compared to the wild type group. In contrast, cells transfected with the Hb Boghé construct showed no significant changes in HBA2 transcription or translation activities when compared to the wild type group. The in silico prediction of possible effects of these mutations on the folding structures of the HBA2 transcripts showed a change of secondary folding pattern in the Hb Flurlingen transcript when compared to those of HBA2-WT and Hb Boghé. Our experimental findings support the clinical presentation of an α-thalassemic phenotype for Hb Flurlingen in contrast with Hb Boghé, despite identical amino acid substitutions. The results confirm the importance of experimental analysis in establishing the impact of novel base substitutions.

Rationally designed, heterologous S. cerevisiae transcripts expose novel expression determinants.

Deducing generic causal relations between RNA transcript features and protein expression profiles from endogenous gene expression data remains a major unsolved problem in biology. The analysis of gene expression from heterologous genes contributes significantly to solving this problem, but has been heavily biased toward the study of the effect of 5' transcript regions and to prokaryotes. Here, we employ a synthetic biology driven approach that systematically differentiates the effect of different regions of the transcript on gene expression up to 240 nucleotides into the ORF. This enabled us to discover new causal effects between features in previously unexplored regions of transcripts, and gene expression in natural regimes. We rationally designed, constructed, and analyzed 383 gene variants of the viral HRSVgp04 gene ORF, with multiple synonymous mutations at key positions along the transcript in the eukaryote S. cerevisiae. Our results show that a few silent mutations at the 5'UTR can have a dramatic effect of up to 15 fold change on protein levels, and that even synonymous mutations in positions more than 120 nucleotides downstream from the ORF 5'end can modulate protein levels up to 160%-300%. We demonstrate that the correlation between protein levels and folding energy increases with the significance of the level of selection of the latter in endogenous genes, reinforcing the notion that selection for folding strength in different parts of the ORF is related to translation regulation. Our measured protein abundance correlates notably(correlation up to r = 0.62 (p=0.0013)) with mean relative codon decoding times, based on ribosomal densities (Ribo-Seq) in endogenous genes, supporting the conjecture that translation elongation and adaptation to the tRNA pool can modify protein levels in a causal/direct manner. This report provides an improved understanding of transcript evolution, design principles of gene expression regulation, and suggests simple rules for engineering synthetic gene expression in eukaryotes.

Secondary structure-stretched forms of long intron RNA products from the view point of initiation of chromosome homologs somatic pairing.

In different types of chromosome pairing (meiotic, somatic, and sister chromatids pairing) initiation stages are less elucidated. In somatic homolog pairing initiation, the long intron RNA products interference may play the essential role. The strongest somatic pairing in Drosophila melanogaster 28B1-B2 locus and its enrichment by long bi-directional transcripts prone us to analyze the pre-mRNA secondary structures. The comparison of sense (pre-mRNA) and antisense (lncRNA) portions corresponding to the lengthy introns for some others' genes loci reveals the significant correlation of stretched folding form lengths with the homologue pairing percentage. Also for 28B1-B2 locus, the most significant homologue pairing is justified by the plurality of sense and antisense RNA variants for lengthy introns. The stretched forms of long intron products with multiple stem-loop clusters widely presented for sense and antisense strands may interact by multiple loops during transcription being connected to different chromosomes and hypothetically may serve for the pairing initiation. Stretched rod-like or V-shape-like are dominating forms for the whole intron RNA products or their central portions while predominantly star-like for others intron fragments.

5'-coding sequence of the nasA gene of Azotobacter vinelandii is required for efficient expression.

The operon nasACBH in Azotobacter vinelandii encodes nitrate and nitrite reductases that sequentially reduce nitrate to nitrite and to ammonium for nitrogen assimilation into organic molecules. Our previous analyses showed that nasACBH expression is subject to antitermination regulation that occurs upstream of the nasA gene in response to the availability of nitrate and nitrite. In this study, we continued expression analyses of the nasA gene and observed that the nasA 5'-coding sequence plays an important role in gene expression, as demonstrated by the fact that deletions caused over sixfold reduction in the expression of the lacZ reporter gene. Further analysis suggests that the nasA 5'-coding sequence promotes gene expression in a way that is not associated with weakened transcript folding around the translational initiation region or codon usage bias. The findings from this study imply that there exists potential to improve gene expression in A. vinelandii by optimizing 5'-coding sequences.

Deciphering the rules by which 5'-UTR sequences affect protein expression in yeast.

The 5'-untranslated region (5'-UTR) of mRNAs contains elements that affect expression, yet the rules by which these regions exert their effect are poorly understood. Here, we studied the impact of 5'-UTR sequences on protein levels in yeast, by constructing a large-scale library of mutants that differ only in the 10 bp preceding the translational start site of a fluorescent reporter. Using a high-throughput sequencing strategy, we obtained highly accurate measurements of protein abundance for over 2,000 unique sequence variants. The resulting pool spanned an approximately sevenfold range of protein levels, demonstrating the powerful consequences of sequence manipulations of even 1-10 nucleotides immediately upstream of the start codon. We devised computational models that predicted over 70% of the measured expression variability in held-out sequence variants. Notably, a combined model of the most prominent features successfully explained protein abundance in an additional, independently constructed library, whose nucleotide composition differed greatly from the library used to parameterize the model. Our analysis reveals the dominant contribution of the start codon context at positions -3 to -1, mRNA secondary structure, and out-of-frame upstream AUGs (uAUGs) to phenotypic diversity, thereby advancing our understanding of how protein levels are modulated by 5'-UTR sequences, and paving the way toward predictably tuning protein expression through manipulations of 5'-UTRs.

Directed evolution study unveiling key sequence factors that affect translation efficiency in Escherichia coli.

Synonymous mutations in protein coding genes significantly impact translation efficiency. We synthesized a pair of genes encoding green fluorescent protein that were separated by 160 synonymous mutations to investigate key factors that affect translation efficiency. One sequence was optimized for Escherichia coli (GFP(Eco)) and the other for Bacillus subtilis (GFP(Bsu)). When the genes were expressed in E. coli, GFP(Eco) fluoresced 12-fold stronger than GFP(Bsu), confirming the suboptimal nature of the GFP(Bsu) gene. We then employed directed evolution to improve the expression of GFP(Bsu). Random mutagenesis and DNA shuffling was used to generate mutant libraries, which were screened for fluorescence. A variant showing 6-fold fluorescence enhancement was identified, which contained a single mutation (G10A) in a rare codon for Gly-4. However, the substitution generated another type of rare codon, AGA, for Arg, suggesting that the improvement was caused by a factor other than the rare codon. We next applied saturation mutagenesis to Gly-4. The darkest variant contained a GGG codon (GFP(Bsu)-G) for Gly-4. Taking the location of the mutation into account, we hypothesized that destabilization of the mRNA secondary structure around the initiation codon improved the expression. We then randomized the nucleotide triplet in 5'-untranslated region (5'UTR) of GFP(Bsu), which is complementary to the Gly-4 codon. A variant showing 6-fold fluorescence enhancement was identified, which exhibited a destabilized secondary structure. When this 5'UTR sequence was combined with GFP(Bsu)-G, 22-fold fluorescent improvement was achieved. Collectively, the stability of the mRNA secondary structure around the initiation codon predominantly affected the translation efficiency.