The myxoma virus EcoRI-O fragment encodes the DNA binding core protein and the major envelope protein of extracellular poxvirus

The genome sequence of Yaba-like disease virus, a yatapoxvirus

The genome sequence of Yaba-like disease virus (YLDV), an unclassified member of the yatapoxvirus genus, has been determined. Excluding the terminal hairpin loops, the YLDV genome is 144,575 bp in length and contains inverted terminal repeats (ITRs) of 1883 bp. Within 20 nucleotides of the termini, there is a sequence that is conserved in other poxviruses and is required for the resolution of concatemeric replicative DNA intermediates. The nucleotide composition of the genome is 73% A+T, but the ITRs are only 63% A+T. The genome contains 151 tightly packed open reading frames (ORFs) that either are > or =180 nucleotides in length or are conserved in other poxviruses. ORFs within 23 kb of each end are transcribed toward the termini, whereas ORFs within the central region of the genome are encoded on either DNA strand. In the central region ORFs have a conserved position, orientation, and sequence compared with vaccinia virus ORFs and encode many enzymes, transcription factors, or structural proteins. In contrast, ORFs near the termini are more divergent and in seven cases are without counterparts in other poxviruses. The YLDV genome encodes several predicted immunomodulators; examples include two proteins with similarity to CC chemokine receptors and predicted secreted proteins with similarity to MHC class I antigen, OX-2, interleukin-10/mda-7, poxvirus growth factor, serpins, and a type I interferon-binding protein. Phylogenic analyses indicated that YLDV is very closely related to yaba monkey tumor virus, but outside the yatapoxvirus genus YLDV is more closely related to swinepox virus and leporipoxviruses than to other chordopoxvirus genera.

Immunomodulatory proteins of myxoma virus

Poxviruses collectively encode an impressive collection of diverse immunomodulatory proteins. In this review we draw attention to some of the new open reading frames (ORFs) discovered during the sequencing of the myxoma virus DNA genome [Cameron C, Hota-Mitchell S, Chen L, Barrett J, Cao J-X, Macaulay C, Willer D, Evans D, McFadden G (1999) The complete DNA sequence of myxoma virus. Virology 264:298-318] that may function to subvert the host immune system. Most of these predicted functions are speculative but some of the deduced primary amino acid sequences contain intriguing similarities to known cellular and viral proteins in the public domain for which immunomodulatory functions have been assigned.

Sequence and analysis of a swinepox virus homologue of the vaccinia virus major envelope protein P37 (F13L)

P37 (F13L gene product), the most abundant protein in the envelope of the extracellular virus form of the prototype poxvirus, vaccinia virus (VV), is a crucial player in the process leading to acquisition of the envelope, virus egress and transmission. We have cloned and sequenced a swinepox virus (SPV) gene homologous to VV F13L. The SPV gene product, termed P42, was 54% identical to P37, the VV F13L gene product, and, among the poxviruses, was most similar (73% identity) to the myxoma virus homologue. The SPV P42 gene contained late transcription signals and was expressed only at late times during infection. The protein was palmitylated, and showed an intracellular distribution similar to that of VV P37, both by immunofluorescence and by subcellular fractionation. As with VV P37, SPV P42 was incorporated in extracellular enveloped SPV particles, but was absent from the intracellular mature virus form. To check the ability of SPV P42 to function in the context of VV infection, we inserted the SPV gene into a VV deficient in P37, which is severely blocked in virus envelopment and cell-to-cell transmission. Despite correct expression of SPV P42, the resulting recombinant VV showed no rescue of extracellular virus formation or cell-to-cell virus spread. The lack of function of SPV P42 in the VV genetic background suggests that specific interactions between SPV P42 or VV P37 and other viral proteins is required to drive the envelopment process.

The complete DNA sequence of myxoma virus

Myxomatosis in European rabbits is a severely debilitating disease characterized by profound systemic cellular immunosuppression and a high rate of mortality. The causative agent, myxoma virus, is a member of the poxvirus family and prototype of the Leporipoxvirus genus. As a major step toward defining the genetic strategies by which the virus circumvents host antiviral responses, the genomic DNA sequence of myxoma virus, strain Lausanne, was determined. A total of 171 open reading frames were assigned to cover the 161.8-kb genome, including two copies each of the 12 genes that map within the 11.5-kb terminal inverted repeats. Database searches revealed a central core of approximately 120 kb that encodes more than 100 genes that exhibit close relationships to the conserved genes of members of other poxvirus genera. Open reading frames with predicted signal sequences, localization motifs, or homology to known proteins with immunomodulatory or host-range functions were examined more extensively for predicted features such as hydrophobic regions, nucleic acid binding domains, ankyrin repeats, serpin signatures, lectin domains. and structural cysteine spacings. As a result, several novel, potentially immunomodulatory proteins have been identified, including a family with multiple ankyrin-repeat domains, an OX-2 like member of the neural cell adhesion molecule family, a third myxoma serpin, a putative chemokine receptor fragment, two natural killer receptor-like species, and a variety of species with domains closely related to diverse host immune regulatory proteins. Coupled with the genomic sequencing of the related leporipoxvirus Shope fibroma virus, this work affirms the existence of a conserved complement of poxvirus-specific core genes and expands the growing repertoire of virus genes that confer the unique capacity of each poxvirus family member to counter the immune responses of the infected host.

The complete genome sequence of shope (rabbit) fibroma virus

We have determined the complete DNA sequence of the Leporipoxvirus Shope fibroma virus (SFV). The SFV genome spans 159.8 kb and encodes 165 putative genes of which 13 are duplicated in the 12.4-kb terminal inverted repeats. Although most SFV genes have homologs encoded by other Chordopoxvirinae, the SFV genome lacks a key gene required for the production of extracellular enveloped virus. SFV also encodes only the smaller ribonucleotide reductase subunit and has a limited nucleotide biosynthetic capacity. SFV preserves the Chordopoxvirinae gene order from S012L near the left end of the chromosome through to S142R (homologs of vaccinia F2L and B1R, respectively). The unique right end of SFV appears to be genetically unstable because when the sequence is compared with that of myxoma virus, five myxoma homologs have been deleted (C. Cameron, S. Hota-Mitchell, L. Chen, J. Barrett, J.-X. Cao, C. Macaulay, D. Willer, D. Evans, and G. McFadden, 1999, Virology 264, 298-318). Most other differences between these two Leporipoxviruses are located in the telomeres. Leporipoxviruses encode several genes not found in other poxviruses including four small hydrophobic proteins of unknown function (S023R, S119L, S125R, and S132L), an alpha 2, 3-sialyltransferase (S143R), a protein belonging to the Ig-like protein superfamily (S141R), and a protein resembling the DNA-binding domain of proteins belonging to the HIN-200 protein family S013L). SFV also encodes a type II DNA photolyase (S127L). Melanoplus sanguinipes entomopoxvirus encodes a similar protein, but SFV is the first mammalian virus potentially capable of photoreactivating ultraviolet DNA damage.

Myxoma virus encodes an alpha2,3-sialyltransferase that enhances virulence

A 4.7-kb region of DNA sequence contained at the right end of the myxoma virus EcoRI-G2 fragment located 24 kb from the right end of the 163-kb genome has been determined. This region of the myxoma virus genome encodes homologs of the vaccinia virus genes A51R, A52R, A55R, A56R, and B1R; the myxoma virus gene equivalents have been given the prefix M. The MA55 gene encodes a protein belonging to the kelch family of actin-binding proteins, while the MA56 gene encodes a member of the immunoglobulin superfamily related to a variety of cellular receptors and adhesion molecules. A novel myxoma virus early gene, MST3N, is a member of the eukaryotic sialyltransferase gene family located between genes MA51 and MA52. Detergent lysates prepared from myxoma virus-infected cell cultures contained a virally encoded sialyltransferase activity that catalyzed the transfer of sialic acid (Sia) from CMP-Sia to an asialofetuin glycoprotein acceptor. Analysis of the in vitro-sialylated glycoprotein acceptor by digestion with N-glycosidase F and by lectin binding suggested that the MST3N gene encodes an enzyme with Galbeta1,3(4)GlcNAc alpha2,3-sialyltransferase specificity for the N-linked oligosaccharide of glycoprotein. Lectin binding assays demonstrated that alpha2,3-sialyltransferase activity is expressed by several known leporipoxviruses that naturally infect Sylvilagus rabbits. The sialyltransferase is nonessential for myxoma virus replication in cell culture; however, disruption of the MST3N gene caused attenuation in vivo. The possible implications of the myxoma virus-expressed sialyltransferase in terms of the host's defenses against infection are discussed.