Reticuloendotheliosis type C and primate type D oncoretroviruses are members of the same receptor interference group.

The reticuloendotheliosis viruses (REVs), originally isolated from avian species, constitute a group of retroviruses which are more closely related to mammalian retroviruses than to other avian retroviruses. The envelope glycoproteins of members of the REV group display a striking amino acid sequence identity with a group of primate oncoretroviruses which belong to a single receptor interference group and include all of the type D and some type C primate oncoretroviruses. Members of the REV group also have a broad host range which covers most avian cells and some mammalian cells, including those of simian and human origin. In view of this broad host range and the envelope sequence similarities, we investigated the cross-interference pattern between REV and primate virus groups to determine whether they utilized the same receptor. Superinfection experiments using a vector virus containing an Escherichia coli lacZ gene showed that reticuloendotheliosis and simian oncoretroviruses constitute a single receptor interference group on both human and canine cells and indicate that the viruses bind to the same receptor to initiate infection. These results suggest that this receptor binding specificity has been maintained over a wide range of retroviruses and may be responsible for the broad spread of these retroviruses between different orders of vertebrates.     

The Extraordinary Evolutionary History of the Reticuloendotheliosis Viruses

The reticuloendotheliosis viruses (REVs) comprise several closely related amphotropic retroviruses isolated from birds. These viruses exhibit several highly unusual characteristics that have not so far been adequately explained, including their extremely close relationship to mammalian retroviruses, and their presence as endogenous sequences within the genomes of certain large DNA viruses. We present evidence for an iatrogenic origin of REVs that accounts for these phenomena. Firstly, we identify endogenous retroviral fossils in mammalian genomes that share a unique recombinant structure with REVs—unequivocally demonstrating that REVs derive directly from mammalian retroviruses. Secondly, through sequencing of archived REV isolates, we confirm that contaminated Plasmodium lophurae stocks have been the source of multiple REV outbreaks in experimentally infected birds. Finally, we show that both phylogenetic and historical evidence support a scenario wherein REVs originated as mammalian retroviruses that were accidentally introduced into avian hosts in the late 1930s, during experimental studies of P. lophurae, and subsequently integrated into the fowlpox virus (FWPV) and gallid herpesvirus type 2 (GHV-2) genomes, generating recombinant DNA viruses that now circulate in wild birds and poultry. Our findings provide a novel perspective on the origin and evolution of REV, and indicate that horizontal gene transfer between virus families can expand the impact of iatrogenic transmission events.     

A universal BMV-based RNA recombination system--how to search for general rules in RNA recombination

At present, there is no doubt that RNA recombination is one of the major factors responsible for the generation of new RNA viruses and retroviruses. Numerous experimental systems have been created to investigate this complex phenomenon. Consequently, specific RNA structural motifs mediating recombination have been identified in several viruses. Unfortunately, up till now a unified model of genetic RNA recombination has not been formulated, mainly due to difficulties with the direct comparison of data obtained for different RNA-based viruses. To solve this problem, we have attempted to construct a universal system in which the recombination activity of various RNA sequences could be tested. To this end, we have used brome mosaic virus, a model (+)RNA virus of plants, for which the structural requirements of RNA recombination are well defined. The effectiveness of the new homomolecular system has been proven in an experiment involving two RNA sequences derived from the hepatitis C virus genome. In addition, comparison of the data obtained with the homomolecular system with those generated earlier using the heteromolecular one has provided new evidence that the mechanisms of homologous and non-homologous recombination are different and depend on the virus' mode of replication.     

Nucleotide Sequences Derived from Pheasant DNA in the Genome of Recombinant Avian Leukosis Viruses with Subgroup F Specificity

Recombination between viral and cellular genes can give rise to new strains of retroviruses. For example, Rous-associated virus 61 (RAV-61) is a recombinant between the Bryan high-titer strain of Rous sarcoma virus (RSV) and normal pheasant DNA. Nucleic acid hybridization techniques were used to study the genome of RAV-61 and another RAV with subgroup F specificity (RAV-F) obtained by passage of RSV-RAV-0 in cells from a ring-necked pheasant embryo. The nucleotide sequences acquired by these two independent isolates of RAV-F that were not shared with the parental virus comprised 20 to 25% of the RAV-F genomes and were indistinguishable by nucleic acid hybridization. (In addition, RAV-F genomes had another set of nucleotide sequences that were homologous to some pheasant nucleotide sequences and also were present in the parental viruses.) A specific complementary DNA, containing only nucleotide sequences complementary to those acquired by RAV-61 through recombination, was prepared. These nucleotide sequences were pheasant derived and were not present in the genomes of reticuloendotheliosis viruses, pheasant viruses, and avian leukosis-sarcoma viruses of subgroups A, B, C, D, and E. They were partially endogenous, however, to avian DNA other than pheasant. The fraction of these nucleotide sequences present in other avian DNAs generally paralleled the genetic relatedness of these avian species to pheasants. However, there was a high degree of homology between these pheasant nucleotide sequences and related nucleotide sequences in the DNA of normal chickens as indicated by the identical melting profiles of the respective hybrids.     

Are RNA Viruses Adapting or Merely Changing?

RNA viruses and retroviruses fix substitutions approximately 1 million-fold faster than their hosts. This diversification could represent an inevitable drift under purifying selection, the majority of substitutions being phenotypically neutral. The alternative is to suppose that most fixed mutations are beneficial to the virus, allowing it to keep ahead of the host and/or host population. Here, relative sequence diversification of different proteins encoded by viral genomes is found to be linear. The examples encompass a wide variety of retroviruses and RNA viruses. The smoothness of relative divergence spans quasispeciation following clonal infection, to variation among different isolates of the same virus, to viruses from different species or those associated with different diseases, indicating that the majority of fixed mutations likely reflects drift. This held for both mammalian and plant viruses, indicating that adaptive immunity doesn't necessarily shape the relative accumulation of amino acid substitutions. When compared to their hosts RNA viruses evolution appears conservative. Received: 16 November 1999 / Accepted: 10 March 2000     

Conserved Region of Mammalian Retrovirus RNA

The viral RNAs of various mammalian retroviruses contain highly conserved sequences close to their 3' ends. This was demonstrated by interviral molecular hybridization between fractionated viral complementary DNA (cDNA) and RNA. cDNA near the 3' end (cDNA(3')) from a rat virus (RPL strain) was fractionated by size and mixed with mouse virus RNA (Rauscher leukemia virus). No hybridization occurred with total cDNA (cDNA(total)), in agreement with previous results, but a cross-reacting sequence was found with the fractionated cDNA(3'). The sequences between 50 to 400 nucleotides from the 3' terminus of heteropolymeric RNA were most hybridizable. The rat viral cDNA(3') hybridized with mouse virus RNA more extensively than with RNA of remotely related retroviruses. The related viral sequence of the rodent viruses (mouse and rat) showed as much divergence in heteroduplex thermal denaturation profiles as did the unique sequence DNA of these two rodents. This suggests that over a period of time, rodent viruses have preserved a sequence with changes correlated to phylogenetic distance of hosts. The cross-reacting sequence of replication-competent retroviruses was conserved even in the genome of the replication-defective sarcoma virus and was also located in these genomes near the 3' end of 30S RNA. A fraction of RD114 cDNA(3'), corresponding to the conserved region, cross-hybridized extensively with RNA of a baboon endogenous virus (M7). Fractions of similar size prepared from cDNA(3') of MPMV, a primate type D virus, hybridized with M7 RNA to a lesser extent. Hybridization was not observed between Mason-Pfizer monkey virus and M7 if total cDNA's were incubated with viral RNAs. The degree of cross-reaction of the shared sequence appeared to be influenced by viral ancestral relatedness and host cell phylogenetic relationships. Thus, the strikingly high extent of cross-reaction at the conserved region between rodent viruses and simian sarcoma virus and between baboon virus and RD114 virus may reflect ancestral relatedness of the viruses. Slight cross-reaction at the site between type B and C viruses of rodents (mouse mammary tumor virus and RPL virus, 58-2T) or type C and D viruses of primates (M7, RD114, and Mason-Pfizer monkey virus) may have arisen at the conserved region through a mechanism that depends more on the phylogenetic relatedness of the host cells than on the viral type or origin. Determining the sequence of the conserved region may help elucidate this mechanism. The conserved sequences in retroviruses described here may be an important functional unit for the life cycle of many retroviruses.     

Serological definition of the lentivirus group of retroviruses.

The major polypeptides of visna viruses and other lentiviruses have been isolated and shown to be closely related if not identical in radioimmunoassays. By this criterion the lentiviruses form a distinct group of retroviruses unrelated to spuma viruses, mammalian and avian retroviruses that cause tumors, and unclassified retroviruses of cattle and horses. Two sera obtained from goats immunized with Mason-Pfizer monkey virus or squirrel monkey virus reacted with visna p30. Additional data suggest that this reaction represents infection of goats with a lentivirus or a new retrovirus closely related to the lentiviruses.     

泡沫病毒Gag蛋白的研究进展

Gag蛋白在逆转录病毒复制周期的许多阶段中发挥重要作用。泡沫病毒基因组结构与其它逆转录病毒类似,但它们的基因组成分和生活周期存在明显差异,这在一定程度上是由其Gag蛋白的结构和功能所决定的。本文对18种不同株型的泡沫病毒gag基因序列进行进化树分析,探究不同来源的泡沫病毒的亲缘关系;并以典型的原型泡沫病毒为代表,阐述泡沫病毒Gag蛋白的结构和功能以及对病毒复制不同阶段的影响。     

Recombinants between endogenous and exogenous avian tumor viruses: role of the C region and other portions of the genome in the control of replication and transformation.

Endogenous retroviruses of chickens are closely related to exogenous viruses isolated from spontaneous tumors in the same species, yet differ in a number of important characteristics, including the ability to transform cells in culture, ability to cause sarcomas or leukemias, host range, and growth rate in cell culture. To correlate these differences with specific sequence differences between the two viral genomes, the genome RNA of transforming subgroup E recombinants between the Prague strain of Rous sarcoma virus, subgroup B (Pr-RSV-B), and the endogenous Rous-associated virus-0 (RAV-0), Subgroup E, and seven nontransforming subgroup E recombinants between the transformation-defective mutant of Pr-RSV-B and RAV-0 was examined by oligonucleotide fingerprinting. The pattern of inheritance among the recombinant viruses of regions of the genome in which Pr-RSV-B and RAV-0 differ allowed us to draw the following conclusions. (i) Nonselected parts of the genome were, with a few exceptions, inherited by the recombinant virus progeny randomly from either parent, with no obvious linkage between neighboring sequences. (ii) A small region in the Pr-RSV-B genome which maps in the 5' region was found in all transforming but only some of the nontransforming recombinants, suggesting that it plays a role in the control of the expression of transformation. (iii) A region of the Pr-RSV-B genome which maps between env and src was similarly linked to the src gene and may be either part of the structural gene for src or a control sequence regulating the expression of src. (iv) The C region at the extreme 3' end of the virus genome which is closely related in all the exogenous avian retroviruses but distinctly different in the endogenous viruses is the major determinant responsible for the differences in growth rate between RAV-0 and Pr-RSV-B. This latter observation allowed us to redefine the C region as a genetic locus, c, with two alleles cn (in RAV-0) and cx (in exogenous viruses).     

Retroviral proteases

SUMMARY: The proteases of retroviruses, such as leukemia viruses, immunodeficiency viruses (including the human immunodeficiency virus, HIV), infectious anemia viruses, and mammary tumor viruses, form a family with the proteases encoded by several retrotransposons in Drosophila and yeast and endogenous viral sequences in primates. Retroviral proteases are key enzymes in viral propagation and are initially synthesized with other viral proteins as polyprotein precursors that are subsequently cleaved by the viral protease activity at specific sites to produce mature, functional units. Active retroviral proteases are homodimers, with each dimer structurally related to the larger class of single-chain aspartic peptidases. Each monomer has four structural elements: two distinct hairpin loops, a wide loop containing the catalytic aspartic acid and an alpha helix. Retroviral gene sequences can vary between infected individuals, and mutations affecting the binding cleft of the protease or the substrate cleavage sites can alter the response of the virus to therapeutic drugs. The need to develop new drugs against HIV will continue to be, to a large extent, the driving force behind further characterization of retroviral proteases.     

Spleen necrosis virus, an avian immunosuppressive retrovirus, shares a receptor with the type D simian retroviruses.

The reticuloendotheliosis viruses (REV) are a family of highly related retroviruses isolated from gallinaceous birds. On the basis of sequence comparison and overall genome organization, these viruses are more similar to the mammalian type C retroviruses than to the avian sarcoma/leukemia viruses. The envelope of a member of the REV family, spleen necrosis virus (SNV), is about 50% identical in amino acid sequence to the envelope of the type D simian retroviruses. Although SNV does not productively infect primate or murine cells, the receptor for SNV is present on a variety of human and murine cells. Moreover, interference assays show that the receptor for SNV is the same as the receptor for the type D simian retroviruses. We propose that adaptation of a mammalian type C virus to an avian host provided the REV progenitor.     

Contribution of the gag and pol sequences to the leukemogenicity of Friend murine leukemia virus.

Friend murine leukemia virus (F-MuLV) is a highly leukemogenic replication-competent murine retrovirus. Both the F-MuLV envelope gene and the long terminal repeat (LTR) contribute to its pathogenic phenotype (A. Oliff, K. Signorelli, and L. Collins, J. Virol. 51:788-794, 1984). To determine whether the F-MuLV gag and pol genes also possess sequences that affect leukemogenicity, we generated recombinant viruses between the F-MuLV gag and pol genes and two other murine retroviruses, amphotrophic clone 4070 (Ampho) and Friend mink cell focus-inducing virus (Fr-MCF). The F-MuLV gag and pol genes were molecularly cloned on a 5.8-kilobase-pair DNA fragment. This 5.8-kilobase-pair F-MuLV DNA was joined to the Ampho envelope gene and LTR creating a hybrid viral DNA, F/A E+L. A second hybrid viral DNA, F/Fr ENV, was made by joining the 5.8-kilobase-pair F-MuLV DNA to the Fr-MCF envelope gene plus the F-MuLV LTR. F/A E+L and F/Fr ENV DNAs generated recombinant viruses upon transfection into NIH 3T3 cells. F/A E+L virus (F-MuLV gag and pol, Ampho env and LTR) induced leukemia in 20% of NIH Swiss mice after 6 months. Ampho-infected mice did not develop leukemia. F/Fr ENV virus (F-MuLV gag and pol, Fr-MCV env, F-MuLV LTR) induced leukemia in 46% of mice after 3 months. Recombinant viruses containing the Ampho gag and pol, Fr-MCF env, and F-MuLV LTR caused leukemia in 38% of mice after 6 months. We conclude that the F-MuLV gag and pol genes contain sequences that contribute to the pathogenicity of murine retroviruses. These sequences can convert a nonpathogenic virus into a leukemia-causing virus or increase the pathogenicity of viruses that are already leukemogenic.     

Reassortant between Human-Like H3N2 and Avian H5 Subtype Influenza A Viruses in Pigs: A Potential Public Health Risk

Background

Human-like H3N2 influenza viruses have repeatedly been transmitted to domestic pigs in different regions of the world, but it is still uncertain whether any of these variants could become established in pig populations. The fact that different subtypes of influenza viruses have been detected in pigs makes them an ideal candidate for the genesis of a possible reassortant virus with both human and avian origins. However, the determination of whether pigs can act as a “mixing vessel” for a possible future pandemic virus is still pending an answer. This prompted us to gather the epidemiological information and investigate the genetic evolution of swine influenza viruses in Jilin, China.

Methods

Nasopharyngeal swabs were collected from pigs with respiratory illness in Jilin province, China from July 2007 to October 2008. All samples were screened for influenza A viruses. Three H3N2 swine influenza virus isolates were analyzed genetically and phylogenetically.

Results

Influenza surveillance of pigs in Jilin province, China revealed that H3N2 influenza viruses were regularly detected from domestic pigs during 2007 to 2008. Phylogenetic analysis revealed that two distinguishable groups of H3N2 influenza viruses were present in pigs: the wholly contemporary human-like H3N2 viruses (represented by the Moscow/10/99-like sublineage) and double-reassortant viruses containing genes from contemporary human H3N2 viruses and avian H5 viruses, both co-circulating in pig populations.

Conclusions

The present study reports for the first time the coexistence of wholly human-like H3N2 viruses and double-reassortant viruses that have emerged in pigs in Jilin, China. It provides updated information on the role of pigs in interspecies transmission and genetic reassortment of influenza viruses.