Regulated expression of the feline panleukopenia virus P38 promoter on extrachromosomal FPV/EBV chimeric plasmids

The 5' Untranslated Region of the Capsid Protein 2 Gene of Mink Enteritis Virus Is Essential for Its Expression

Mink enteritis virus (MEV), as a parvovirus, is among the smallest of the animal DNA viruses. The limited genome leads to multifunctional sequences and complex gene expression regulation. Here, we show that the expression of viral capsid protein 2 (VP2) of MEV requires its 5' untranslated regions (5' UTR) which promote VP2 gene expression at both transcriptional and translational levels. The expression of VP2 was inhibited in several common eukaryotic expression vectors. Our data showed that the 5' UTR of VP2 enhanced capsid gene transcription but not increased stability or promotes nucleocytoplasmic export of VP2 mRNA. Analysis of the functions of 5' UTR fragments showed that the proximal region (nucleotides [nt] 1 to 270; that is, positions +1 to +270 relative to the transcription initiation site, nt 2048 to 2317 of MEV-L) of 5' UTR of VP2 was necessary for VP2 transcription and also promoted the activity of P38 promoter. Unexpectedly, further analysis showed that deletion of the distal region (nt 271 to 653) of the 5' UTR of VP2 almost completely abolished VP2 translation in the presence of P38, whereas the transcription was still induced significantly. Furthermore, using a luciferase reporter bicistronic system, we identified that the 5' UTR had an internal ribosome entry site-like function which could be enhanced by NS1 via the site at nt 382 to 447. Mutation of the 5' UTR in the MEV full-length clones further showed that the 5' UTR was required for VP2 gene expression. Together, our data reveal an undiscovered function of 5' UTR of MEV VP2 in regulating viral gene expression.IMPORTANCE MEV, a parvovirus, causes acute enteritis in mink. In the present report, we describe an untranslated sequence-dependent mechanism by which MEV regulates capsid gene expression. Our results highlight the roles of untranslated sequences in regulating the transcriptional activity of P38 promoter and translation of capsid genes. These data also reveal the possibility of an unusual translation mechanism in capsid protein expression and the multiple functions of nonstructural protein. A better understanding of the gene expression regulation mechanism of this virus will help in the design of new vaccines and targets for antiviral agents against MEV.

Characterization of the structural gene promoter of Aedes aegypti densovirus

Aedes aegypti densonucleosis virus (AeDNV) has two promoters that have been shown to be active by reporter gene expression analysis (B. N. Afanasiev, Y. V. Koslov, J. O. Carlson, and B. J. Beaty, Exp. Parasitol. 79:322-339, 1994). Northern blot analysis of cells infected with AeDNV revealed two transcripts 1,200 and 3,500 nucleotides in length that are assumed to express the structural protein (VP) gene and nonstructural protein genes, respectively. Primer extension was used to map the transcriptional start site of the structural protein gene. Surprisingly, the structural protein gene transcript began at an initiator consensus sequence, CAGT, 60 nucleotides upstream from the map unit 61 TATAA sequence previously thought to define the promoter. Constructs with the beta-galactosidase gene fused to the structural protein gene were used to determine elements necessary for promoter function. Deletion or mutation of the initiator sequence, CAGT, reduced protein expression by 93%, whereas mutation of the TATAA sequence at map unit 61 had little effect. An additional open reading frame was observed upstream of the structural protein gene that can express beta-galactosidase at a low level (20% of that of VP fusions). Expression of the AeDNV structural protein gene was shown to be stimulated by the major nonstructural protein NS1 (Afanasiev et al., Exp. parasitol., 1994). To determine the sequences required for transactivation, expression of structural protein gene-beta-galactosidase gene fusion constructs differing in AeDNV genome content was measured with and without NS1. The presence of NS1 led to an 8- to 10-fold increase in expression when either genomic end was present, compared to a 2-fold increase with a construct lacking the genomic ends. An even higher (37-fold) increase in expression occurred with both genomic ends present; however, this was in part due to template replication as shown by Southern blot analysis. These data indicate the location and importance of various elements necessary for efficient protein expression and transactivation from the structural protein gene promoter of AeDNV.

Expression of parvovirus LuIII NS1 from a Sindbis replicon for production of LuIII-luciferase transducing virus

In order to develop an alternative packaging system for recombinant parvoviruses, the gene for the major nonstructural protein (NS1) of parvovirus LuIII was inserted into a Sindbis replicon vector. Cells infected with recombinant SinNS1 virus produced NS1 RNA from the Sindbis 26S promoter and expressed NS1 protein which was able to transactivate a parvovirus P38 promoter. Co-transfections of Sindbis-NS1 RNA together with a packageable LuIII transducing genome and a coat protein expression plasmid generated detectable levels of LuIII-luciferase transducing virus. These levels could be increased by a capsid expression plasmid that was also capable of expressing NS2. These results show that a multi-functional parvovirus protein expressed from a Sindbis RNA molecule can be used to produce recombinant parvoviruses.

Role of protein kinase C and tyrosine kinase activity in IFN-gamma-induced expression of the class II MHC gene

Astrocytes are induced by interferon-gamma (IFN-gamma) to express class II major histocompatibility complex (MHC) antigens. Our previous studies demonstrated that IFN-gamma-initiated signaling events important for class II expression include activation of protein kinase C (PKC) and the Na+/H+ antiporter. We have extended these studies and found that protein tyrosine kinase (PTK) activity is also required for class II expression. Treatment of astrocytes with inhibitors specific for PKC and PTK blocked INF-gamma-induced class II gene transcription, mRNA expression, and protein expression. Immunoblotting and immunoprecipitation experiments demonstrated that IFN-gamma induced tyrosine phosphorylation of the p91 component of ISGF3, which is blocked by preincubation of cells with PTK inhibitors. Treatment of astrocytes with IFN-gamma and either PKC and PTK inhibitors changed the mobility and intensity of a nuclear factor, IFN-gamma-enhanced factor X, which binds to the X box of the class II MHC promoter. Taken together, these data provide evidence that activation of both PTK and PKC is required for IFN-gamma-induced expression of the class II gene.

Recombinant LuIII autonomous parvovirus as a transient transducing vector for human cells

Recombinants based on the genome of the autonomous parvovirus, LuIII, were constructed by replacing the viral coding sequences in an infectious clone (pGLu883) by a luciferase or beta-galactosidase reporter, which was linked to the viral P4 promoter. In cells cotransfected with either of these constructs, together with a plasmid supplying LuIII nonstructural and capsid proteins, excision and replication of the recombinant genome occurred. Transducing virions accumulated in the culture medium of the cotransfected cells, as assayed by reporter activity in recipient cells exposed to this medium. Transducing activity could be neutralized by antiserum to LuIII. Production of replicative form DNA and transducing virions were observed following cotransfection of HeLa, 293, or NB324K cells, in increasing order of efficiency. When homology existed between the recombinant genome and sequences flanking the viral genes in the helper construct, concomitant production of replication-competent, cytopathic virus was sometimes observed. This could be minimized by removal of the left end homology from the helper; by this means, preparations of luciferase transducing virus were obtained free from replication-competent virus. With such preparations, we observed luciferase expression (declining after 3 days) for up to 7 days in recipient HeLa cells. Hybridization of the recombinant viral DNA with strand-specific luciferase probes indicated packaging of both strands (as reported for LuIII), but with a several-fold excess of the (-) strand. We suggest that transducing-autonomous parvoviruses will be useful in gene transfer applications, possibly including gene therapy when only transient expression is desired.

cis-acting sequences in the Aleutian mink disease parvovirus late promoter important for transcription: comparison to the canine parvovirus and minute virus of mice

We are currently investigating the regulation of transcription of the Aleutian mink disease parvovirus (ADV). ADV causes a chronic immune complex-mediated condition known as classical Aleutian disease, characterized by slow viral replication. This slow replication is an intrinsic property of ADV and distinguishes it from the more prototypic parvoviruses such as minute virus of mice (MVM) and canine parvovirus (CPV). We have previously suggested a role for the weak ADV promoters in the slow replication and thereby the absence of acute cytopathology and instead establishment of persistent ADV infection with progressive immune complex-mediated chronic lesions. In this study, we have mapped the cis-acting sequences around the ADV P36 promoter responsible for both constitutive transcription and transactivation mediated by the nonstructural protein 1. The mapping was performed by using endpoint deletions of the ADV P36 promoter and by making chimeras between the ADV P36 and MVM P38 promoters. We found the weak constitutive activity of the ADV P36 promoter to be caused by suboptimal promoter proximal sequences, while the low level of transactivation was caused mainly by an upstream region including sequences with homology to the transactivation responsive element (tar) of the H-1 parvovirus (M.-L. Gu, F.-X. Chen, and S. L. Rhode, Virology 187:10-17, 1992). We also found the corresponding regions in the MVM and CPV P38 promoters to be important for transactivation of these promoters by making 5' deletions of the promoter region. In addition, it was found that MVM tar-like and upstream sequences could transfer high nonstructural protein 1 responsiveness to the ADV promoter even though the distance between the tar-like element and the TATA box was significantly changed. On the basis of comparative data for ADV, MVM, CPV, and H-1, a new clustered motif (TTGGTT) is proposed to be the responsive cis-acting element for transactivation. Homology comparison of the specific transcriptional elements of the ADV P36, MVM P38, and CPV P38 promoters suggests that few, but crucial, changes in the ADV P36 promoter and upstream region are responsible for the weak constitutive activity and low level of transactivation of the ADV P36 promoter.

Expression of Aleutian mink disease parvovirus capsid proteins by a recombinant vaccinia virus: self-assembly of capsid proteins into particles

A portion of a cDNA clone containing coding sequences for both structural proteins (VP1 and VP2) of Aleutian mink disease parvovirus (ADV) was inserted into recombinant vaccinia viruses, VV:ADSP. Immunohistochemical staining of VV:ADSP-infected cells revealed that the ADV antigen was readily detected and localized in the nuclei of infected cells. Analysis of VV:ADSP-infected cell lystates indicated that both VP1 and VP2 were produced and comigrated with authentic VP1 and VP2 from ADV-infected Crandell feline kidney cells. These results suggested, therefore, that both VP1 and VP2 were synthesized from a single cloned transcript. CsCl density gradient centrifugation of partially purified VV:ADSP-infected cell lysates indicated that the majority of the antigen was located in a fraction with a density near 1.33 g/ml, indicative of empty ADV particles. Subsequent electron microscopic examination revealed the presence of 27-nm icosahedral virion-like structures at the same density, suggesting that the proteins self-assembled into empty virions. Furthermore, sera from eight of eight mice inoculated with VV:ADSP contained ADV-specific antibodies and two of these eight serum samples had neutralizing activity, indicating that the particles produced in VV:ADSP-infected cells were immunogenic. Finally, when lysates from VV:ADSP-infected cells were compared with standard ADV antigens in counterimmunoelectrophoresis assays, a similar pattern of specific reactivity was observed for sera from normal and infected mink.