Rescue of Sendai virus from viral ribonucleoprotein-transfected cells by infection with recombinant vaccinia viruses carrying Sendai virus L and P/C genes

Influence of glycosylation on the efficacy of an Env-based vaccine against simian immunodeficiency virus SIVmac239 in a macaque AIDS model

The envelope glycoprotein (Env) of human immunodeficiency viruses (HIVs) and simian immunodeficiency viruses (SIVs) is heavily glycosylated, and this feature has been speculated to be a reason for the insufficient immune control of these viruses by their hosts. In a macaque AIDS model, we demonstrated that quintuple deglycosylation in Env altered a pathogenic virus, SIVmac239, into a novel attenuated mutant virus (delta5G). In delta5G-infected animals, strong protective immunity against SIVmac239 was elicited. These HIV and SIV studies suggested that an understanding of the role of glycosylation is critical in defining not only the virological properties but also the immunogenicity of Env, suggesting that glycosylation in Env could be modified for the development of effective vaccines. To examine the effect of deglycosylation, we constructed prime-boost vaccines consisting of Env from SIVmac239 and delta5G and compared their immunogenicities and vaccine efficacies by challenge infection with SIVmac239. Vaccination-induced immune responses differed between the two vaccine groups. Both Env-specific cellular and humoral responses were higher in wild-type (wt)-Env-immunized animals than in delta5G Env-immunized animals. Following the challenge, viral loads in SIVmac239 Env (wt-Env)-immunized animals were significantly lower than in vector controls, with controlled viral replication in the chronic phase. Unexpectedly, viral loads in delta5G Env-immunized animals were indistinguishable from those in vector controls. This study demonstrated that the prime-boost Env vaccine was effective against homologous SIVmac239 challenge. Changes in glycosylation affected both cell-mediated and humoral immune responses and vaccine efficacy.

Sendai virus wild-type and mutant C proteins show a direct correlation between L polymerase binding and inhibition of viral RNA synthesis

The Sendai virus C proteins, C', C, Y1, and Y2, are a nested set of four independently initiated carboxy-coterminal proteins encoded on the P mRNA from an alternate reading frame. Together the C proteins have been shown to inhibit viral transcription and replication in vivo and in vitro and C' binds the Sendai virus L protein, the presumed catalytic subunit of the viral RNA polymerase. To identify amino acids within the C' protein that are important for binding L, site-directed mutagenesis of the gstC' gene was used to change conserved charged amino acids to alanine, generating nine mutants. Additionally, a tenth natural mutant, gstF170S, was also constructed. Six of the gstC' mutants, primarily in the C-terminal half of C', exhibited a defect in the ability to bind L protein. The mutants were assayed for their effect on in vitro transcription and replication from the antigenomic promoter, and the data suggest in all but one case a direct correlation between the ability of C to bind L and to inhibit these steps in RNA synthesis. Further studies with two nonfusion C mutants showed that this correlation was specifically due to the C' portion, and not the gst portion, of the fusion proteins. To study their individual functions, each of the four C proteins was fused downstream of glutathione S-transferase. The gstC', gstC, gstY1, and gstY1 fusion proteins were all able to bind L protein and to inhibit viral mRNA and (+)-leader RNA synthesis, and antigenome replication in vitro. In addition, the nonfusion C, Y1, and Y2 proteins all inhibited transcription. The inhibition of (+)-leader RNA and mRNA synthesis by wt C proteins (nonfusion) showed nearly identical dose-response curves, suggesting that inhibition occurs early in RNA synthesis.

Activation of HIV-1-specific immune responses to an HIV-1 vaccine constructed from a replication-defective adenovirus vector using various combinations of immunization protocols

We constructed a recombinant replication defective adenovirus vector containing the env gene (Ad-Bal) derived from macrophage-trophic HIV-1 (HIV-1 Bal). We then immunized mice with this vector using several administration routes and protocols, and examined the immune response. When the Ad-Bal viral vector (over 1 x 10(7) pfu) was injected subcutaneously, both humoral and cell-mediated immunities were induced. However, immune response induced by the Ad-Bal vector alone was weaker than that induced by the recombinant vaccinia viral vector. We then employed the following three immunization protocols: (l) DNA vaccination followed by immunization with the Ad-Bal; (2) vaccination using the Ad-Bal vector followed by DNA vaccination; and (3) DNA vaccination followed by Ad-Bal infection and passive transfer of dendritic cells (DCs) infected with the Ad-Bal. Among the three protocols, the last gave the strongest humoral and cell-mediated immunity. These results suggest that the combination of DNA vaccination, Ad-Bal vector infection and passive transfer of Ad-Bal-infected DCs can induce strong immunity against HIV-1 Bal.

Sendai virus C protein physically associates with Stat1

BACKGROUND

The P/C gene of the Sendai virus (SeV), a member of the family Paramyxoviridae, encodes C protein, which plays a crucial role in counteracting the antiviral effect of interferon (IFN). The C protein blocks IFN signalling to prevent the activation of IFN stimulated genes. However, its underlying molecular mechanism remains to be defined.

RESULTS

Signal transducer and activator of transcription 1 (Stat1) is a critical component of IFN-alpha/beta and IFN-gamma signalling. We found that both unphosphorylated Stat1 and tyrosine-phosphorylated (pY) Stat1 were present in a form of aberrant high molecular weight complexes (HMWCs) of over 2 MDa in infected cell extracts under low-salt conditions. Of recombinant vaccinia viruses carrying each SeV gene, only those expressing the C gene induced Stat1-HMWC. SeV infected cell extracts further displayed an in vitro ability to convert the pY-Stat1 homodimer to pY-Stat1-HMWC. This cell extract activity was not seen after removal of the C protein from the extracts. C protein was therefore involved in the formation of HMWCs. The HMWCs decomposed into smaller complexes in a high-salt buffer, and under this stringent (high-salt) condition, as well as a physiological (isotonic) condition, both unphosphorylated Stat1 and pY-Stat1 were co-precipitated with anti-C antibody.

CONCLUSION

The C protein physically associates with Stat1. This suggests that SeV C protein directly targets Stat1 for inhibitory control on the transcriptional activation of IFN stimulated genes.

Comparison of substrate specificities against the fusion glycoprotein of virulent Newcastle disease virus between a chick embryo fibroblast processing protease and mammalian subtilisin-like proteases

The fusion (F) protein precursor of virulent Newcastle disease virus (NDV) strains has two pairs of basic amino acids at the cleavage site, and its intracellular cleavage activation occurs in a variety of cells; therefore, the viruses cause systemic infections in poultry. To explore the protease responsible for the cleavage in the natural host, we examined detailed substrate specificity of the enzyme in chick embryo fibroblasts (CEF) using a panel of the F protein mutants at the cleavage site expressed by vaccinia virus vectors, and compared the specificity with those of mammalian subtilisin-like proteases such as furin, PC6 and PACE4 which are candidates for F protein processing enzymes. It was demonstrated in CEF cells that Arg residues at the -4, -2 and -1 positions upstream of the cleavage site were essential, and that at the -5 position was required for maximal cleavage. Phe at the +1 position was also important for efficient cleavage. On the other hand, furin and PC6 expressed by vaccinia virus vectors showed cleavage specificities against the F protein mutants consistent with that shown by the processing enzyme of CEF cells, but PACE4 hardly cleaved the F proteins including the wild type. These results indicate that the proteolytic processing enzymes of poultry for virulent NDV F proteins could be furin and/or PC6 but not PACE4. The significance of individual contribution of the three amino acids at the -5, -2 and +1 positions to cleavability was discussed in relation to the evolution of virulent and avirulent NDV strains.

Long-term replication of Sendai virus defective interfering particle nucleocapsids in stable helper cell lines

An essential prerequisite for generating a stable helper cell line, which constitutively expresses functional Sendai virus RNA-dependent RNA polymerase, is the expression of all three Sendai virus nucleocapsid (NC) proteins, NP, P, and L, simulataneously. Generating a stable helper cell line was accomplished by cotransfecting cell line 293 with all three corresponding viral genes under the control of cytomegalovirus promoter-enhancer elements. Cotransfection with a dominant selectable marker enabled selection for stably transfected cells. The levels of the expressed P and NP proteins reached up to 1/10th and 1/20th of the protein levels in Sendai virus-infected cells, respectively. The Sendai virus polymerase activity of the coexpressed proteins was demonstrated by an in vivo polymerase assay. The cell clone H29 gave the strongest signal and produced DI genomes continuously for at least 3 months. This result demonstrates that it is possible to stably express adequate levels of all three viral NC proteins to form Sendai virus polymerase activity, thereby performing the replication and encapsidation of viral RNA, essential prerequisites for a helper cell line to be competent in producing recombinant viruses.

Mapping of a region of the paramyxovirus L protein required for the formation of a stable complex with the viral phosphoprotein P

The paramyxovirus large protein (L) and phosphoprotein (P) are both required for viral RNA-dependent RNA polymerase activity. Previous biochemical experiments have shown that L and P can form a complex when expressed from cDNA plasmids in vivo. In this report, L and P proteins of the paramyxovirus simian virus 5 (SV5) were coexpressed in HeLa T4 cells from cDNA plasmids, and L-P complexes were examined. To identify regions of the SV5 L protein that are required for L-P complex formation, 16 deletion mutants were constructed by mutagenesis of an SV5 L cDNA. Following coexpression of these L mutants with cDNA-derived P and radiolabeling with 35S-amino acids, cell lysates were analyzed for stable L-P complexes by a coimmunoprecipitation assay and by sedimentation on 5 to 20% glycerol gradients. Mutant forms of L containing deletions that removed as much as 1,008 residues from the C-terminal half of the full-length 2,255-residue L protein were detected in complexes with P by these two assays. In contrast, large deletions in the N-terminal half of L resulted in proteins that were defective in the formation of stable L-P complexes. Likewise, L mutants containing smaller deletions that individually removed N-terminal regions which are conserved among paramyxovirus and rhabdovirus L proteins (domain I, II, or III) were also defective in stable interactions with P. These results suggest that the N-terminal half of the L protein contains sequences important for stable L-P complex formation and that the C-terminal half of L is not directly involved in these interactions. SV5-infected HeLa T4 cells were pulse-labeled with 35S-amino acids, and cell extracts were examined by gradient sedimentation. Solubilized L protein was detected as an approximately 8 to 10S species, while the P protein was found as both a approximately 4S form (approximately 85%) and a species that cosedimented with L (approximately 15%). These data provide the first biochemical evidence in support of a simple domain structure for an L protein of the nonsegmented negative-sense RNA viruses. The results are discussed in terms of a structural model for the L protein and the interactions of L with the second viral polymerase subunit P.

The hypervariable C-terminal tail of the Sendai paramyxovirus nucleocapsid protein is required for template function but not for RNA encapsidation

The paramyxovirus nucleocapsid proteins (NPs) are relatively well conserved, except for the C-terminal 20% (or ca. 100 amino acids), referred to as the tail. We have examined whether this hypervariable tail is required for genome synthesis, both in vitro, where synthesis is predominantly from the input templates, and in vivo, where multiple rounds of amplification occur. In these viruses, genome synthesis and assembly of the nascent chain are coupled. We find that the tail is required in vivo but not in vitro. Closer examination of the in vivo system showed that the tailless NP could encapsidate the genome chain but that amplification did not occur. We interpret these results as indicating that the tail is not required for RNA assembly but is required for the template to function in RNA synthesis. Relatively small deletions within the conserved N-terminal 80% of the protein, on the other hand, rendered the protein nonfunctional in either system. The possible functions of the tail in RNA synthesis are discussed.

Assembly of functional Sindbis virus RNA replication complexes: requirement for coexpression of P123 and P34

A vaccinia virus transient expression system was used to determine which of the Sindbis virus (SIN) proteins and/or polyproteins are necessary for the formation of active replication complexes and, in particular, to analyze the role of nsP4, the putative polymerase, versus P34 in RNA replication. We generated vaccinia virus recombinants in which the cDNA for the entire SIN nonstructural coding region as well as cDNA copies of the individual nonstructural proteins (nsPs) and several intermediate polyproteins were placed downstream of the promoter for T7 RNA polymerase and the encephalomyocarditis virus 5' untranslated region. The proteins expressed by the vaccinia virus recombinants comigrate with authentic proteins synthesized in SIN-infected cells, and the polyproteins appear to be processed to the individual proteins of the correct size. To examine the replication efficiencies of different protein combinations, a vaccinia virus recombinant was designed to express an engineered substrate RNA which could serve as a template for replication and subgenomic mRNA transcription by the SIN nsPs. Expression of the entire SIN nonstructural coding region resulted in the synthesis of high levels of both genomic and subgenomic RNAs derived from the engineered template. No RNA replication could be detected during coexpression of the four individual nsPs, although the proteins were indistinguishable, in terms of electrophoretic mobility, from those synthesized in SIN-infected cells. Coexpression of polyproteins P12, P23, and/or P34 with the individual nsPs also did not result in detectable levels of RNA replication. However, when P123 and P34 were coexpressed, efficient RNA replication and subgenomic mRNA transcription of the substrate RNA was observed. Coexpression of nsP4 with P123 resulted in the synthesis of only minus-strand RNAs. These studies show that expression of both P123 and P34 is necessary for establishment of functional RNA replication and transcription complexes and raise the possibility that the polyproteins themselves may be functional components of these complexes. In addition, these data indicate that an nsP4 moiety expressed independently with an additional N-terminal methionine is capable of functioning in minus-strand but not plus-strand RNA synthesis.

Expression and properties of the V protein in acute measles virus and subacute sclerosing panencephalitis virus strains

Measles virus (MV) inserts one guanosine (G) residue at a specific site in a subpopulation of the mRNA transcribed from the phosphoprotein (P) gene to produce V mRNA. Using an antiserum against the unique carboxyl-terminal region of the predicted V protein, we found that a phosphorylated V protein was expressed in two acute MV strains (Edmonston and Nagahata) and three SSPE virus strains (Biken, Yamagata, and Niigata). The V protein of Biken strain SSPE virus was electrophoretically and antigenically indistinguishable from the V protein of Nagahata strain acute MV, the likely progenitor of the Biken strain. The V protein of these two viruses was not present in the intracellular viral nucleocapsids, but was found only in the cytosolic free protein pool. Pulse-chase experiments failed to show transport of the V protein to the plasma membrane. The V protein was also absent in the extracellular virions. The P protein synthesized from the cloned gene associated with the MV nucleocapsids in vitro, but the V protein had no affinity to the MV nucleocapsids. These results suggest that expression and properties of the V protein are conserved in chronic MV infection.

The Sendai virus nonstructural C proteins specifically inhibit viral mRNA synthesis

An in vitro transcription system for paramyxoviruses is described, in which polymerase-free templates are combined with cell extracts containing polymerase made in vivo via transfected plasmids. Both P and L are required for polymerase activity, and both must be coexpressed for optimum activity. mRNA synthesis here was found to be inversely proportional to the level of C expression, whereas defective interfering genome replication was largely unaffected by the level of C in the extract. The inhibition of transcription appeared to be due to the C' and C, but not the Y1 and Y2 proteins, and only occurred when C'/C was coexpressed with P and L. C'/C appears to intervene during polymerase formation, possibly by forming polymerase complexes which are inactive for transcription, but still competent for genome replication.

Complexes of Sendai virus NP-P and P-L proteins are required for defective interfering particle genome replication in vitro

We present evidence that the formation of NP-P and P-L protein complexes is essential for replication of the genome of Sendai defective interfering (DI-H) virus in vitro, using extracts of cells expressing these viral proteins from plasmids. Optimal replication of DI-H nucleocapsid RNA required extracts of cells transfected with critical amounts and ratios of each of the plasmids and was three- to fivefold better than replication with a control extract prepared from a natural virus infection. Extracts in which NP and P proteins were coexpressed supported replication of the genome of purified DI-H virus which contained endogenous polymerase proteins, but extracts in which NP and P were expressed separately and then mixed were inactive. Similarly, the P and L proteins must be coexpressed for biological activity. The replication data thus suggest that two protein complexes, NP-P and P-L, are required for nucleocapsid RNA replication and that these complexes must form during or soon after synthesis of the proteins. Biochemical evidence in support of the formation of each complex includes coimmunoprecipitation of both proteins of each complex with an antibody specific for one component and cosedimentation of the subunits of each complex. We propose that the P-L complex serves as the RNA polymerase and NP-P is required for encapsidation of newly synthesized RNA.

RNA packaging signal of human immunodeficiency virus type 1

Cells infected with a recombinant vaccinia virus carrying the gag and pol regions of the human immunodeficiency virus type 1 genome (Vac-gag/pol) released human immunodeficiency virus (HIV)-like particles containing HIV-specific RNA. However, cells infected with another recombinant vaccinia, Vac-gag/pol-dP, derived through the deletion of an 85-base region (nucleotide positions 679-763) of the HIV genome between the primer binding site and the gag initiation codon of Vac-gag/pol, produced HIV-like particles devoid of the HIV-specific RNA. This 85-base deletion was suggested to cause the collapse of a stable stem-loop structure of 46 bases (751-796) around the gag initiation codon. To examine the role of the stem-loop structure in the packaging of RNAs, we constructed a vaccinia vector plasmid that carried this 46-base sequence followed by the Sendai virus nucleocapsid (NP) gene. When both Vac-gag/pol-dP and this plasmid were introduced into cells, HIV-like particles released from the cells contained the NP gene RNA. However, another vaccinia vector plasmid, which carried the 46-base sequence in the midst of the NP gene, could not supply RNA for incorporation into HIV-like particles. Computer analysis of this plasmid sequence suggested that the 46-base sequence cannot form the stem-loop structure. These findings suggest that the stem-loop structure formed by the 46-base sequence is crucial as a packaging signal.

Immune response of mice infected with recombinant vaccinia viruses carrying the HIV gag gene

We examined mouse immune response to 4 kinds of recombinant vaccinia viruses carrying the HIV gag gene, including vac-gag/pol, which produces HIV-like particles with processed gag proteins; vac-gag, which also produces HIV-like particles but with unprocessed gag protein; and vac-gag-pol-fuse and vac-es-gag/pol, neither of which produces such particles but releases reverse transcriptase and gag protein, respectively, from infected cells. Although infection of mice with recombinant vaccinia viruses induced production of the anti-p24 antibody in all mice, vac-gag/pol and vac-es-pol induced higher production than the other two recombinants. Increase in [3H]thymidine uptake by splenic lymphocytes following p24 antigen stimulation was most evident in mice infected with vac-gag/pol. Thus, the highest immune reaction, both humoral and cellular, was elicited by vac-gag/pol, indicating that among those tested, this recombinant vaccinia virus is the best candidate for a vaccine that induces anti-HIV gag immunity.

Expression of functional Bunyamwera virus L protein by recombinant vaccinia viruses

A cDNA containing the complete coding sequence of the Bunyamwera virus (family Bunyaviridae) L genome segment has been constructed and cloned into two recombinant vaccinia virus expression systems. In the first, the L gene is under control of vaccinia virus P7.5 promoter; in the second, the L gene is under control of the bacteriophage T7 phi 10 promoter, and expression of the L gene requires coinfection with a second recombinant vaccinia virus which synthesizes T7 RNA polymerase. Both systems express a protein which is the same size as the Bunyamwera virus L protein and is recognized by a monospecific L antiserum. The expressed L protein was shown to be functional in synthesizing Bunyamwera virus RNA in a nucleocapsid transfection assay: recombinant vaccinia virus-infected cells were transfected with purified Bunyamwera virus nucleocapsids, and subsequently, total cellular RNA was analyzed by Northern (RNA) blotting. No Bunyamwera virus RNA was detected in control transfections, but in cells which had previously been infected with recombinant vaccinia viruses expressing the L protein, both positive- and negative-sense Bunyamwera virus S segment RNA was detected. The suitability of this system to delineate functional domains within the Bunyamwera virus L protein is discussed.

Rescue of a Sendai virus DI genome by other parainfluenza viruses: implications for genome replication

Using a defective interfering Sendai virus stock (DIH4) freed of nondefective helper virus, we found that the closely related parainfluenza viruses 1 and 3 could substitute for the Sendai virus helper in replicating DIH4, creating chimeric nucleocapsids. The morbillivirus measles and the rhabdovirus VSV could not substitute. When DIH4 is incubated intracellularly for 5 days in the absence of help, the ability of PIV3 to rescue DIH4 at this time depended on fresh Sendai virus polymerase. The PIV3 polymerase apparently can only copy the chimeric template, but not that wrapped in the homologous Sendai NP protein. These results suggest that the cis-acting RNA sequences important for genome replication, e.g., the promoter and the encapsidation site, have been conserved among these viruses, but that the interactions between the polymerase and the template protein NP are unique for each virus.

Studies on the regulation of influenza virus RNA replication: a differential inhibition of the synthesis of vRNA segments in shift-up experiments with ts mutants

The regulation of influenza virus vRNA synthesis in the course of the reproduction cycle was studied with the use of a series of ts mutants in shift-up experiments. The synthesis of vRNA segments was registered by means of polyacrylamide gel electrophoresis of nucleocapsid-associated RNA isolated from the infected cells labelled with [3H]uridine after the shift-up to a semi-permissive temperature. Each mutant exhibited a specific differential pattern of vRNA synthesis inhibition after the shift-up. The most affected segments were either vRNA 4, vRNAs 4 and 7, or vRNAs 4, 6, and 7 in cells infected, respectively, with ts mutants C15 (ts lesion in PB1 gene), C45 (ts lesion in PA gene) and CmN3 (ts lesion in NS gene). The synthesis of vRNAs 1, 2, and 3 was relatively resistant to the shift-up in the cells infected with C15 or C45 and more sensitive in the cells infected with C44 (ts lesion in PB2 gene) or CmN3. The replication of the "early" genes (vRNAs 5 and 8) was generally least affected by the shift-up. The results are discussed in connection with the "early-late" transition of vRNA synthesis pattern in the course of infection.

Sendai virus protein-protein interactions studied by a protein-blotting protein-overlay technique: mapping of domains on NP protein required for binding to P protein

Proteins from Sendai virus particles and from infected cells were analyzed in a protein-blotting protein-overlay assay for their interaction with in vitro-synthesized, [35S]methionine-labeled viral proteins NP, P, and M. After separation by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, transfer onto polyvinylidene difluoride membranes, and renaturation, the immobilized proteins were found to interact specifically with radiolabeled proteins. NP proteins from virus particles and from infected cells retained 35S-P protein equally well. Conversely, P protein from virus particles and from infected cells retained 35S-NP protein. 35S-M protein was retained mainly by NP protein but also by several cellular proteins. To determine the domains on NP protein required for binding to immobilized P protein, a series of truncated and internally deleted 35S-NP proteins was constructed. The only deletion that did not affect binding resides between residues 426 and 497. The carboxyl-terminal 27 residues (positions 498 to 524) contribute significantly to the binding affinity. Removal of 20 residues (positions 225 to 244) in the hydrophobic middle part of NP protein completely abolished its binding to P protein.

Conditional expression of foreign genes by temperature-sensitive mutants of vaccinia virus

To assess the utility of two temperature-sensitive (ts) mutant vaccinia viruses as vectors for the conditional in vitro expression of recombinant foreign genes, we have studied the kinetics of expression of foreign genes incorporated into these viruses. At nonpermissive temperature, 40 degrees C, these viruses were defective either in DNA synthesis or in virus assembly. Foreign gene expression was affected by the nature of the ts lesion and by the nature of the vaccinia promoter positioned upstream from the foreign gene. With both vector viruses, a foreign gene controlled by the p7.5 early-late promoter was expressed at both 33 degrees and 40 degrees C. With the DNA synthesis-defective vector virus, foreign gene expression controlled by the p11 DNA synthesis-dependent late promoter was inhibited at 40 degrees C, but could be turned on by shift to 33 degrees C. This ts expression system provides an alternative to use of drugs that inhibit DNA synthesis as a means for experimental manipulation of gene expression. Both vector viruses can be used with existing vaccinia virus expression technology.

Determination of influenza virus proteins required for genome replication

An artificial vaccinia virus vector-driven replication system for influenza virus RNA has been developed. In this system, a synthetic NS-like gene is replicated and expressed by influenza virus proteins supplied through infection with vaccinia virus recombinant vectors. The minimum subset of influenza virus proteins needed for specific replication and expression of the viral ribonucleoprotein was found to be the three polymerase proteins (PB1, PB2, and PA) and the nucleoprotein.

Production of human immunodeficiency virus (HIV)-like particles from cells infected with recombinant vaccinia viruses carrying the gag gene of HIV

We constructed a recombinant vaccinia virus carrying the entire gag and pol genes of human immunodeficiency virus type 1 (HIV-1). The main gene product detected in the lysates of infected CV-1 and SW480 cells was the gag precursor protein. However, in the culture fluid of infected SW480 cells, but not of infected CV-1 cells, reverse transcriptase (RT) activity was detected. The highest RT activity was found at a density of 1.15 g/ml and this fraction contained many round particles with diameters of 100-150 nm. In contrast to the infected cell lysates, the particles contained the processed gag and pol proteins, suggesting that particle formation may be a prerequisite for efficient processing of the gag precursor by the HIV protease encoded in the pol gene. Particles were also recovered from the culture fluid of SW480 cells infected with another recombinant vaccinia virus carrying only the gag gene. These particles contained the unprocessed gag precursor, indicating that the gag precursor alone was sufficient for particle production.