Methylation pattern of fish lymphocystis disease virus DNA

Dinucleotide biases in the genomes of prokaryotic and eukaryotic dsDNA viruses and their hosts

The genomes of cellular organisms display CpG and TpA dinucleotide composition biases. Such biases have been poorly investigated in dsDNA viruses. Here, we show that in dsDNA virus, bacterial, and eukaryotic genomes, the representation of TpA and CpG dinucleotides is strongly dependent on genomic G + C content. Thus, the classical observed/expected ratios do not fully capture dinucleotide biases across genomes. Because a larger portion of the variance in TpA frequency was explained by G + C content, we explored which additional factors drive the distribution of CpG dinucleotides. Using the residuals of the linear regressions as a measure of dinucleotide abundance and ancestral state reconstruction across eukaryotic and prokaryotic virus trees, we identified an important role for phylogeny in driving CpG representation. Nonetheless, phylogenetic ANOVA analyses showed that few host associations also account for significant variations. Among eukaryotic viruses, most significant differences were observed between arthropod-infecting viruses and viruses that infect vertebrates or unicellular organisms. However, an effect of viral DNA methylation status (either driven by the host or by viral-encoded methyltransferases) is also likely. Among prokaryotic viruses, cyanobacteria-infecting phages resulted to be significantly CpG-depleted, whereas phages that infect bacteria in the genera Burkolderia and Staphylococcus were CpG-rich. Comparison with bacterial genomes indicated that this effect is largely driven by the general tendency for phages to resemble the host's genomic CpG content. Notably, such tendency is stronger for temperate than for lytic phages. Our data shed light into the processes that shape virus genome composition and inform manipulation strategies for biotechnological applications.

Hypermethylated genome of a fish vertebrate iridovirus ISKNV plays important roles in viral infection

Iridoviruses are nucleocytoplasmic large dsDNA viruses that infect invertebrates and ectothermic vertebrates. The hypermethylated genome of vertebrate iridoviruses is unique among animal viruses. However, the map and function of iridovirus genomic methylation remain unknown. Herein, the methylated genome of Infectious spleen and kidney necrosis virus (ISKNV, a fish iridovirus), and its role in viral infection, are investigated. The methylation level of ISKNV is 23.44%. The hypermethylated genome is essential for ISKNV amplification, but there is no correlation between hypermethylation and viral gene expression. The hypomethylated ISKNV (obtained via 5-Azacytidine) activates a strong immunoreaction in vitro and reduces its pathogenicity in vivo. The unmethylated viral DNA can induce a stronger immunoreaction in vitro, whereas inactivated hypomethylated ISKNV can induce a stronger immunoreaction in vivo, suggesting ISKNV may evade from immune system by increasing its genome methylation level. Our work provides new insights into the role of genome methylation in viral infection.

Complete genome sequence of a Megalocytivirus (family Iridoviridae) associated with turbot mortality in China

Background

Turbot reddish body iridovirus (TRBIV) causes serious systemic diseases with high mortality in the cultured turbot, Scophthalmus maximus. We here sequenced and analyzed the complete genome of TRBIV, which was identified in Shandong province, China.

Results

The genome of TRBIV is a linear double-stranded DNA of 110,104 base pairs, comprising 55% G + C. Total 115 open reading frames were identified, encoding polypeptides ranging from 40 to 1168 amino acids. Amino acid sequences analysis revealed that 39 of the 115 potential gene products of TRBIV show significant homology to other iridovirus proteins. Phylogenetic analysis of conserved genes indicated that TRBIV is closely related to infectious spleen and kidney necrosis virus (ISKNV), rock bream iridovirus (RBIV), orange-spotted grouper iridovirus (OSGIV), and large yellow croaker iridovirus (LYCIV). The results indicated that TRBIV belongs to the genus Megalocytivirus (family Iridoviridae).

Conclusions

The determination of the genome of TRBIV will provide useful information for comparative study of Megalocytivirus and developing strategies to control outbreaks of TRBIV-induced disease.

Presence and role of cytosine methylation in DNA viruses of animals

Nucleotide composition varies greatly among DNA viruses of animals, yet the evolutionary pressures and biological mechanisms driving these patterns are unclear. One of the most striking discrepancies lies in the frequency of CpG (the dinucleotide CG, linked by a phosphate group), which is underrepresented in most small DNA viruses (those with genomes below 10 kb) but not in larger DNA viruses. Cytosine methylation might be partially responsible, but research on this topic has focused on a few virus groups. For several viruses that integrate their genome into the host genome, the methylation status during this stage has been studied extensively, and the relationship between methylation and viral-induced tumor formation has been examined carefully. However, for actively replicating viruses—particularly small DNA viruses—the methylation status of CpG motifs is rarely known and the effects on the viral life cycle are obscure. In vertebrate host genomes, most cytosines at CpG sites are methylated, which in vertebrates acts to regulate gene expression and facilitates the recognition of unmethylated, potentially pathogen-associated DNA. Here we briefly introduce cytosine methylation before reviewing what is currently known about CpG methylation in DNA viruses.

Evolutionary basis of codon usage and nucleotide composition bias in vertebrate DNA viruses

Understanding the extent and causes of biases in codon usage and nucleotide composition is essential to the study of viral evolution, particularly the interplay between viruses and host cells or immune responses. To understand the common features and differences among viruses we analyzed the genomic characteristics of a representative collection of all sequenced vertebrate-infecting DNA viruses. This revealed that patterns of codon usage bias are strongly correlated with overall genomic GC content, suggesting that genome-wide mutational pressure, rather than natural selection for specific coding triplets, is the main determinant of codon usage. Further, we observed a striking difference in CpG content between DNA viruses with large and small genomes. While the majority of large genome viruses show the expected frequency of CpG, most small genome viruses had CpG contents far below expected values. The exceptions to this generalization, the large gammaherpesviruses and iridoviruses and the small dependoviruses, have sufficiently different life-cycle characteristics that they may help reveal some of the factors shaping the evolution of CpG usage in viruses.

Complete genome sequence analysis of an iridovirus isolated from the orange-spotted grouper, Epinephelus coioides

Orange-spotted grouper iridovirus (OSGIV) was the causative agent of serious systemic diseases with high mortality in the cultured orange-spotted grouper, Epinephelus coioides. Here we report the complete genome sequence of OSGIV. The OSGIV genome consists of 112,636 bp with a G+C content of 54%. 121 putative open reading frames (ORF) were identified with coding capacities for polypeptides varying from 40 to 1168 amino acids. The majority of OSGIV shared homologies to other iridovirus genes. Phylogenetic analysis of the major capsid protein, ATPase, cytosine DNA methyl transferase and DNA polymerase indicated that OSGIV was closely related to infectious spleen and kidney necrosis virus (ISKNV) and rock bream iridovirus (RBIV), but differed from lymphocytisvirus and ranavirus. The determination of the genome of OSGIV will facilitate a better understanding of the molecular mechanism underlying the pathogenesis of the OSGIV and may provide useful information to develop diagnosis method and strategies to control outbreak of OSGIV.

[Red sea bream iridoviral disease]

The first outbreak of red sea bream iridoviral disease caused by red sea bream iridovirus (RSIV) was recorded in cultured red sea bream Pagrus major in Shikoku Island, Japan in 1990. Since 1991, the disease has caused mass mortalities of cultured marine fishes not only red sea bream but also many other species. The affected fish were lethargic and exhibited severe anemia, petechiae of the gills, and enlargement of the spleen. The causative agent was a large, icosahedral, cytoplasmic DNA virus classified as a member of the family Iridoviridae and was designated as red sea bream iridovirus (RSIV). The genome of RSIV is liner dsDNA and considered to be circularly permitted and terminally redundant like other iridoviruses. The length of physical map of RSIV genome is 112,415bp. An indirect immunofluorescence test with a monoclonal antibody and PCR are commonly used for the rapid diagnosis of RSIV infected fish in the field. For the control of this disease, a formalin-killed vaccine against red sea bream iridoviral disease was developed and now commercially available.

Sequence analysis of the complete genome of an iridovirus isolated from the tiger frog

We have isolated a tiger frog virus (TFV) from diseased tiger frogs, Rana tigrina rugulosa. The genome was a linear double-stranded DNA of 105,057 basepairs in length with a base composition of 55.01% G+C. About 105 open reading frames were identified with coding capacities for polypeptides ranging from 40 to 1294 amino acids. Computer-assisted analyses of the deduced amino acid sequences revealed that 39 of 105 putative gene products showed significant homology to functionally characterized proteins of other species in the GenBank/EMBL/DDBJ databases. These proteins included enzymes and structural proteins involved in virus replication, transcription, modification, and virus--host interaction. The deduced amino acid sequences of TFV gene products showed more than 90% identity to FV3, but a low degree of similarity among TFV, ISKNV, and LCDV-1. The results from this study indicated that TFV may belong to the genus Ranavirus of the family Iridoviridae.

Complete genome analysis of the mandarin fish infectious spleen and kidney necrosis iridovirus

The nucleotide sequence of the infectious spleen and kidney necrosis virus (ISKNV) genome was determined and found to comprise 111,362 bp with a G+C content of 54.78%. It contained 124 potential open reading frames (ORFs) with coding capacities ranging from 40 to 1208 amino acids. The analysis of the amino acid sequences deduced from the individual ORFs revealed that 35 of the 124 potential gene products of ISKNV show significant homology to functionally characterized proteins of other species. Some of the putative gene products of ISKNV showed significant homologies to proteins in the GenBank/EMBL/DDBJ databases including enzymes and structural proteins involved in virus replication, transcription, protein modification, and virus-host interaction. In addition, one major repeated sequence showing significant homology to the Red Sea bream iridovirus (RSIV) genome was identified. Based on the information obtained from biological properties (including histopathology, tissue tropisms, natural host range, and geographic distribution), physiochemical and physical properties, and genome analysis, we suggest that ISKNV, RSIV, sea bass iridovirus, grouper iridovirus, and African lampeye iridovirus may belong to a new genus of the Iridoviridae family and are tentatively referred to as cell hypertrophy iridoviruses.

Characterization of repetitive DNA regions and methylated DNA in ascovirus genomes

The accompanying phylogenetic study of large double-stranded DNA viruses based on their delta DNA polymerase genes suggests that ascoviruses (family ASCOVIRIDAE:) and iridoviruses (family IRIDOVIRIDAE:) are closely related and may share a common ancestor. This relationship was unexpected because of marked differences between these viruses. Iridoviruses produce icosahedral virions and occur broadly among vertebrates and invertebrates, whereas ascoviruses typically produce reniform or bacilliform virions and are restricted to insect hosts, primarily lepidopterans. Detailed comparisons of these two virus types are not possible because fundamental information on the properties of the virions and their genomes is lacking, especially for ascoviruses. To facilitate further investigation of the putative evolutionary relationship between ascoviruses and iridoviruses, the genomes of representative viruses from each family were compared with respect to physical configuration, presence of DNA repeats and degree of DNA methylation. Genomes from Spodoptera frugiperda (SfAV1), Heliothis virescens (HvAV3) and Diadromus pulchellus (DpAV4) ascoviruses were all found to be circular and partially superhelical and to contain large interspersed repeats of 1-3 kbp. Mosquito (IV type 3), lepidopteran (IV type 6) and isopod (IV type 31) iridovirus genomes were all linear and lacked large regions of repetitive DNA. Ascovirus and iridovirus genomes were methylated and one, DpAV4, had the highest degree of methylation of any reported animal DNA virus. The major differences in the physical and biochemical characteristics of ascoviruses and iridoviruses reported here provide a foundation for further studies of their relatedness while making their possible close relationship and divergence during evolution of even greater interest.

Chlorella virus NY-2A encodes at least 12 DNA endonuclease/methyltransferase genes

The 380-kb chlorella virus NY-2A genome is highly methylated; 45% of the cytosines are 5-methylcytosine (5mC) and 37% of the adenines are N6-methyladenine (6mA). Based on the sensitivity/resistance of NY-2A DNA to 80 methylation-sensitive DNA restriction endonucleases, the virus is predicted to encode at least 10 DNA methyltransferases: 7 6mA-specific methyltransferases, M.CviQI (GTmAC), M.CvQII (RmAR), M.CviQIII (TCGmA), M.CviQIV (GmATC), M.CviQV (TGCmA), M.CviQVI (GmANTC), and M.CviQVII (CmATG): and 3 5mC-specific methyltransferases, M.CviQVIII [RGmC(T/C/G)], M.CviQIX (mCC), and M.CviQX (mCGR). Five of the 6mA methyltransferase genes, M.CviQI, M.CviQIII, M.CviQV, M.CviQVI, and M.CviQVII, were cloned and sequenced. In addition, 2 site-specific endonuclease activities, R.CviQI (G/TAC) and NY2A-nickase (R/AG), were detected in cell-free extracts from NY-2A virus-infected chlorella. Therefore, the NY-2A genome contains at least 12 DNA methyltransferase and endonuclease genes which, altogether, compose about 3-4% of the virus genome.

Molecular anatomy of lymphocystis disease virus

Lymphocystis disease (LD) has been reported to occur in over one hundred different species of fish worldwide. The disease is caused by lymphocystis disease virus (LCDV), a member of the iridovirus family. Numerous fish species that play an important role in fishery and fish farming are highly susceptible to LCDV infection. The infected animals develop disseminated clusters of aberrant hypertrophied cells within their connective tissue, the so-called lymphocystis cells. In the cytoplasm of these cells a massive accumulation of virions can be observed. As a first step towards understanding the mechanisms of viral infection and pathogenesis the complete genomic nucleotide sequence of lymphocystis disease virus type 1 (LCDV-1; flounder isolate) was determined. LCDV-1 is the type species of the genus Lymphocystivirus within the family Iridoviridae. The virions contain a single linear double-stranded DNA molecule that is circularly permuted, terminally redundant and heavily methylated. Since there is no convenient cell system for virus replication we determined the complete nucleotide sequence of the viral genome (102,653 base pairs). Computer assisted analyses of 195 potential open reading frames resulted in the identification of a number of putative gene products with significant homology to functionally characterized proteins of other species.

The complete DNA sequence of lymphocystis disease virus

Lymphocystis disease virus (LCDV) is the causative agent of lymphocystis disease, which has been reported to occur in over 100 different fish species worldwide. LCDV is a member of the family Iridoviridae and the type species of the genus Lymphocystivirus. The virions contain a single linear double-stranded DNA molecule, which is circularly permuted, terminally redundant, and heavily methylated at cytosines in CpG sequences. The complete nucleotide sequence of LCDV-1 (flounder isolate) was determined by automated cycle sequencing and primer walking. The genome of LCDV-1 is 102.653 bp in length and contains 195 open reading frames with coding capacities ranging from 40 to 1199 amino acids. Computer-assisted analyses of the deduced amino acid sequences led to the identification of several putative gene products with significant homologies to entries in protein data banks, such as the two major subunits of the viral DNA-dependent RNA polymerase, DNA polymerase, several protein kinases, two subunits of the ribonucleoside diphosphate reductase, DNA methyltransferase, the viral major capsid protein, insulin-like growth factor, and tumor necrosis factor receptor homolog.

Identification of the gene encoding the DNA (cytosine-5) methyltransferase of lymphocystis disease virus

The gene encoding the DNA (cytosine-5) methyltransferase (m5C-MTase) of lymphocystis disease virus (flounder isolate, LCDV-1) has been identified by polymerase chain reaction (PCR) using oligonucleotide primers synthesized corresponding to different regions of the m5C-MTase gene of frog virus 3 (FV3). A DNA fragment of 487 bp was amplified using oligonucleotide primers L3 and R4 which correspond to the nucleotide positions 87 to 109 and 530 to 550 of the m5C-MTase gene of FV3, respectively. The DNA nucleotide sequence of the PCR product was determined by direct cycle sequencing. The alignment of the deduced amino acid sequence derived from the PCR product and the m5C-MTase protein of FV3 revealed a homology of 55.4% identity and 29.1% similarity. The amino acid sequence which was found to be significantly homologous to the amino acid sequence deduced from the nucleotide sequence of the PCR product was located at the amino acid position 37 to 175 of the m5C-MTase of FV3 indicating the specificity of the amplified PCR product. The DNA nucleotide sequence of the LCDV-1 genome corresponding to the 5' and 3' termini of the m5C-MTase gene was determined by primer walking. The locus of the m5C-MTase gene of LCDV-1 was identified within the EcoRI DNA fragment G of LCDV-1 (7.9 kbp; 0.947 to 0.034 map units). The m5C-MTase gene of LCDV-1 comprises 684 nucleotides coding for a putative protein of 228 amino acid residues. A high degree of amino acid sequence homology (53.3% identity and 25.8% similarity) was detected between the m5C-MTase of LCDV-1 and FV3.

Patterns of frog virus 3 DNA methylation and DNA methyltransferase activity in nuclei of infected cells

The iridovirus frog virus 3 (FV3) can replicate in culture in fat head minnow (FHM) fish cells or in BHK-21 hamster cells. Viral DNA replication commences about 3 h after infection of FHM cells with FV3. Between 3 and 6 h postinfection (p.i.), a portion of the intranuclear FV3 DNA is partly unmethylated. At later times, p.i., all of the viral DNA in the nuclear and cytoplasmic compartments is methylated at the 5'-CCGG-3' sequences. Cytoplasmic FV3 DNA has not been found unmethylated. We have cloned viral DNA fragments from methylated virion DNA. By using the genomic sequencing technique, it has been demonstrated for segments of the FV3 DNA replicated both in FHM fish and BHK21 hamster cells that in a stretch encompassing a total of 350 bp, all of the analyzed 5'-CG-3' dinucleotides are methylated. The modified nucleotide 5-methyldeoxycytidine is present exclusively in the 5'-CG-3' dinucleotide combination. In the cloned FV3 DNA fragment p21A, an open reading frame has been located. The 5' region of this presumptive viral gene is also methylated in all 5'-CG-3' positions. DNA methyltransferase activity has been detected in the nuclei of FV3-infected FHM cells at 4, 11, and 20 h p.i. In the cytoplasmic fraction, comparable activity has not been observed. These data are consistent with the interpretation that FV3 DNA is newly synthesized and de novo methylated in the nuclei of infected FHM cells and subsequently exported into the cytoplasm for viral assembly.

Characterization of the repetitive DNA elements in the genome of fish lymphocystis disease viruses

The complete DNA nucleotide sequence of the repetitive DNA elements in the genome of fish lymphocystis disease virus (FLDV) isolated from two different species (flounder and dab) was determined. The size of these repetitive DNA elements was found to be 1413 bp which corresponds to the DNA sequences of the 5' terminus of the EcoRI DNA fragment B (0.034 to 0.052 m.u.) and to the EcoRI DNA fragment M (0.718 to 0.736 m.u.) of the FLDV genome causing lymphocystis disease in flounder and plaice. The degree of DNA nucleotide homology between both regions was found to be 99%. The repetitive DNA element in the genome of FLDV isolated from other fish species (dab) was identified and is located within the EcoRI DNA fragment B and J of the viral genome. The DNA nucleotide sequence of one duplicate of this repetition (EcoRI DNA fragment J) was determined (1410 bp) and compared to the DNA nucleotide sequences of the repetitive DNA elements of the genome of FLDV isolated from flounder. It was found that the repetitive DNA elements of the genome of FLDV derived from two different fish species are highly conserved and possess a degree of DNA sequence homology of 94%. The DNA sequences of each strand of the individual repetitive element possess one open reading frame.

Transcription of methylated viral DNA by eukaryotic RNA polymerase II

The genome of the large icosahedral DNA virus, frog virus 3 (FV3), is heavily methylated at the cytosine residues of dCdG dinucleotide pairs, with more than 22% of the total cytosine residues in the form of 5-methylcytosine (5mC). This methylation is carried out postreplicatively in the cytoplasm of infected cells by a virus-encoded DNA methyltransferase. DNA methyltransferase activity was shown to copurify with a 26 kD virus-induced, DNA-binding protein that had an altered mobility in extracts from cells infected with a DNA-methyl-transferase deficient mutant of FV3. Immediately after infection, the highly methylated parental DNA is transcribed in the nucleus by the host cell RNA polymerase II. As FV3 induces the synthesis of a protein that can override the inhibitory effect of methylation on the transcription of exogenous promoters methylation in vitro, we suggest that this protein is a factor evolved by this virus to allow transcription from methylated promoters by eukaryotic RNA polymerase II.

Non-C-G recognition sequences of DNA cytosine-5-methyltransferase from rat liver

The eukaryotic DNA cytosine-5-methyltransferase (E.C.2.1.1.37) is known to methylate cytosine in DNA mainly, but not exclusively in C-G. In the present study the minor, non-C-G recognition sequences of a rat DNA methyltransferase were analyzed by Maxam-Gilbert sequencing of in vitro methylated SV40 DNA. The enzyme methylates C-A and C-T at a 50-fold lower initial rate than C-G. Methylation of C-C at the 5'C was not observed in the piece of DNA sequenced. The methylation of C-A is very low in the trinucleotides ACA and CAC, the other C-A containing trinucleotides in DNA are much better methylacceptors. C-T was found methylated predominantly in the sequences CCTAA, ACTAA, and ACTGT. A comparison of the activity with different substrates is in favour of the enzyme making its recognition in the major groove of the DNA.

A comparison of viruses infecting two different Chlorella-like green algae

Five plaque-forming viruses (Pbi viruses) of the unicellular, eukaryotic, exsymbiotic Chlorella-like green alga strain Pbi were isolated from fresh water collected in Germany. The viruses were compared to two previously characterized plaque-forming viruses (NC64A viruses) of Chlorella strain NC64A. The Pbi viruses do not infect Chlorella NC64A and vice versa. Like the NC64A viruses the Pbi viruses are large polyhedron with a diameter of 140 to 150 nm, are chloroform sensitive, have many structural proteins, and have large dsDNA genomes of at least 300 kb. However, the Pbi viruses are serologically distinct from the NC64A viruses. The five Pbi virus genomes contain 5-methylcytosine, which varied from 14.2 to 43.1% of the cytosine, and two of them also contained N6-methyladenine. DNAs from the Pbi viruses hybridized poorly with the two NC64A virus DNAs and they have a higher guanine plus cytosine content (ca. 46%) than the NC64A virus DNAs (ca. 40%).

Identification, mapping and cloning of the thymidine kinase gene of fish lymphocystis disease virus

The thymidine kinase (TK) gene of fish lymphocystis disease virus (FLDV) was identified by biochemical transformation of 3T3 TK negative (TK-) to 3T3 TK positive (TK+) cells using specific viral DNA sequences. DNA fragments of the viral genome used in this study were obtained from a defined gene library of FLDV genome containing the complete viral DNA sequences. The selection of the converted cells was carried out under the condition of the HAT selection procedure. The results of these experiments revealed that the EcoRI FLDV DNA fragment C (11.2 kbp; 0.611 to 0.718 map units) is able to transform 3T3 TK- to 3T3 TK+ cells. Additional experiments using the subclones of EcoRI DNA fragment C revealed that DNA sequences of 4.1 kbp size between the coordinates 0.669 to 0.718 of the FLDV genome possessed the ability for biochemical transformation, indicating that the TK gene locus is located in this particular region.

Identification and nucleotide sequence analysis of the repetitive DNA element in the genome of fish lymphocystis disease virus

The genome of the fish lymphocystis disease virus (FLDV) was screened for the existence of repetitive DNA sequences using a defined and complete gene library of the viral genome (98 kbp) by DNA-DNA hybridization, heteroduplex analysis, and restriction fine mapping. A repetitive DNA sequence was detected at the coordinates 0.034 to 0.057 and 0.718 to 0.736 map units (m.u.) of the FLDV genome. The first region (0.034 to 0.057 m.u.) corresponds to the 5' terminus of the EcoRI FLDV DNA fragment B (0.034 to 0.165 m.u.) and the second region (0.718 to 0.736 m.u.) is identical to the EcoRI DNA fragment M of the viral genome. The DNA nucleotide sequence of the EcoRI FLDV DNA fragment M was determined. This analysis revealed the presence of many short direct and inverted repetitions, e.g., a 18-mer direct repetition (TTTAAAATTTAATTAA) that started at nucleotide positions 812 and 942 and a 14-mer inverted repeat (TTAAATTTAAATTT) at nucleotide positions 820 and 959. Only short open reading frames were detected within this region. The DNA repetitions are discussed as sequences that play a possible regulatory role for virus replication. Furthermore, hybridization experiments revealed that the repetitive DNA sequences are conserved in the genome of different strains of fish lymphocystis disease virus isolated from two species of Pleuronectidae (flounder and dab).

DNA methyltransferase induced by PBCV-1 virus infection of a Chlorella-like green alga

A DNA methyltransferase was isolated from a eucaryotic, Chlorella-like green alga infected with the virus PBCV-1. The enzyme recognized the sequence GATC and methylated deoxyadenosine solely in GATC sequences. Host DNA, which contains GATC sequences, but not PBCV-1 DNA, which contains GmATC sequences, was a good substrate for the enzyme in vitro. The DNA methyltransferase activity was first detected about 1 h after viral infection; PBCV-1 DNA synthesis and host DNA degradation also began at about this time. The appearance of the DNA methyltransferase activity required de novo protein synthesis, and the enzyme was probably virus encoded. Methylation of DNAs with the PBCV-1-induced methyltransferase conferred resistance of the DNAs to a PBCV-1-induced restriction endonuclease enzyme described previously (Y. Xia, D. E. Burbank, L. Uher, D. Rabussay, and J. L. Van Etten, Mol. Cell. Biol. 6:1430-1439). We propose that the PBCV-1-induced methyltransferase protects viral DNA from the PBCV-1-induced restriction endonuclease and is part of a virus-induced restriction and modification system in PBCV-1-infected Chlorella cells.

DNA methyltransferase induced by PBCV-1 virus infection of a Chlorella-like green alga

A DNA methyltransferase was isolated from a eucaryotic, Chlorella-like green alga infected with the virus PBCV-1. The enzyme recognized the sequence GATC and methylated deoxyadenosine solely in GATC sequences. Host DNA, which contains GATC sequences, but not PBCV-1 DNA, which contains GmATC sequences, was a good substrate for the enzyme in vitro. The DNA methyltransferase activity was first detected about 1 h after viral infection; PBCV-1 DNA synthesis and host DNA degradation also began at about this time. The appearance of the DNA methyltransferase activity required de novo protein synthesis, and the enzyme was probably virus encoded. Methylation of DNAs with the PBCV-1-induced methyltransferase conferred resistance of the DNAs to a PBCV-1-induced restriction endonuclease enzyme described previously (Y. Xia, D. E. Burbank, L. Uher, D. Rabussay, and J. L. Van Etten, Mol. Cell. Biol. 6:1430-1439). We propose that the PBCV-1-induced methyltransferase protects viral DNA from the PBCV-1-induced restriction endonuclease and is part of a virus-induced restriction and modification system in PBCV-1-infected Chlorella cells.

Characterization of viruses infecting a eukaryotic Chlorella-like green alga

Nineteen plaque-forming viruses of the unicellular, eukaryotic Chlorella-like green alga, strain NC64A, were isolated from various geographic regions in the United States and characterized. Like the previously described virus, PBCV-1, all of the new viruses were large polyhedrons, sensitive to chloroform, and contained large dsDNA genomes of ca. 300 kbp. All of the viral DNAs contained 5-methyldeoxycytidine which varied from 0.1 to 47% of the deoxycytidine. In addition, 10 of the viral DNAs contained N6-methyldeoxyadenosine which varied from 8.1 to 37% of the deoxyadenosine. These viruses, along with 11 previously described viruses which replicate in the same Chlorella host, were grouped into 11 classes based on at least one of the following properties: plaque size, reaction with PBCV-1 antiserum, or the nature and abundance of methylated bases in their genomic DNA.

Insect iridescent virus type 6 induced toxic degenerative hepatitis in mice

The toxic effect of insect iridescent virus type 6 - chilo iridescent virus - (CIV) was investigated using Balb/c mice (strain ByJ Ico and Kisslegg). The animals were inoculated with CIV intraperitoneally (1 X 10(9) to 9.2 X 10(11) TCID50/animal). The animals which were administered with 1 X 10(11) to 9 X 10(11) TCID50 of CIV per animal, developed acute clinical illness and died during 18 to 80 h post infection. Histopathological and electronmicroscopic examinations of the liver tissues of those animals which died and/or were sacrificed when moribund showed acute degenerative hepatitis leading to death. No evidence for viral replication was found in the liver cells affected. A mortality rate between 21.1% and 100% was recorded for CIV, depending on the strain and number of mice used and the dose of virus administered. The toxic effect of CIV was eliminated or reduced extensively using heat denaturation or treatment of CIV with sodium dodecylsulphate or proteinase K. This indicates that the nature of the factor causing toxic degenerative cell damage is a protein.

Molecular cloning and physical mapping of the genome of fish lymphocystis disease virus

A defined and complete gene library of the fish lymphocystis disease virus (FLDV) genome was established. FLDV DNA was cleaved with EcoRI, BamHI, EcoRI/BamHI and EcoRI/HindIII and the resulting fragments were inserted into the corresponding sites of the pACYC184 or pAT153 plasmid vectors using T4 DNA ligase. Since FLDV DNA is highly methylated at CpG sequences (Darai et al., 1983; Wagner et al., 1985), an Escherichia coli GC-3 strain was required to amplify the recombinant plasmids harboring the FLDV DNA fragments. Bacterial colonies harboring recombinant plasmids were selected. All cloned fragments were individually identified by digestion of the recombinant plasmid DNA with different restriction enzymes and screened by hybridization of recombinant plasmid DNA to viral DNA. This analysis revealed that sequences representing 100% of the viral genome were cloned. Using these recombinant plasmids, the physical maps of the genome were constructed for BamHI, EcoRI, BestEII, and PstI restriction endonucleases. Although the FLDV genome is linear, due to circular permutation the restriction maps are circular.