The analysis of codon bias of foot-and-mouth disease virus and the adaptation of this virus to the hosts

Genetic characterization of four bacteriophages of Salmonella enterica derived from different geographic regions in China via genomic comparison

Based on the AT content > GC content in four Salmonella enterica lytic bacteriophage genomes, information entropy analysis revealed that overall nucleotide usage bias is shaped in the gene population. This genetic feature directly contributes to synonymous codons tending toward the A/T end rather than the C/G end. Furthermore, the interplay between the nucleotide composition constraint from the bacteriophage itself and the natural selection caused by outside environments forces our bacteriophages into similar evolutionary trends in terms of overall codon usage patterns. We identified the nucleotide composition constraint which plays an important role in shaping synonymous codon usage patterns including the keto skew at the first codon position, the pyrimidine skew at the second position and the AT skew at the third position. Although the four bacteriophages were isolated from different geographical regions in China, they display similar evolutionary trends in terms of genomic organization and synonymous codon usage, which are strongly influenced by the nucleotide composition constraint of the bacteriophage. The findings of the present study reveal important details of the evolutionary and host-pathogen interactions of Salmonella enterica, which will benefit the efficient utilization of phages for therapeutic and other applications.

Role of nucleotide pair frequency and synonymous codon usage in the evolution of bovine viral diarrhea virus

Synonymous codon usage plays an important role in the adaptation of viruses to their hosts. Bovine viral diarrhea virus (BVDV) relies on a high mutation rate in its genome to achieve the necessary fitness in a particular host. However, the question of which selective forces influence nucleotide pair and synonymous codon usage patterns in different BVDV genotypes remains unresolved. Here, 169 BVDV strains isolated at different times in various countries were analyzed to compare their dinucleotide frequency and synonymous codon usage. Examination of the nucleotide usage pattern in the open reading frame (ORF) of BVDV revealed a significantly higher frequency of purine than pyrimidine, with the highest extent of nucleotide usage bias observed in the first codon position. Moreover, a nucleotide pair bias, especially favoring CpG dinucleotides, was observed in all of the genotypes. Together, the nucleotide composition constraints and nucleotide pair bias appear to have influenced the overall codon usage pattern. Nucleotide pair and synonymous codon usage biases were associated with individual genotypes to different degrees. Of particular note, BVDV-1 exhibited more variation in its nucleotide pair and synonymous codon usage than BVDV-2 and BVDV-3, suggesting that these patterns are shaped both by selection of mutations in the viral genome and translational selection in the host.

Pathobiological characterization of a novel duck picornavirus in duck in China

A novel strain of duck picornavirus was isolated from duck tissue in Taian, Shandong Province, in 2017 in our laboratory. The virus was amplified in specific-pathogen-free (SPF) chicken embryos, purified and then analyzed by whole genome sequencing, which revealed a new duck-derived small RNA virus that was designated as Duck/FC22/China/2017 (FC22, GenBank accession no. MN102111) based on its genome structure and phylogenetic relationship. An in-depth study revealed that the virus grew well on the Leghorn male hepatoma (LMH) cell line. After propagation of the virus, SPF ducks were inoculated for pathogenicity tests, and their mental state, growth and development were observed after inoculation; the ducks were dissected to observe the organs and histopathological changes. A TaqMan fluorescence quantitative PCR method was utilized to detect the proliferation and shedding patterns of the virus within the ducks, while the SYBR Green I fluorescence quantitative PCR method was used to assess cytokine expression levels in the organs. The results showed that following inoculation with the FC22 strain, the mental status of the SPF ducks remained unchanged. Mild oedema was observed in some tissue organs during dissection; however, no pathological changes, such as congestion or degeneration, were noted. Histopathological analysis revealed cellular necrosis in organs, including the heart, liver, and bursa of Fabricius, as well as a reduced volume and deep staining of certain neurons in the cerebrum. Following infection, the virus titres in various organs and in cloacal swabs of the SPF ducks peaked on the first day before gradually decreasing daily. By the 10th d, the virus titres in all organs had decreased to less than 101 copies/μL. Additionally, notable alterations were observed in the expression levels of the cytokines IFNα, IL6, IL10, and TNFα. This indicates that the FC22 strain does not cause significant disease in ducks. This report presents the initial study on a recently discovered picornavirus, offering a thorough and methodical examination of its pathogenic characteristics and provides a reference for the clinical evaluation and scientific strategies for the prevention and treatment of this particular picornavirus.

Analysis for codon usage bias in membrane anchor of nonstructural protein 5A from BVDV

The nonstructural protein 5A (NS5A) of the bovine viral diarrhea virus (BVDV) is a monotopic membrane protein. This protein can anchor to the cell membrane by an in-plane amphipathic ⍺-helix, which participates in the viral replication complex. In this study, the effects of synonymous codon usage pattern of NS5A and the overall transfer RNA (tRNA) abundance in cells on the formation of the in-plane membrane anchor of NS5A were analyzed, based on NS5A coding sequences of different BVDV genotypes. BVDV NS5A coding sequences represent the most potential for BVDV genotyping. Moreover, the nucleotide usage of BVDV NS5A dominates the genotype-specific pattern of synonymous codon usage. There is an obvious relationship between synonymous codon usage bias and the spatial conformation of the in-plane membrane anchor. Furthermore, the overall tRNA abundance profiling displays that codon positions with a high level of tRNA abundance are more than ones with a low level of tRNA abundance in the in-plane membrane anchor, implying that high translation speed probably acts on the spatial conformation of in-plane membrane anchor of BVDV NS5A. These results give a new opinion on the effect of codon usage bias in the formation of the in-plane membrane anchor of BVDV NS5A.

Directed Evolution of Seneca Valley Virus in Tumorsphere and Monolayer Cell Cultures of a Small-Cell Lung Cancer Model

The Seneca Valley virus (SVV) is an oncolytic virus from the picornavirus family, characterized by a 7.3-kilobase RNA genome encoding for all the structural and functional viral proteins. Directed evolution by serial passaging has been employed for oncolytic virus adaptation to increase the killing efficacy towards certain types of tumors. We propagated the SVV in a small-cell lung cancer model under two culture conditions: conventional cell monolayer and tumorspheres, with the latter resembling more closely the cellular structure of the tumor of origin. We observed an increase of the virus-killing efficacy after ten passages in the tumorspheres. Deep sequencing analyses showed genomic changes in two SVV populations comprising 150 single nucleotides variants and 72 amino acid substitutions. Major differences observed in the tumorsphere-passaged virus population, compared to the cell monolayer, were identified in the conserved structural protein VP2 and in the highly variable P2 region, suggesting that the increase in the ability of the SVV to kill cells over time in the tumorspheres is acquired by capsid conservation and positively selecting mutations to counter the host innate immune responses.

Insights into Synonymous Codon Usage Bias in Hepatitis C Virus and Its Adaptation to Hosts

Hepatitis C virus (HCV) is enveloped RNA virus, encoding for a polyprotein that is processed by cellular proteases. The virus is responsible for liver cirrhosis, allograft rejection, and human hepatocellular carcinoma. Based on studies including compositional analysis, odds ratio analysis, parity analysis, skew analysis, relative synonymous codon usage, codon bias, and protein properties, it was evident that codon usage bias in HCV is dependent upon the nucleotide composition. Codon context analysis revealed CTC-CTG as a preferred codon pair. While CGA and CGT codons were rare, none of the codons were rare in HCV-like viruses envisaged in the present study. Many of the preferred codon pairs were valine amino acid-initiated, which possibly infers viral infectivity; hence the role of selection forces appears to act on the HCV genome, which was further validated by neutrality analysis where selection accounted for 87.28%, while mutation accounted for 12.72% force shaping codon usage. Furthermore, codon usage was correlated with the length of the genome. HCV viruses prefer valine-initiated codon pairs, while HCV-like viruses prefer alanine-initiated codon pairs. The HCV host range is very narrow and is confined to only humans and chimpanzees. Based on indices including codon usage correlation analysis, similarity index, and relative codon deoptimization index, it is evident in the study that the chimpanzee is the primary host of the virus. The present study helped elucidate the preferred host for HCV. The information presented in the study paved the way for generating an attenuated vaccine candidate through viral recoding, with finely tuned nucleotide composition and a perfect balance of preferred and rare codons.

Nucleotide and codon usage biases involved in the evolution of African swine fever virus: A comparative genomics analysis

Since African swine fever virus (ASFV) replication is closely related to its host's machinery, codon usage of viral genome can be subject to selection pressures. A better understanding of codon usage can give new insights into viral evolution. We implemented information entropy and revealed that the nucleotide usage pattern of ASFV is significantly associated with viral isolation factors (region and time), especially the usages of thymine and cytosine. Despite the domination of adenine and thymine in the viral genome, we found that mutation pressure alters the overall codon usage pattern of ASFV, followed by selective forces from natural selection. Moreover, the nucleotide skew index at the gene level indicates that nucleotide usages influencing synonymous codon bias of ASFV are significantly correlated with viral protein hydropathy. Finally, evolutionary plasticity is proved to contribute to the weakness in synonymous codons with A- or T-end serving as optimal codons of ASFV, suggesting that fine-tuning translation selection plays a role in synonymous codon usages of ASFV for adapting host. Taken together, ASFV is subject to evolutionary dynamics on nucleotide selections and synonymous codon usage, and our detailed analysis offers deeper insights into the genetic characteristics of this newly emerging virus around the world.

Evolution of codon usage in Taenia saginata genomes and its impact on the host

The beef tapeworm, also known as Taenia saginata, is a zoonotic tapeworm from the genus Taenia in the order Cyclophyllidea. Taenia saginata is a food-borne zoonotic parasite with a worldwide distribution. It poses serious health risks to the host and has a considerable negative socioeconomic impact. Previous studies have explained the population structure of T. saginata within the evolutionary time scale and adaptive evolution. However, it is still unknown how synonymous codons are used by T. saginata. In this study, we used 90 T. saginata strains, applying the codon usage bias (CUB). Both base content and relative synonymous codon usage (RSCU) analysis revealed that AT-ended codons were more frequently used in the genome of T. saginata. Further low CUB was observed from the effective number of codons (ENC) value. The neutrality plot analysis suggested that the dominant factor of natural selection was involved in the structuring of CUB in T. saginata. Further analysis showed that T. saginata has adapted host-specific codon usage patterns to sustain successful replication and transmission chains within hosts (Bos taurus and Homo sapiens). Generally, both natural selection and mutational pressure have an impact on the codon usage patterns of the protein-coding genes in T. saginata. This study is important because it characterized the codon usage pattern in the T. saginata genomes and provided the necessary data for a basic evolutionary study on them.

Analysis of codon usage bias of lumpy skin disease virus causing livestock infection

Lumpy skin disease virus (LSDV) causes lumpy skin disease (LSD) in livestock, which is a double-stranded DNA virus that belongs to the genus Capripoxvirus of the family Poxviridae. LSDV is an important poxvirus that has spread out far and wide to become distributed worldwide. It poses serious health risks to the host and causes considerable negative socioeconomic impact on farmers financially and on cattle by causing ruminant-related diseases. Previous studies explained the population structure of the LSDV within the evolutionary time scale and adaptive evolution. However, it is still unknown and remains enigmatic as to how synonymous codons are used by the LSDV. Here, we used 53 LSDV strains and applied the codon usage bias (CUB) analysis to them. Both the base content and the relative synonymous codon usage (RSCU) analysis revealed that the AT-ended codons were more frequently used in the genome of LSDV. Further low codon usage bias was calculated from the effective number of codons (ENC) value. The neutrality plot analysis suggested that the dominant factor of natural selection played a role in the structuring of CUB in LSDV. Additionally, the results from a comparative analysis suggested that the LSDV has adapted host-specific codon usage patterns to sustain successful replication and transmission chains within hosts (Bos taurus and Homo sapiens). Both natural selection and mutational pressure have an impact on the codon usage patterns of the protein-coding genes in LSDV. This study is important because it has characterized the codon usage pattern in the LSDV genomes and has provided the necessary data for a basic evolutionary study on them.

Genetic features of bovine viral diarrhea virus subgenotype 1c in newborn calves at nucleotide and synonymous codon usages

Bovine viral diarrhea virus (BVDV), serving as an important pathogen for newborn calves, poses threat to reproductive and economic losses in the cattle industry. To survey the infection rate and genetic diversity of BVDV in newborn calves in northern China, a total of 676 sera samples of newborn calves were collected from four provinces between 2021 and 2022. All sera samples were individually detected for BVDV infection by RT-PCR and ELISA. Our results showed that the overall serological rate was 9.76% (66/676) and the average positive rate of BVDV RNA was 8.14% (55/676) in the newborn calves. Eight BVDV strains were successfully isolated from RT-PCR positive sera samples, and four isolates displayed the cytopathic effect (CPE). Based on phylogenetic tree at the genome level, the eight strains were classified into subgenotype 1c. Moreover, the BVDV isolates had a close genetic relationship with the GSTZ strain at either nucleotide or codon usage level. Interestingly, in comparison of synonymous codon usage patterns between the BVDV isolates with CPE and ones without CPE, there were four synonymous codons (UCG, CCC, GCA, and AAC) which displayed the significant differences (p < 0.05) at codon usage pattern, suggesting that synonymous codon usage bias might play a role in BVDV-1c biotypes. In addition, the usage of synonymous codons containing CpG dinucleotides was suppressed by the BVDV-1c isolates, reflecting one of strategies of immune evasion of BVDV to its host. Taken together, our study provided data for monitoring and vaccination strategies of BVDV for newborn calves in northern China.

Evolutionary dynamics of codon usages for peste des petits ruminants virus

Peste des petits ruminants virus (PPRV) is an important agent of contagious, acute and febrile viral diseases in small ruminants, while its evolutionary dynamics related to codon usage are still lacking. Herein, we adopted information entropy, the relative synonymous codon usage values and similarity indexes and codon adaptation index to analyze the viral genetic features for 45 available whole genomes of PPRV. Some universal, lineage-specific, and gene-specific genetic features presented by synonymous codon usages of the six genes of PPRV that encode N, P, M, F, H and L proteins reflected evolutionary plasticity and independence. The high adaptation of PPRV to hosts at codon usages reflected high viral gene expression, but some synonymous codons that are rare in the hosts were selected in high frequencies in the viral genes. Another obvious genetic feature was that the synonymous codons containing CpG dinucleotides had weak tendencies to be selected in viral genes. The synonymous codon usage patterns of PPRV isolated during 2007–2008 and 2013–2014 in China displayed independent evolutionary pathway, although the overall codon usage patterns of these PPRV strains matched the universal codon usage patterns of lineage IV. According to the interplay between nucleotide and synonymous codon usages of the six genes of PPRV, the evolutionary dynamics including mutation pressure and natural selection determined the viral survival and fitness to its host.

Dispersion of synonymous codon usage patterns in hepatitis E virus genomes derived from various hosts

Hepatitis E virus (HEV) is an important zoonotic pathogen infecting a wide range of host species. It has a positive-sense, single-stranded RNA genome encoding three open reading frames (ORFs). Synonymous codon usages of viruses essentially determine their survival and adaptation to susceptible hosts. To better understand the interplay between the ever-expanding host range and synonymous codon usages of HEV, we quantified the dispersion of synonymous codon usages of HEV genomes isolated from different hosts via V_s calculation and information entropy. HEV ORFs show species-specific synonymous codon usage patterns. Ruminant-derived HEV ORFs own the most synonymous codons with stable usage patterns (V_s value <0.1) which leads to the stable overall codon usage patterns (R value being close to zero). Swine-derived HEV ORFs own more concentrated synonymous codons than those from wild boar. Compared with HEV strains isolated from other hosts, the human-derived HEV exhibits a distinct pattern at the overall codon usage (R < 0). Generally, ORF1 contains more synonymous codons with stable usage patterns (V_s  < 0.1) than those of ORFs 2 and 3. Moreover, ORF3 contains more synonymous codons with varied patterns (V_s  > 1.0) than ORFs 1 and 2. The host factor serving as one of the evolutionary dynamics probably influences synonymous codon usage patterns of the HEV genome. Taken together, synonymous codons with stable usage patterns in ORF1 might help to sustain the infection, while that with varied usage patterns in ORF3 may facilitate cross-species infection and expand the host range.

Codon Usage Bias in Autophagy-Related Gene 13 in Eukaryotes: Uncovering the Genetic Divergence by the Interplay Between Nucleotides and Codon Usages

Synonymous codon usage bias is a universal characteristic of genomes across various organisms. Autophagy-related gene 13 (atg13) is one essential gene for autophagy initiation, yet the evolutionary trends of the atg13 gene at the usages of nucleotide and synonymous codon remains unexplored. According to phylogenetic analyses for the atg13 gene of 226 eukaryotic organisms at the nucleotide and amino acid levels, it is clear that their nucleotide usages exhibit more genetic information than their amino acid usages. Specifically, the overall nucleotide usage bias quantified by information entropy reflected that the usage biases at the first and second codon positions were stronger than those at the third position of the atg13 genes. Furthermore, the bias level of nucleotide ‘G’ usage is highest, while that of nucleotide ‘C’ usage is lowest in the atg13 genes. On top of that, genetic features represented by synonymous codon usage exhibits a species-specific pattern on the evolution of the atg13 genes to some extent. Interestingly, the codon usages of atg13 genes in the ancestor animals (Latimeria chalumnae, Petromyzon marinus, and Rhinatrema bivittatum) are strongly influenced by mutation pressure from nucleotide composition constraint. However, the distributions of nucleotide composition at different codon positions in the atg13 gene display that natural selection still dominates atg13 codon usages during organisms’ evolution.

The nucleotide usages significantly impact synonymous codon usage in Mycoplasma hyorhinis

Five annotated genomes of Mycoplasma hyorhinis were analyzed for clarifying evolutionary dynamics driving the overall codon usage pattern. Information entropy used for estimating nucleotide usage pattern at the gene level indicates that multiple evolutionary dynamics participate in forcing nucleotide usage bias at every codon position. Moreover, nucleotide usage bias directly contributes to synonymous codon usage biases with two different extremes. The overrepresented codons tended to have A/T in the third codon position, and the underrepresented codons strongly used G/C in the third position. Furthermore, correspondence analysis and neutrality plot reflect an obvious interplay between mutation pressure and natural selection mediating codon usage in M. hyorhinis genome. Due to significant bias in usages between A/T and G/C at the gene level, different selective forces have been proposed to contribute to codon usage preference in M. hyorhinis genome, including nucleotide composition constraint derived from mutation pressure, translational selection involved in natural selection, and strand-specific mutational bias represented by different nucleotide skew index. The systemic analyses of codon usage for M. hyorhinis can enable us to better understand the mechanisms of evolution in this species.

Viral adaption of staphylococcal phage: A genome-based analysis of the selective preference based on codon usage Bias

Given the high therapeutic value of the staphylococcal phage, the genome co-evolution of the phage and the host has gained great attention. Though the genome-wide AT richness in staphylococcal phages has been well-studied with nucleotide usage bias, here we proved that host factor, lifestyle and taxonomy are also important factors in understanding the phage nucleotide usages bias using information entropy formula. Such correlation is especially prominent when it comes to the synonymous codon usages of staphylococcal phages, despite the overall scattered codon usage pattern represented by principal component analysis. This strong relationship is explained by nucleotide skew which testified that the usage biases of nucleotide at different codon positions are acting on synonymous codons. Therefore, our study reveals a hidden relationship of genome evolution with host limitation and phagic phenotype, providing new insight into phage genome evolution at genetic level.

The effects of synonymous codon usages on genotypic formation of open reading frames in hepatitis E virus

Hepatitis E virus (HEV) infection has emerged as an important public health issue. As a zoonotic RNA virus, new strains are continuously discovered from human or various animal species. However, the capability of cross-species infection varies largely among different strains. Because the classical nucleotide-based genotyping system provides little functional insight, this study aimed to comprehensively investigate codon usage of the HEV coding regions for better understanding the evolutional orientation, virus-host interaction and cross-species transmission. We observed significant differences of the four nucleotide usages in the three open reading frames, indicating that the evolutional tendency of HEV caused by mutation pressure is modified by the evolutional dynamic related to positive selection. Furthermore, significant differences of nucleotide usages were found among HEV isolated from different host species, suggesting an important role of natural selection related to the host. Analysis of effective number of codons revealed distinct degrees of biased codon usage caused by mutation pressure or the host. Finally, we have mapped the similarity levels of the overall codon usage between the virus and the host to assess the potential of cross-species infection. Thus, this study has provided a novel aspect for better understanding the HEV genetic orientation and the zoonotic nature.

Tracing Brucella evolutionary dynamics in expanding host ranges through nucleotide, codon and amino acid usages in genomes

The host range of Brucella organisms has expanded from terrestrial and marine mammals to fish and amphibians. The high homology genomes of different Brucella organisms promote us to investigate evolutionary patterns for nucleotide, codon and amino acid usage patterns at gene levels among Brucella species. Although the similar patterns for nucleotide and synonymous codon usages exist in gene population, GC composition at the first codon position has significant correlations to that of the second and third codon positions, respectively, suggesting that nucleotide usages surrounding one codon influence synonymous codon usage patterns. Evolutionary patterns represented by synonymous codon and amino acid usages reflect host factor impacting Brucella speciation. As for genetic variations of important virulent factors involved with different biological functions, genes encoding lipoplysaccharides (LPSs) display more distinctive codon adaptation to Brucella than those of the BvrR/BvrS system and type IV secretion system. By Bayesian analysis, the polygenetic constructions for these genes of virulent factors shared by Brucella species display the purifying/positive selections and partially host factor in mediating genetic variations of these genes. The systemic analyses for nucleotide, synonymous codon and amino acid usages at gene level and genetic variations of important virulent factor genes display that host limitation influences either genetic characterizations at gene level or a particular gene involved in virulent factors of Brucella.Communicated by Ramaswamy H. Sarma.

Emerging Roles of tRNAs in RNA Virus Infections

Viruses rely on the host cell translation machinery for efficient synthesis of their own proteins. Emerging evidence highlights different roles for host transfer RNAs (tRNAs) in the process of virus replication. For instance, different RNA viruses manipulate host tRNA pools to favor viral protein translation. Interestingly, specific host tRNAs are used as reverse transcription primers and are packaged into retroviral virions. Recent data also demonstrate the formation of tRNA-derived fragments (tRFs) upon infection to facilitate viral replication. Here, we comprehensively discuss how RNA viruses exploit distinct aspects of the host tRNA biology for their benefit. In light of the recent advances in the field, we propose that host tRNA-related pathways and mechanisms represent promising cellular targets for the development of novel antiviral strategies.

The characteristic of the synonymous codon usage and phylogenetic analysis of hepatitis B virus

Background

Hepatitis B virus (HBV) infection is a crucial medical issue worldwide. The dependence of HBV replication on host cell machineries and their co-evolutionary interactions prompt the codon usage pattern of viral genes to translation selection and mutation pressure.

Objective

The evolutionary characteristics of HBV and the natural selection effects of the human genome on the codon usage characteristics were analyzed to provide a basis for medication development for HBV infection.

Methods

The codon usage pattern of sequences from different HBV genotypes of our isolates and reference HBV genome sequences downloaded from the National Center for Biotechnology Information (NCBI) database were analyzed by computing the relative synonymous codon usage (RSCU), nucleotide content, codon adaptation index (CAI) and the effective number of codons (ENC).

Results

The highest ENC values were observed in the C genotypes, followed by the B genotypes. The ENC values indicated a weak codon usage bias (CUB) in HBV genome. The number of codons differentially used between the three genotypes was markedly higher than that of similarly used codons. High CAI values indicated a good adaptability of HBV to its host. The ENC plot indicated the occurrence of mutational pressure in the three genotypes. The mean Ka/Ks ratios in the three genotypes were lower than 1, which indicated a negative selection pressure. The CAI and GC3% plot indicated the existence of CUB in the HBV genome.

Conclusions

Nucleotide composition, mutation bias, negative selection and mutational pressure are key factors influencing the CUB and phylogenetic diversity in HBV genotypes. The data provided here could be useful for developing drugs for HBV infection.

Electronic supplementary material

The online version of this article (10.1007/s13258-020-00932-w) contains supplementary material, which is available to authorized users.

Significance and roles of synonymous codon usage in the evolutionary process of Proteus

Proteus spp. bacteria frequently serve as opportunistic pathogens that can infect many animals and show positive survival and existence in various natural environments. The evolutionary pattern of Proteus spp. is an unknown topic, which benefits understanding the different evolutionary dynamics for excellent bacterial adaptation to various environments. Here, the eight whole genomes of different Proteus species were analyzed for the interplay between nucleotide usage and synonymous codon usage. Although the orthologous average nucleotide identity and average nucleotide identity display the genetic diversity of these Proteus species at the genome level, the principal component analysis further shows that these species sustain the specific genetic niche at the aspect of synonymous codon usage patterns. Interestingly, although these Proteus species have A/T rich genes with underrepresented G (guanine) or C (cytosine) at the third codon positions and overrepresented A or T at these positions, some synonymous codons with A or T end are obviously suppressed in usage. The overall codon usage pattern reflected by the effective number of codons (ENC) has a significantly positive correlation with GC3 content (GC content at the third codon position), and ENC has a significantly negative correlation with the adaptation index for these species. These results suggest that the mutation pressure caused by nucleotide composition constraint serves as a dominant evolutionary dynamic driving evolutionary trend of Proteus spp., along with other selections related to natural selection, replication and fine-tune translation, and so on. Taken together, the analyses help to understand the evolutionary interplay between nucleotide and codon usage at the gene level of Proteus.

Codon usage pattern and its influencing factors in different genomes of hepadnaviruses

Codon usage bias (CUB) arises from the preference for a codon over codons for the same amino acid. The major factors contributing to CUB are evolutionary forces, compositional properties, gene expression, and protein properties. The present analysis was performed to investigate the compositional properties and the extent of CUB across the genomes of members of the family Hepadnaviridae, as previously no work using bioinformatic tools has been reported. The viral genes were found to be AT rich with low CUB. Analysis of relative synonymous codon usage (RSCU) was used to identify overrepresented and underrepresented codons for each amino acid. Correlation analysis of overall nucleotide composition and its composition at the third codon position suggested that mutation pressure might influence the CUB. A highly significant correlation was observed between GC12 and GC3 (r = 0.910, p < 0.01), indicating that directional mutation affected all three codon positions across the genome. Translational selection (P2) and mutational responsive index (MRI) values of genes suggested that mutation plays a more important role than translational selection in members of the family Hepadnaviridae.

Synonymous Codon Usages as an Evolutionary Dynamic for Chlamydiaceae

The family of Chlamydiaceae contains a group of obligate intracellular bacteria that can infect a wide range of hosts. The evolutionary trend of members in this family is a hot topic, which benefits our understanding of the cross-infection of these pathogens. In this study, 14 whole genomes of 12 Chlamydia species were used to investigate the nucleotide, codon, and amino acid usage bias by synonymous codon usage value and information entropy method. The results showed that all the studied Chlamydia spp. had A/T rich genes with over-represented A or T at the third positions and G or C under-represented at these positions, suggesting that nucleotide usages influenced synonymous codon usages. The overall codon usage trend from synonymous codon usage variations divides the Chlamydia spp. into four separate clusters, while amino acid usage divides the Chlamydia spp. into two clusters with some exceptions, which reflected the genetic diversity of the Chlamydiaceae family members. The overall codon usage pattern represented by the effective number of codons (ENC) was significantly positively correlated to gene GC3 content. A negative correlation exists between ENC and the codon adaptation index for some Chlamydia species. These results suggested that mutation pressure caused by nucleotide composition constraint played an important role in shaping synonymous codon usage patterns. Furthermore, codon usage of T3ss and Pmps gene families adapted to that of the corresponding genome. Taken together, analyses help our understanding of evolutionary interactions between nucleotide, synonymous codon, and amino acid usages in genes of Chlamydiaceae family members.

Type I Interferon Induced and Antagonized by Foot-and-Mouth Disease Virus

Viral infections trigger the innate immune system, serving as the first line of defense, and are characterized by the production of type I interferon (IFN). Type I IFN is expressed in a broad spectrum of cells and tissues in the host and includes various subtypes (IFN-α, IFN-β, IFN-δ, IFN-ε, IFN-κ, IFN-τ, IFN-ω, IFN-ν, and IFN-ζ). Since the discovery of type I IFN, our knowledge of the biology of type I IFN has accumulated immensely, and we now have a substantial amount of information on the molecular mechanisms of the response and induction of type I IFN, as well as the strategies utilized by viruses to evade the type I IFN response. Foot-and-mouth disease virus (FMDV) can selectively alter cellular pathways to promote viral replication and evade antiviral immune activation of type I IFN. RNA molecules generated by FMDV are sensed by the cellular receptor for pathogen-associated molecular patterns (PAMPs). FMDV preferentially activates different sensor molecules and various signal transduction pathways. Based on knowledge of the virus or RNA pathogen specificity as well as the function-structure relationship of RNA sensing, it is necessary to summarize numerous signaling adaptors that are reported to participate in the regulation of IFN gene activation.

Analyses of nucleotide, synonymous codon and amino acid usages at gene levels of Brucella melitensis strain QY1

Brucella melitensis is the causative pathogen of the zoonotic disease brucellosis in China. This work focused on analyses of genetic features represented by nucleotide, synonymous codon and amino acid usages at gene levels of B. melitensis strain QY1 isolated from China. Although nucleotide usage biases at different codon positions all work on synonymous codon usage bias, nucleotide usage biases at the 1st and 3rd positions play more important roles in codon usages. Mutation pressure caused by nucleotide composition constraint influences the formation of over-representative synonymous codons, but neighboring nucleotides surrounding a codon strongly influence synonymous codon usage bias for B. melitensis strain QY1. There is significant correlation between amino acid usage bias and hydropathicity of proteins for B. melitensis strain QY1. Compared with different Brucella species about synonymous codon usage patterns, synonymous codon usages are not obviously influenced by hosts. Due to nucleotide usage bias at the 1st codon position influencing synonymous codon and amino acid usages, good interactions among nucleotide, synonymous codon and amino acid usages exist in the evolutionary process of B. melitensis.

Evolution of Synonymous Codon Usage Bias in West African and Central African Strains of Monkeypox Virus

The evolution of bias in synonymous codon usage in chosen monkeypox viral genomes and the factors influencing its diversification have not been reported so far. In this study, various trends associated with synonymous codon usage in chosen monkeypox viral genomes were investigated, and the results are reported. Identification of factors that influence codon usage in chosen monkeypox viral genomes was done using various codon usage indices, such as the relative synonymous codon usage, the effective number of codons, and the codon adaptation index. The Spearman rank correlation analysis and a correspondence analysis were used for correlating various factors with codon usage. The results revealed that mutational pressure due to compositional constraints, gene expression level, and selection at the codon level for utilization of putative optimal codons are major factors influencing synonymous codon usage bias in monkeypox viral genomes. A cluster analysis of relative synonymous codon usage values revealed a grouping of more virulent strains as one major cluster (Central African strains) and a grouping of less virulent strains (West African strains) as another major cluster, indicating a relationship between virulence and synonymous codon usage bias. This study concluded that a balance between the mutational pressure acting at the base composition level and the selection pressure acting at the amino acid level frames synonymous codon usage bias in the chosen monkeypox viruses. The natural selection from the host does not seem to have influenced the synonymous codon usage bias in the analyzed monkeypox viral genomes.

Analyses of nucleotide, codon and amino acids usages between peste des petits ruminants virus and rinderpest virus

Peste des petits ruminants virus (PPRV) and rinderpest virus (RPV) are two causative agents of an economically important disease for ruminants (i.e., sheep, cattle and goat). In this study, the nucleotide, codon and amino acid usages for PPRV and RPV have been analyzed by multivariate statistical methods. Relative synonymous codon usage (RSCU) analysis represents that ACG for Thr and GCG for Ala are selected with under-representation in both PPRV and RPV, and AGA for Arg in PPRV and AGG for Arg in RPV are used with over-representation. The usage of nucleotide pair (CpG) tends to be removed from viral genes of the two viruses, suggesting that other evolutionary forces take part in evolutionary processes for viral genes in addition to mutation pressure from nucleotide usage at the third codon position. The overall nucleotide usage of viral gene is not major factor in shaping synonymous codon usage patterns, while the nucleotide usages at the third codon position and the nucleotide pairs play important roles in shaping synonymous codon usage patterns. Although PPRV and RPV are closely related antigenically, the codon and amino acid usage patterns for viral genes represent a significant genetic diversity between PPRV and RPV. Moreover, the overall codon usage trends for viral genes between PPRV and RPV are mainly influenced by mutation pressure from nucleotide usage at the third codon position and translation selection from hosts. Taken together, this is first comprehensive analyses for nucleotide, codon and amino acid usages of viral genes of PPRV and RPV and the findings are expected to increase our understanding of evolutionary forces influencing viral evolutionary pathway and adaptation toward hosts.

Comprehensive analysis of the codon usage patterns in the envelope glycoprotein E2 gene of the classical swine fever virus

The classical swine fever virus (CSFV), circulating worldwide, is a highly contagious virus. Since the emergence of CSFV, it has caused great economic loss in swine industry. The envelope glycoprotein E2 gene of the CSFV is an immunoprotective antigen that induces the immune system to produce neutralizing antibodies. Therefore, it is essential to study the codon usage of the E2 gene of the CSFV. In this study, 140 coding sequences of the E2 gene were analyzed. The value of effective number of codons (ENC) showed low codon usage bias in the E2 gene. Our study showed that codon usage could be described mainly by mutation pressure ENC plot analysis combined with principal component analysis (PCA) and translational selection-correlation analysis between the general average hydropathicity (Gravy) and aromaticity (Aroma), and nucleotides at the third position of codons (A3s, T3s, G3s, C3s and GC3s). Furthermore, the neutrality analysis, which explained the relationship between GC12s and GC3s, revealed that natural selection had a key role compared with mutational bias during the evolution of the E2 gene. These results lay a foundation for further research on the molecular evolution of CSFV.

Codon usage bias in the N gene of rabies virus

Since its emergence, rabies virus (RABV) has been a major worldwide concern especially in developing countries. The nucleoprotein (N) of RABV is highly conserved and key for genetic typing, thus a better understanding of the N gene evolutionary trajectory can assist the development of control measures. We found that the N gene of RABV has a low codon usage bias with a mean effective number of codons (ENC) value of 56.33 influenced by both mutation pressure and natural selection. However, neutrality analysis indicated that natural selection dominates over mutation pressure. Additionally, we found that dinucleotide bias partly contributed to RABV codon usage bias. On the other hand, based on the clades of phylogenetic tree, we found that the evolutionary rate of the Africa 2 clade was the highest with a mean value of 3.75×10^-3 substitutions per site per year. Above all, our results regarding N gene of RABV codon usage will serve future RABV evolution research.

Analysis of the codon usage of the ORF2 gene of feline calicivirus

Feline calicivirus (FCV) is a highly prevalent pathogen of the domestic cat that causes acute infections of the oral and upper respiratory tract. The E region of the ORF2 protein is responsible for the induction of virus-neutralizing antibodies, thus it is important to understand the codon usage of this gene. Here, analysed 90 coding sequences of ORF2 and show that it undergoes a low codon usage bias. In addition, although mutational bias is one of the factors shaping the codon usage bias of this gene, natural selection plays a more significant role. Our results reveal part of the mechanisms driving FCV evolution, which will lay foundation for the further research of FCV.

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The codon usage of the ORF2 gene of FCV was first analysed in this study.

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The ORF2 gene of FCV shows a low codon usage bias.

Dinucleotide Composition in Animal RNA Viruses Is Shaped More by Virus Family than by Host Species

Viruses use the cellular machinery of their hosts for replication. It has therefore been proposed that the nucleotide and dinucleotide compositions of viruses should match those of their host species. If this is upheld, it may then be possible to use dinucleotide composition to predict the true host species of viruses sampled in metagenomic surveys. However, it is also clear that different taxonomic groups of viruses tend to have distinctive patterns of dinucleotide composition that may be independent of host species. To determine the relative strength of the effect of host versus virus family in shaping dinucleotide composition, we performed a comparative analysis of 20 RNA virus families from 15 host groupings, spanning two animal phyla and more than 900 virus species. In particular, we determined the odds ratios for the 16 possible dinucleotides and performed a discriminant analysis to evaluate the capability of virus dinucleotide composition to predict the correct virus family or host taxon from which it was isolated. Notably, while 81% of the data analyzed here were predicted to the correct virus family, only 62% of these data were predicted to their correct subphylum/class host and a mere 32% to their correct mammalian order. Similarly, dinucleotide composition has a weak predictive power for different hosts within individual virus families. We therefore conclude that dinucleotide composition is generally uniform within a virus family but less well reflects that of its host species. This has obvious implications for attempts to accurately predict host species from virus genome sequences alone.IMPORTANCE Determining the processes that shape virus genomes is central to understanding virus evolution and emergence. One question of particular importance is why nucleotide and dinucleotide frequencies differ so markedly between viruses. In particular, it is currently unclear whether host species or virus family has the biggest impact on dinucleotide frequencies and whether dinucleotide composition can be used to accurately predict host species. Using a comparative analysis, we show that dinucleotide composition has a strong phylogenetic association across different RNA virus families, such that dinucleotide composition can predict the family from which a virus sequence has been isolated. Conversely, dinucleotide composition has a poorer predictive power for the different host species within a virus family and across different virus families, indicating that the host has a relatively small impact on the dinucleotide composition of a virus genome.

Nucleotide composition bias and codon usage trends of gene populations in Mycoplasma capricolum subsp. capricolum and M. Agalactiae

Because of the low GC content of the gene population, amino acids of the two mycoplasmas tend to be encoded by synonymous codons with an A or T end. Compared with the codon usage of ovine, Mycoplasma capricolum and M. agalactiae tend to select optimal codons, which are rare codons in ovine. Due to codon usage pattern caused by genes with key biological functions, the overall codon usage trends represent a certain evolutionary direction in the life cycle of the two mycoplasmas. The overall codon usage trends of a gene population of M. capricolum subsp. capricolum can be obviously separated from other mycoplasmas, and the overall codon usage trends of M. agalactiae are highly similar to those of M. bovis. These results partly indicate the independent evolution of the two mycoplasmas without the limits of the host cell's environment. The GC and AT skews estimate nucleotide composition bias at different positions of nucleotide triplets and the protein consideration caused by the nucleotide composition bias at codon positions 1 and 2 largely take part in synonymous codon usage patterns of the two mycoplasmas. The correlation between the codon adaptation index and codon usage variation indicates that the effect of codon usage on gene expression in M. capricolum subsp. capricolum is opposite to that of M. agalactiae, further suggesting independence of the evolutionary process influencing the overall codon usage trends of gene populations of mycoplasmas.

Exploiting tRNAs to Boost Virulence

Transfer RNAs (tRNAs) are powerful small RNA entities that are used to translate nucleotide language of genes into the amino acid language of proteins. Their near-uniform length and tertiary structure as well as their high nucleotide similarity and post-transcriptional modifications have made it difficult to characterize individual species quantitatively. However, due to the central role of the tRNA pool in protein biosynthesis as well as newly emerging roles played by tRNAs, their quantitative assessment yields important information, particularly relevant for virus research. Viruses which depend on the host protein expression machinery have evolved various strategies to optimize tRNA usage—either by adapting to the host codon usage or encoding their own tRNAs. Additionally, several viruses bear tRNA-like elements (TLE) in the 5′- and 3′-UTR of their mRNAs. There are different hypotheses concerning the manner in which such structures boost viral protein expression. Furthermore, retroviruses use special tRNAs for packaging and initiating reverse transcription of their genetic material. Since there is a strong specificity of different viruses towards certain tRNAs, different strategies for recruitment are employed. Interestingly, modifications on tRNAs strongly impact their functionality in viruses. Here, we review those intersection points between virus and tRNA research and describe methods for assessing the tRNA pool in terms of concentration, aminoacylation and modification.

The effects of the context-dependent codon usage bias on the structure of the nsp1α of porcine reproductive and respiratory syndrome virus

The information about the crystal structure of porcine reproductive and respiratory syndrome virus (PRRSV) leader protease nsp1α is available to analyze the roles of tRNA abundance of pigs and codon usage of the nsp1 α gene in the formation of this protease. The effects of tRNA abundance of the pigs and the synonymous codon usage and the context-dependent codon bias (CDCB) of the nsp1 α on shaping the specific folding units (α-helix, β-strand, and the coil) in the nsp1α were analyzed based on the structural information about this protease from protein data bank (PDB: 3IFU) and the nsp1 α of the 191 PRRSV strains. By mapping the overall tRNA abundance along the nsp1 α, we found that there is no link between the fluctuation of the overall tRNA abundance and the specific folding units in the nsp1α, and the low translation speed of ribosome caused by the tRNA abundance exists in the nsp1 α. The strong correlation between some synonymous codon usage and the specific folding units in the nsp1α was found, and the phenomenon of CDCB exists in the specific folding units of the nsp1α. These findings provide an insight into the roles of the synonymous codon usage and CDCB in the formation of PRRSV nsp1α structure.

Codon usage bias of the phosphoprotein gene of spring viraemia of carp virus and high codon adaptation to the host

In this study, we calculated the relative synonymous codon usage (RSCU) value and the effective number of codons (ENC) value to carry out principal component analysis (PCA) and correlation analysis of the codon usage pattern of the phosphoprotein gene (P gene) of spring viraemia of carp virus (SVCV). The synonymous codon usage pattern in P genes is geography-specific, based on PCA analysis. The high correlation between (G + C)1,2 % and (G + C)3 % suggests that mutational pressure rather than natural selection is the main factor that determines the codon usage and base components in P genes. At least 40 out of 59 synonymous codons are similarly selected in all functional genes within five complete SVCV genomes, and the hosts based on the RSCU data. These results not only provide insight into variations in the codon usage pattern of SVCV but also may help in understanding the processes governing the evolution of SVCV.

The distribution of synonymous codon choice in the translation initiation region of dengue virus

Dengue is the most common arthropod-borne viral (Arboviral) illness in humans. The genetic features concerning the codon usage of dengue virus (DENV) were analyzed by the relative synonymous codon usage, the effective number of codons and the codon adaptation index. The evolutionary distance between DENV and the natural hosts (Homo sapiens, Pan troglodytes, Aedes albopictus and Aedes aegypti) was estimated by a novel formula. Finally, the synonymous codon usage preference for the translation initiation region of this virus was also analyzed. The result indicates that the general trend of the 59 synonymous codon usage of the four genotypes of DENV are similar to each other, and this pattern has no link with the geographic distribution of the virus. The effect of codon usage pattern of Aedes albopictus and Aedes aegypti on the formation of codon usage of DENV is stronger than that of the two primates. Turning to the codon usage preference of the translation initiation region of this virus, some codons pairing to low tRNA copy numbers in the two primates have a stronger tendency to exist in the translation initiation region than those in the open reading frame of DENV. Although DENV, like other RNA viruses, has a high mutation to adapt its hosts, the regulatory features about the synonymous codon usage have been 'branded' on the translation initiation region of this virus in order to hijack the translational mechanisms of the hosts.

The effects of the synonymous codon usage and tRNA abundance on protein folding of the 3C protease of foot-and-mouth disease virus

The 3C protease of foot-and-mouth disease virus (FMDV) has a conserved amino acid sequence and is responsible for most cleavage in the viral polyprotein. The effects of the synonymous codon usage of FMDV 3C gene and tRNA abundance of the hosts on shaping different folding units (α-helix, β-strand and the coil) in the 3C protease were analyzed based on the structural information of the FMDV 3C protease from Protein Data Bank (PDB: 2BHG) and 210 genes of 3C for all serotypes of FMDV. The strong correlation between some codons usage and the specific folding unit in the FMDV 3C protease is found. As for the effect of translation speed caused by tRNA abundance on the formation of folding units, the codon positions with lowly abundant tRNA scatter in the 3C gene and there is the obvious fluctuation of tRNA abundance locating in the transition boundaries from the β-strand to the α-helix and the coil, respectively. However, codon positions with lowly abundant tRNA clustering into these boundaries are not found, suggesting that the adjustment of translation speed is likely also achieved by the single codon position with low tRNA abundance rather than a cluster. The observations can provide the information for insight into the role of the synonymous codon usage in the formation of 3C protease of FMDV and effect of the tRNA abundance of the hosts on this formation of protease.

An infectious recombinant foot-and-mouth disease virus expressing a fluorescent marker protein

Foot-and-mouth disease virus (FMDV) is one of the most extensively studied animal pathogens because it remains a major threat to livestock economies worldwide. However, the dynamics of FMDV infection are still poorly understood. The application of reverse genetics provides the opportunity to generate molecular tools to further dissect the FMDV life cycle. Here, we have used reverse genetics to determine the capsid packaging limitations for a selected insertion site in the FMDV genome. We show that exogenous RNA up to a defined length can be stably introduced into the FMDV genome, whereas larger insertions are excised by recombination events. This led us to construct a recombinant FMDV expressing the fluorescent marker protein, termed iLOV. Characterization of infectious iLOV-FMDV showed the virus has a plaque morphology and rate of growth similar to the parental virus. In addition, we show that cells infected with iLOV-FMDV are easily differentiated by flow cytometry using the inherent fluorescence of iLOV and that cells infected with iLOV-FMDV can be monitored in real-time with fluorescence microscopy. iLOV-FMDV therefore offers a unique tool to characterize FMDV infection in vitro, and its applications for in vivo studies are discussed.