A model for the development of the tandem repeat units in the EBV ori-P region and a discussion of their possible function

Horizontal gene transfer and nucleotide compositional anomaly in large DNA viruses

Background

DNA viruses have a wide range of genome sizes (5 kb up to 1.2 Mb, compared to 0.16 Mb to 1.5 Mb for obligate parasitic bacteria) that do not correlate with their virulence or the taxonomic distribution of their hosts. The reasons for such large variation are unclear. According to the traditional view of viruses as gifted "gene pickpockets", large viral genome sizes could originate from numerous gene acquisitions from their hosts. We investigated this hypothesis by studying 67 large DNA viruses with genome sizes larger than 150 kb, including the recently characterized giant mimivirus. Given that horizontally transferred DNA often have anomalous nucleotide compositions differing from the rest of the genome, we conducted a detailed analysis of the inter- and intra-genome compositional properties of these viruses. We then interpreted their compositional heterogeneity in terms of possible causes, including strand asymmetry, gene function/expression, and horizontal transfer.

Results

We first show that the global nucleotide composition and nucleotide word usage of viral genomes are species-specific and distinct from those of their hosts. Next, we identified compositionally anomalous (cA) genes in viral genomes, using a method based on Bayesian inference. The proportion of cA genes is highly variable across viruses and does not exhibit a significant correlation with genome size. The vast majority of the cA genes were of unknown function, lacking homologs in the databases. For genes with known homologs, we found a substantial enrichment of cA genes in specific functional classes for some of the viruses. No significant association was found between cA genes and compositional strand asymmetry. A possible exogenous origin for a small fraction of the cA genes could be confirmed by phylogenetic reconstruction.

Conclusion

At odds with the traditional dogma, our results argue against frequent genetic transfers to large DNA viruses from their modern hosts. The large genome sizes of these viruses are not simply explained by an increased propensity to acquire foreign genes. This study also confirms that the anomalous nucleotide compositions of the cA genes is sometimes linked to particular biological functions or expression patterns, possibly leading to an overestimation of recent horizontal gene transfers.

Sequence requirements of the Epstein-Barr virus latent origin of DNA replication

The Epstein-Barr virus (EBV) latent origin of DNA replication (oriP) is composed of two elements that contain binding sites for the sole viral gene product required for latent cycle replication, EBNA-1. One of these elements, region I, functions as an EBNA-1-dependent enhancer for RNA polymerase II-transcribed genes, may play a role in plasmid segregation, and is required for origin function in B cells latently infected with EBV. The second element, region II, contains or is very near the site of initiation of DNA replication. A genetic approach was taken to determine the contribution of the EBNA-1 binding sites in oriP to origin function. Although region I is required for the transient replication of plasmids bearing region II in EBV-infected B cells, a plasmid lacking region I but containing region II, was observed to replicate transiently in both D98/Raji and HeLa cells expressing EBNA-1. Thus, binding of EBNA-1 to region I is not absolutely required for the molecular events that lead to initiation of DNA replication at region II. Site-directed mutagenesis of the four EBNA-1-binding sites in region II, individually and in various combinations, demonstrated that only two EBNA-1-binding sites are required for region II function. The results obtained with these mutants, together with the analysis of the replicative ability of plasmids containing insertions between EBNA-1-binding sites, suggested that the spatial relationship of the two sites is critical. Mutants that contain only two EBNA-1-binding sites separated by 26 to 31 bp in region II were not maintained as plasmids over many cell generations and were greatly reduced in their ability to replicate transiently in D98/Raji cells. The EBNA-1-induced bending or untwisting of the DNA in EBNA-1-binding sites 1 and 4 in region II did not, however, demonstrate this spatial constraint. It may be concluded from these results that specific protein-protein interactions between EBNA-1 and/or between EBNA-1 and a cellular protein(s) are required for origin function.

Contrasts in codon usage of latent versus productive genes of Epstein-Barr virus: data and hypotheses

Epstein-Barr virus (EBV) has two different modes of existence: latent and productive. There are eight known genes expressed during latency (and hardly at all during the productive phase) and about 70 other ("productive") genes. It is shown that the EBV genes known to be expressed during latency display codon usage strikingly different from that of genes that are expressed during lytic growth. In particular, the percentage of S3 (G or C in codon site 3) is persistently lower (about 20%) in all latent genes than in nonlatent genes. Moreover, S3 is lower in each multicodon amino acid form. Also, the percentage of S in silent codon sites 1 of leucine and arginine is lower in latent than in nonlatent genes. The largest absolute differences in amino acid usage between latent and nonlatent genes emphasize codon types SSN and WWN (W means nucleotide A or T and N is any nucleotide). Two principal explanations to account for the EBV latent versus productive gene codon disparity are proposed. Latent genes have codon usage substantially different from that of host cell genes to minimize the deleterious consequences to the host of viral gene expression during latency. (Productive genes are not so constrained.) It is also proposed that the latency genes of EBV were acquired recently by the viral genome. Evidence and arguments for these proposals are presented.

Plasmid origin of replication of herpesvirus papio: DNA sequence and enhancer function

Herpesvirus papio (HVP) is a lymphotropic virus of baboons which is related to Epstein-Barr virus (EBV) and produces latent infection. The nucleotide sequence of the 5,775-base-pair (bp) EcoRI K fragment of HVP, which has previously been shown to confer the ability to replicate autonomously, has been determined. Within this DNA fragment is a region which bears structural and sequence similarity to the ori-P region of EBV. The HVP ori-P region has a 10- by 26-bp tandem array which is related to the 20- by 30-bp tandem array from the EBV ori-P region. In HVP there is an intervening region of 764 bp followed by five partial copies of the 26-bp monomer. Both the EBV and HVP 3' regions have the potential to form dyad structures which, however, differ in arrangement. We also demonstrate that a transcriptional enhancer which requires transactivation by a virus-encoded factor is present in the HVP ori-P.

Scrambled duplications in the feline leukemia virus gag gene: a putative pattern for molecular evolution

The present study is a detailed computer-assisted analysis of the feline leukemia virus gag gene nucleotide sequence together with its flanking sequences (ST-FeLV GAG) that is compared with the aligned sectors of the Moloney strain of murine leukemia virus (Mo-MuLV GAG) and of three strains of feline sarcoma virus. It shows that perfectly matched repeated oligomers up to 13 nucleotides long are overrepresented and scattered throughout both ST-FeLV GAG and Mo-MuLV GAG, in noncoding and coding sectors, with no stringent correlation to codon usage in ST-FeLV gPr80gag. Many repeated oligomers share a core consensus that is intriguingly part of the inverted repeat at the termini of the long terminal repeat. Local scrambled repetitions of nucleotide subsequences have been found; they suggest a model of molecular evolution by slippage-like mechanisms. Thus, viral genomes could be subject to the same evolutionary mechanisms that are now known to be operating extensively in eukaryotic genomes. The data are discussed in light of putative patterns of molecular evolution.

Isolation of human sequences that replicate autonomously in human cells

We have isolated a heterogeneous collection of human genomic sequences which replicate autonomously when introduced into human cells. The novel strategy for the isolation of these sequences involved cloning random human DNA fragments into a defective Epstein-Barr virus vector. This vector alone was unable to replicate in human cells, but appeared to provide for the nuclear retention of linked DNA. The human sequences persist in a long-term replication assay (greater than 2 months) in the presence of the viral nuclear retention sequences. Using a short-term (4-day) assay, we showed that the human sequences are able to replicate in the absence of all viral sequences. The plasmids bearing human sequences were shown to replicate based on the persistence of MboI-sensitive plasmid DNA in the long-term assay and the appearance of DpnI-resistant DNA in the short-term assay. The human sequences were shown to be responsible for the replication activity and may represent authentic human origins of replication.

Isolation of human sequences that replicate autonomously in human cells

We have isolated a heterogeneous collection of human genomic sequences which replicate autonomously when introduced into human cells. The novel strategy for the isolation of these sequences involved cloning random human DNA fragments into a defective Epstein-Barr virus vector. This vector alone was unable to replicate in human cells, but appeared to provide for the nuclear retention of linked DNA. The human sequences persist in a long-term replication assay (greater than 2 months) in the presence of the viral nuclear retention sequences. Using a short-term (4-day) assay, we showed that the human sequences are able to replicate in the absence of all viral sequences. The plasmids bearing human sequences were shown to replicate based on the persistence of MboI-sensitive plasmid DNA in the long-term assay and the appearance of DpnI-resistant DNA in the short-term assay. The human sequences were shown to be responsible for the replication activity and may represent authentic human origins of replication.

Distinctive charge configurations in proteins of the Epstein-Barr virus and possible functions

The protein products of several open reading frames (ORFs) of the Epstein-Barr virus (EBV) are remarkable in their distribution of charged residues. The nuclear antigen proteins EBNA1-EBNA4 of the EBV latent state contain separate significant clusters of charge of each sign. They (excepting EBNA4) also feature distinctive periodic charge patterns [e.g., (+, O)8, (O, -, -)7] and significant tandem repeats. None of the other ORFs (about 80) of the genome possess the conjunction of these properties. Only the protein encoded from BMLF1, the first immediate early transactivator protein, contains significant multiple charge clusters and periodic charge patterns. All proteins that contain significant repeats also contain at least one significant charge cluster of a single sign. These include EBNA5 and LYDMA produced during latency and BZLF1, whose expression terminates latency and initiates productive growth. It is reasonable to conclude that these aggregate significant charge configurations and repeats are important functionally for the latent existence and for the initiation of the lytic cycle and may be characteristic of these conditions. We discuss how large multimeric protein structures bound together by clusters of unlike charge may provide a mechanism for regulation of the expression of these proteins.