On the nucleotide composition and structure of retroviral RNA genomes

Different Patterns of Codon Usage and Amino Acid Composition across Primate Lentiviruses

A common feature of the mammalian Lentiviruses (family Retroviridae) is an RNA genome that contains an extremely high frequency of adenine (31.7-38.2%) while being extremely poor in cytosine (13.9-21.2%). Such a biased nucleotide composition has implications for codon usage, causing a striking difference between the frequency of synonymous codons in Lentiviruses and that in their hosts. To test whether primate Lentiviruses present differences in codon and amino acid composition, we assembled a dataset of genome sequences that includes SIV species infecting Old-World monkeys and African apes, HIV-2, and the four groups of HIV-1. Using principal component analysis, we found that HIV-1 shows a significant enrichment in adenine plus thymine in the third synonymous codon position and in adenine and guanine in the first and second nonsynonymous codon positions. Similarly, we observed an enrichment in adenine and in guanine in nonsynonymous first and second codon positions, which affects the amino acid composition of the proteins Gag, Pol, Vif, Vpr, Tat, Rev, Env, and Nef. This result suggests an effect of natural selection in shaping codon usage. Under the hypothesis that the use of synonyms in HIV-1 could reflect adaptation to that of genes expressed in specific cell types, we found a highly significant correlation between codon usage in HIV-1 and monocytes, which was remarkably higher than that with B and T lymphocytes. This finding is in line with the notion that monocytes represent an HIV-1 reservoir in infected patients, and it could help understand how this reservoir is established and maintained.

An endogenous lentivirus in the germline of a rodent

Lentiviruses (genus Lentivirus) are complex retroviruses that infect a broad range of mammals, including humans. Unlike many other retrovirus genera, lentiviruses have only rarely been incorporated into the mammalian germline. However, a small number of endogenous retrovirus (ERV) lineages have been identified, and these rare genomic "fossils" can provide crucial insights into the long-term history of lentivirus evolution. Here, we describe a previously unreported endogenous lentivirus lineage in the genome of the South African springhare (Pedetes capensis), demonstrating that the host range of lentiviruses has historically extended to rodents (order Rodentia). Furthermore, through comparative and phylogenetic analysis of lentivirus and ERV genomes, considering the biogeographic and ecological characteristics of host species, we reveal broader insights into the long-term evolutionary history of the genus.

Nucleotide Composition and Codon Usage Across Viruses and Their Respective Hosts

The genetic material of the three domains of life (Bacteria, Archaea, and Eukaryota) is always double-stranded DNA, and their GC content (molar content of guanine plus cytosine) varies between ≈ 13% and ≈ 75%. Nucleotide composition is the simplest way of characterizing genomes. Despite this simplicity, it has several implications. Indeed, it is the main factor that determines, among other features, dinucleotide frequencies, repeated short DNA sequences, and codon and amino acid usage. Which forces drive this strong variation is still a matter of controversy. For rather obvious reasons, most of the studies concerning this huge variation and its consequences, have been done in free-living organisms. However, no recent comprehensive study of all known viruses has been done (that is, concerning all available sequences). Viruses, by far the most abundant biological entities on Earth, are the causative agents of many diseases. An overview of these entities is important also because their genetic material is not always double-stranded DNA: indeed, certain viruses have as genetic material single-stranded DNA, double-stranded RNA, single-stranded RNA, and/or retro-transcribing. Therefore, one may wonder if what we have learned about the evolution of GC content and its implications in prokaryotes and eukaryotes also applies to viruses. In this contribution, we attempt to describe compositional properties of ∼ 10,000 viral species: base composition (globally and according to Baltimore classification), correlations among non-coding regions and the three codon positions, and the relationship of the nucleotide frequencies and codon usage of viruses with the same feature of their hosts. This allowed us to determine how the base composition of phages strongly correlate with the value of their respective hosts, while eukaryotic viruses do not (with fungi and protists as exceptions). Finally, we discuss some of these results concerning codon usage: reinforcing previous results, we found that phages and hosts exhibit moderate to high correlations, while for eukaryotes and their viruses the correlations are weak or do not exist.

Genome-Wide Analysis of Codon Usage Patterns of SARS-CoV-2 Virus Reveals Global Heterogeneity of COVID-19

The ongoing outbreak of coronavirus disease COVID-19 is significantly implicated by global heterogeneity in the genome organization of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). The causative agents of global heterogeneity in the whole genome of SARS-CoV-2 are not well characterized due to the lack of comparative study of a large enough sample size from around the globe to reduce the standard deviation to the acceptable margin of error. To better understand the SARS-CoV-2 genome architecture, we have performed a comprehensive analysis of codon usage bias of sixty (60) strains to get a snapshot of its global heterogeneity. Our study shows a relatively low codon usage bias in the SARS-CoV-2 viral genome globally, with nearly all the over-preferred codons' A.U. ended. We concluded that the SARS-CoV-2 genome is primarily shaped by mutation pressure; however, marginal selection pressure cannot be overlooked. Within the A/U rich virus genomes of SARS-CoV-2, the standard deviation in G.C. (42.91% ± 5.84%) and the GC3 value (30.14% ± 6.93%) points towards global heterogeneity of the virus. Several SARS-CoV-2 viral strains were originated from different viral lineages at the exact geographic location also supports this fact. Taking all together, these findings suggest that the general root ancestry of the global genomes are different with different genome's level adaptation to host. This research may provide new insights into the codon patterns, host adaptation, and global heterogeneity of SARS-CoV-2.

A comprehensive analysis of genome composition and codon usage patterns of emerging coronaviruses

An outbreak of atypical pneumonia caused by a novel Betacoronavirus (βCoV), named SARS-CoV-2 has been declared a public health emergency of international concern by the World Health Organization. In order to gain insight into the emergence, evolution and adaptation of SARS-CoV-2 viruses, a comprehensive analysis of genome composition and codon usage of βCoV circulating in China was performed. A biased nucleotide composition was found for SARS-CoV-2 genome. This bias in genomic composition is reflected in its codon and amino acid usage patterns. The overall codon usage in SARS-CoV-2 is similar among themselves and slightly biased. Most of the highly frequent codons are A- and U-ending, which strongly suggests that mutational bias is the main force shaping codon usage in this virus. Significant differences in relative synonymous codon usage frequencies among SARS-CoV-2 and human cells were found. These differences are due to codon usage preferences.

Structure of the zinc-finger antiviral protein in complex with RNA reveals a mechanism for selective targeting of CG-rich viral sequences

Infection of animal cells by numerous viruses is detected and countered by a variety of means, including recognition of nonself nucleic acids. The zinc finger antiviral protein (ZAP) depletes cytoplasmic RNA that is recognized as foreign in mammalian cells by virtue of its elevated CG dinucleotide content compared with endogenous mRNAs. Here, we determined a crystal structure of a protein-RNA complex containing the N-terminal, 4-zinc finger human (h) ZAP RNA-binding domain (RBD) and a CG dinucleotide-containing RNA target. The structure reveals in molecular detail how hZAP is able to bind selectively to CG-rich RNA. Specifically, the 4 zinc fingers create a basic patch on the hZAP RBD surface. The highly basic second zinc finger contains a pocket that selectively accommodates CG dinucleotide bases. Structure guided mutagenesis, cross-linking immunoprecipitation sequencing assays, and RNA affinity assays show that the structurally defined CG-binding pocket is not required for RNA binding per se in human cells. However, the pocket is a crucial determinant of high-affinity, specific binding to CG dinucleotide-containing RNA. Moreover, variations in RNA-binding specificity among a panel of CG-binding pocket mutants quantitatively predict their selective antiviral activity against a CG-enriched HIV-1 strain. Overall, the hZAP RBD RNA structure provides an atomic-level explanation for how ZAP selectively targets foreign, CG-rich RNA.

Human T-cell leukemia virus type 1 Gag domains have distinct RNA-binding specificities with implications for RNA packaging and dimerization

Human T-cell leukemia virus type 1 (HTLV-1) is the first retrovirus that has conclusively been shown to cause human diseases. In HIV-1, specific interactions between the nucleocapsid (NC) domain of the Gag protein and genomic RNA (gRNA) mediate gRNA dimerization and selective packaging; however, the mechanism for gRNA packaging in HTLV-1, a deltaretrovirus, is unclear. In other deltaretroviruses, the matrix (MA) and NC domains of Gag are both involved in gRNA packaging, but MA binds nucleic acids with higher affinity and has more robust chaperone activity, suggesting that this domain may play a primary role. Here, we show that the MA domain of HTLV-1, but not the NC domain, binds short hairpin RNAs derived from the putative gRNA packaging signal. RNA probing of the HTLV-1 5' leader and cross-linking studies revealed that the primer-binding site and a region within the putative packaging signal form stable hairpins that interact with MA. In addition to a previously identified palindromic dimerization initiation site (DIS), we identified a new DIS in HTLV-1 gRNA and found that both palindromic sequences bind specifically the NC domain. Surprisingly, a mutant partially defective in dimer formation in vitro exhibited a significant increase in RNA packaging into HTLV-1-like particles, suggesting that efficient RNA dimerization may not be strictly required for RNA packaging in HTLV-1. Moreover, the lifecycle of HTLV-1 and other deltaretroviruses may be characterized by NC and MA functions that are distinct from those of the corresponding HIV-1 proteins, but together provide the functions required for viral replication.

Retroviral nucleocapsid proteins and DNA strand transfers

An infectious retroviral particle contains 1000-1500 molecules of the nucleocapsid protein (NC) that cover the diploid RNA genome. NC is a small zinc finger protein that possesses nucleic acid chaperone activity that enables NC to rearrange DNA and RNA molecules into the most thermodynamically stable structures usually those containing the maximum number of base pairs. Thanks to the chaperone activity, NC plays an essential role in reverse transcription of the retroviral genome by facilitating the strand transfer reactions of this process. In addition, these reactions are involved in recombination events that can generate multiple drug resistance mutations in the presence of anti-HIV-1 drugs. The strand transfer reactions rely on base pairing of folded DNA/RNA structures. The molecular mechanisms responsible for NC-mediated strand transfer reactions are presented and discussed in this review. Antiretroviral strategies targeting the NC-mediated strand transfer events are also discussed.

Within-patient mutation frequencies reveal fitness costs of CpG dinucleotides and drastic amino acid changes in HIV

HIV has a high mutation rate, which contributes to its ability to evolve quickly. However, we know little about the fitness costs of individual HIV mutations in vivo, their distribution and the different factors shaping the viral fitness landscape. We calculated the mean frequency of transition mutations at 870 sites of the pol gene in 160 patients, allowing us to determine the cost of these mutations. As expected, we found high costs for non-synonymous and nonsense mutations as compared to synonymous mutations. In addition, we found that non-synonymous mutations that lead to drastic amino acid changes are twice as costly as those that do not and mutations that create new CpG dinucleotides are also twice as costly as those that do not. We also found that G→A and C→T mutations are more costly than A→G mutations. We anticipate that our new in vivo frequency-based approach will provide insights into the fitness landscape and evolvability of not only HIV, but a variety of microbes.

Retroviral Gag protein-RNA interactions: Implications for specific genomic RNA packaging and virion assembly

Retroviral Gag proteins are responsible for coordinating many aspects of virion assembly. Gag possesses two distinct nucleic acid binding domains, matrix (MA) and nucleocapsid (NC). One of the critical functions of Gag is to specifically recognize, bind, and package the retroviral genomic RNA (gRNA) into assembling virions. Gag interactions with cellular RNAs have also been shown to regulate aspects of assembly. Recent results have shed light on the role of MA and NC domain interactions with nucleic acids, and how they jointly function to ensure packaging of the retroviral gRNA. Here, we will review the literature regarding RNA interactions with NC, MA, as well as overall mechanisms employed by Gag to interact with RNA. The discussion focuses on human immunodeficiency virus type-1, but other retroviruses will also be discussed. A model is presented combining all of the available data summarizing the various factors and layers of selection Gag employs to ensure specific gRNA packaging and correct virion assembly.

CG dinucleotide suppression enables antiviral defence targeting non-self RNA

Vertebrate genomes exhibit marked CG-suppression, that is lower than expected numbers of 5′-CG-3′ dinucleotides¹. This feature is likely due to C-to-T mutations that have accumulated over hundreds of millions of years, driven by CG-specific DNA methyl transferases and spontaneous methyl-cytosine deamination. Remarkably, many RNA viruses of vertebrates that are not substrates for DNA methyl transferases mimic the CG-suppression of their hosts^2–4. This striking property of viral genomes is unexplained^4–6. In a synonymous mutagenesis experiment, we found that CG-suppression is essential for HIV-1 replication. The deleterious effect of CG dinucleotides on HIV-1 replication was cumulative, evident as cytoplasmic RNA depletion, and exerted by CG dinucleotides in both translated and non-translated exonic RNA sequences. A focused siRNA screen revealed that zinc finger antiviral protein (ZAP)⁷ inhibited virion production by cells infected with CG-enriched HIV-1. Crucially, HIV-1 mutants containing segments whose CG-content mimicked random sequence were defective in unmanipulated cells, but replicated normally in ZAP-deficient cells. Crosslinking-immunoprecipitation-sequencing assays demonstrated that ZAP binds directly and selectively to RNA sequences containing CG dinucleotides. These findings suggest that ZAP exploits host CG-suppression to discriminate non-self RNA. The dinucleotide composition of HIV-1, and perhaps other RNA viruses, appears to have adapted to evade this host defense.

HIV-1 tolerates changes in A-count in a small segment of the pol gene

Background

The HIV-1 RNA genome has a biased nucleotide composition with a surplus of As. Several hypotheses have been put forward to explain this striking phenomenon, but the A-count of the HIV-1 genome has thus far not been systematically manipulated. The reason for this reservation is the likelihood that known and unknown sequence motifs will be affected by such a massive mutational approach, thus resulting in replication-impaired virus mutants. We present the first attempt to increase and decrease the A-count in a relatively small polymerase (pol) gene segment of HIV-1 RNA.

Results

To minimize the mutational impact, a new mutational approach was developed that is inspired by natural sequence variation as present in HIV-1 isolates. This phylogeny-instructed mutagenesis allowed us to create replication-competent HIV-1 mutants with a significantly increased or decreased local A-count. The local A-count of the wild-type (wt) virus (40.2%) was further increased to 46.9% or reduced to 31.7 and 26.3%. These HIV-1 variants replicate efficiently in vitro, despite the fact that the pol changes cause a quite profound move in HIV–SIV sequence space.

Conclusions

Extrapolating these results to the complete 9 kb RNA genome, we may cautiously suggest that the A-rich signature does not have to be maintained. This survey also provided clues that silent codon changes, in particular from G-to-A, determine the subtype-specific sequence signatures.

Biased Nucleotide Composition and Differential Codon Usage Pattern in HIV-1 and HIV-2

HIV-1 and HIV-2 are closely related retroviruses with differences in pathogenicity and geographic distribution. HIV-2 infection is associated with slower disease progression and transmission, longer latency period, low or undetectable plasmatic viral loads, and reduced likelihood of progression to AIDS, compared to HIV-1. In this investigation, we analyzed HIV-2 genes and genomes and compared them with that of HIV-1 belonging to various subtypes. Comparative analysis of the effective number of codons (ENC) for each of the nine genes of the two viruses revealed that the tat gene of HIV-2 had a higher ENC value compared to HIV-1 tat, reflecting lower levels of expression of HIV-2 tat. Lower levels of tat protein particularly during the early stages of infection could result in a lower viral load, lower viral set point, and delayed progression of disease in HIV-2-infected individuals compared to HIV-1-infected subjects. Furthermore, the GC3 composition of the regulatory genes of HIV-2 was ≥50%, suggesting a firm effort by these viruses to adapt themselves to evolutionary survival. We hypothesize that differential codon usage could be one of the possible factors that could contribute to the diminished pathogenicity of HIV-2 in the host as compared to HIV-1.

Sequence-Specific Sensing of Nucleic Acids

Innate immune cells are endowed with many nucleic acid receptors, but the role of sequence in the detection of foreign organisms remains unclear. Can sequence patterns influence recognition? In addition, how can we infer those patterns from sequence data? Here, we detail recent computational and experimental evidence associated with sequence-specific sensing. We review the mechanisms underlying the detection and discrimination of foreign sequences from self. We also describe quantitative approaches used to infer the stimulatory capacity of a given pathogen nucleic acid species, and the influence of sequence-specific sensing on host-pathogen coevolution, including endogenous sequences of foreign origin. Finally, we speculate how further studies of sequence-specific sensing will be useful to improve vaccine design, gene therapy and cancer treatment.

Impact of the biased nucleotide composition of viral RNA genomes on RNA structure and codon usage

We are interested in the influence of nucleotide composition on the fundamental characteristics of the virus RNA genome. Most RNA viruses have genomes with a distinct nucleotide composition, e.g. ranging from minimally 12.9 % to maximally 40.3 % (C- and U-count, respectively, in coronavirus HKU). We present a global analysis of diverse virus types, including plus-strand, minus-strand and double-strand RNA viruses, for the impact of this nucleotide preference on the predicted structure of the RNA genome that is packaged in virion particles and on the codon usage in the viral open reading frames. Several virus-specific features will be described, but also some general conclusions were drawn. Without exception, the virus-specific nucleotide bias was enriched in the unpaired, single-stranded regions of the RNA genome, thus creating an even more striking virus-specific signature. We present a simple mechanism that is based on elementary aspects of RNA structure folding to explain this general trend. In general, the nucleotide bias was the major determinant of the virus-specific codon usages, thus limiting a role for codon selection and translational control. We will discuss molecular and evolutionary scenarios that may be responsible for the diverse nucleotide biases of RNA viruses.

N(6)-methyladenosine of HIV-1 RNA regulates viral infection and HIV-1 Gag protein expression

The internal N⁶-methyladenosine (m⁶A) methylation of eukaryotic nuclear RNA controls post-transcriptional gene expression, which is regulated by methyltransferases (writers), demethylases (erasers), and m⁶A-binding proteins (readers) in cells. The YTH domain family proteins (YTHDF1–3) bind to m⁶A-modified cellular RNAs and affect RNA metabolism and processing. Here, we show that YTHDF1–3 proteins recognize m⁶A-modified HIV-1 RNA and inhibit HIV-1 infection in cell lines and primary CD4⁺ T-cells. We further mapped the YTHDF1–3 binding sites in HIV-1 RNA from infected cells. We found that the overexpression of YTHDF proteins in cells inhibited HIV-1 infection mainly by decreasing HIV-1 reverse transcription, while knockdown of YTHDF1–3 in cells had the opposite effects. Moreover, silencing the m⁶A writers decreased HIV-1 Gag protein expression in virus-producing cells, while silencing the m⁶A erasers increased Gag expression. Our findings suggest an important role of m⁶A modification of HIV-1 RNA in viral infection and HIV-1 protein synthesis.

DOI:http://dx.doi.org/10.7554/eLife.15528.001

HIV infection is a global health challenge. The antiviral drugs that are currently available limit the ability of the virus to multiply in infected individuals, but they rarely eliminate the virus entirely. A better understanding of how HIV behaves in the cell would help researchers to find a cure for persistent HIV infection. When HIV enters an immune cell, its genetic material – in the form of molecules of ribonucleic acid (RNA) – is used as a template to make molecules of DNA. This viral DNA can integrate into the host cell’s DNA, where it is used as a template to make more viral RNA molecules, which are then used to make viral proteins. Some of the viral RNAs are also packaged into new virus particles.

In cells, RNA molecules are often subject to a chemical modification called adenosine methylation, which regulates how that RNA is translated into proteins. Specific enzymes add molecules called methyl tags to particular locations on the RNA, while other enzymes remove them. A family of proteins called YTHDF1–3 recognize and bind to these methyl tags on the RNA and influence how much protein is produced from the target RNA. There is evidence to suggest that the cell can add methyl tags to HIV RNA. However, the extent to which this happens, and what effects this modification has on HIV replication and viral protein production, are not clear.

Tirumuru et al. addressed these questions by analyzing how changing the levels of YTHDF1–3 proteins and the enzymes that add or remove methyl tags in human cells affected HIV infection. The experiments show that YTHDF1–3 proteins inhibited HIV infection in immune cells called T-lymphocytes by recognizing HIV RNA that had been methylated, mainly by targeting the step where the viral RNA is copied into DNA.

Altering the levels of the enzymes that add or remove methyl tags in the cells can change the amount of methyl tags attached to RNA molecules, which alters the amount of HIV protein produced. For example, when more RNA molecules had methyl tags, the cells produced more HIV proteins. These findings suggest that adenosine methylation plays an important role in regulating the ability of HIV to thrive and multiply in T-lymphocytes, which are an important target for HIV. Since the RNAs of other human viruses, such as influenza virus, can also be modified by adenosine methylation, drugs that target this pathway could have the potential to be used to fight a variety of viral illnesses.

DOI:http://dx.doi.org/10.7554/eLife.15528.002

Nucleotide composition of the Zika virus RNA genome and its codon usage

BACKGROUND

RNA viruses have genomes with a distinct nucleotide composition and codon usage. We present the global characteristics of the RNA genome of Zika virus (ZIKV), an emerging pathogen within the Flavivirus genus. ZIKV was first isolated in 1947 in Uganda, caused a widespread epidemic in South and Central America and the Caribbean in 2015 and has recently been associated with microcephaly in newborns.

METHODS

The nearly 11 kb positive-stranded RNA genome of ZIKV was analyzed for its nucleotide composition, also in the context of the folded RNA molecule. Nucleotide trends were investigated along the genome length by skew analyses and we analyzed the codons used for translation of the ZIKV proteins.

RESULTS

ZIKV RNA has a biased nucleotide composition in being purine-rich and pyrimidine-poor. This preference for purines is a general characteristic of the mosquito-borne and tick-borne flaviviruses. The virus-specific nucleotide bias is further enriched in the unpaired, single-stranded regions of the structured ZIKV RNA genome, thus further imposing this ZIKV-specific signature. The codons used for translation of the ZIKV proteins is also unusual, but we show that it is the underlying bias in nucleotide composition of the viral RNA that largely dictates these codon preferences.

CONCLUSIONS

The ZIKV RNA genome has a biased nucleotide composition that dictates the codon usage of this flavivirus. We discuss the evolutionary scenarios and molecular mechanisms that may be responsible for these distinctive ZIKV RNA genome features.

Silent mutations at codons 65 and 66 in reverse transcriptase alleviate indel formation and restore fitness in subtype B HIV-1 containing D67N and K70R drug resistance mutations

Resistance to combined antiretroviral therapy (cART) in HIV-1-infected individuals is typically due to nonsynonymous mutations that change the protein sequence; however, the selection of synonymous or ‘silent’ mutations in the HIV-1 genome with cART has been reported. These silent K65K and K66K mutations in the HIV-1 reverse transcriptase (RT) occur in over 35% of drug-experienced individuals and are highly associated with the thymidine analog mutations D67N and K70R, which confer decreased susceptibility to most nucleoside and nucleotide RT inhibitors. However, the basis for selection of these silent mutations under selective drug pressure is unknown. Using Illumina next-generation sequencing, we demonstrate that the D67N/K70R substitutions in HIV-1 RT increase indel frequency by 100-fold at RT codons 65–67, consequently impairing viral fitness. Introduction of either K65K or K66K into HIV-1 containing D67N/K70R reversed the error-prone DNA synthesis at codons 65–67 in RT and improved viral replication fitness, but did not impact RT inhibitor drug susceptibility. These data provide new mechanistic insights into the role of silent mutations selected during antiretroviral therapy and have broader implications for the relevance of silent mutations in the evolution and fitness of RNA viruses.

On the biased nucleotide composition of the human coronavirus RNA genome

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The nucleotide composition of a coronaviral RNA genome is biased (high U, low C).

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This bias is a relatively stable property along the viral genome, but less prominent in the last 1/3 of the genome.

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This bias is even more pronounced in the single-stranded, unpaired RNA domains.

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The bias dictates the atypical codon usage of the coronaviruses.

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The RNA genome of the zoonotic viruses MERS and SARS is extremely biased.

We investigated the nucleotide composition of the RNA genome of the six human coronaviruses. Some general coronavirus characteristics were apparent (e.g. high U, low C count), but we also detected species-specific signatures. Most strikingly, the high U and low C proportions are quite variable and act like communicating vessels, C goes down when U goes up and vice versa. U ranges among virus isolates from 30.7% to 40.3%, and C makes the opposite movement from 20.0% to 12.9%, respectively. The nucleotide biases are more pronounced in the unpaired regions of the structured RNA genome, which may suggest a certain biological function for these distinctive sequence signatures. Coronaviruses have an atypical codon usage that has been linked to mutational events operating on the viral RNA genome on an evolutionary time scale. We suggest that the atypical nucleotide bias may serve a distinct biological function and that it is the direct cause of the characteristic codon usage in these viruses. The relevance for evolution of the novel human pathogens MERS and SARS is discussed.