[Show abstract][Hide abstract] ABSTRACT: The feline immunodeficiency virus (FIV) long terminal repeat (LTR), compared with some primate lentiviral LTRs, is quite a strong basal promoter. However, it seems to be highly species-specific in function and generally not very efficient in cells of non-feline origin. This study systematically explored the function of the FIV LTR in simian Cos cells compared with its activity in feline and human cells. Our studies, using biologically relevant two- and three-plasmid trans complementation assays followed by semi-quantitative reverse transcriptase PCR, show that the FIV LTR is functional in Cos cells. The results of the Cos experiment are different from previously reported literature and suggest that the strain specificity of the FIV LTR is an important determinant of whether the LTR will be functional in a particular cell type.
Full-text · Article · Mar 2005 · Microbes and Infection
[Show abstract][Hide abstract] ABSTRACT: The constitutive transport element (CTE) of Mason-Pfizer monkey virus (MPMV) is a short cis-acting sequence element critical for virus gene expression. Analogous to the Rev/Rev Responsive Element (RRE) of primate lentiviruses, CTE allows the nucleocytoplasmic transport of unspliced viral mRNAs. In fact, CTE can functionally replace Rev/RRE in the genomic context and has been used successfully in the expression of viral and cellular genes from expression vectors as well. However, unlike RRE, CTE accomplishes this by interacting with cellular factors, making CTE function independent of co-expressed trans factors. Thus, CTE has proven to be a valuable tool in the expression of heterologous genes. Our previous studies have shown that close proximity of CTE to the polyadenylation sequences is important for CTE function in the genomic context. However, it is controversial whether CTE needs to be located spatially close to the polyadenylation sequences in the subgenomic context. Since CTE is being frequently used in expression vectors, we investigated the position dependency of CTE in the heterologous, subgenomic background using both genetic and structural analyses. Our results reveal that similar to the genomic situation, close proximity of CTE to the polyadenylation sequences is important for its function in the heterologous subgenomic context.
[Show abstract][Hide abstract] ABSTRACT: The 5' end of the Mason-Pfizer monkey virus (MPMV) genomic RNA has been predicted to fold into a complex stem/loop structure that is thought to play a role in specific RNA encapsidation. In this study, we used a set of mutations that either abrogated or recreated the first four stem loops predicted within the 5' untranslated region (5' UTR) for effects on RNA packaging. Test of these mutations in our biological assay revealed that only stem loop 1 (SL1) was important for the packaging potential of MPMV, while mutations in none of the other stem loops affected packaging significantly. Interestingly, it was the primary sequence of SL1 RNA and not its secondary structure that affected packaging since compensatory mutations that reformed SL1 were unable to restore the packaging efficiency of the retroviral vector. Additionally, our mutational analysis reveals that stem loop 4, predicted to be the major packaging determinant of MPMV, does not seem to have a significant role in packaging. Finally, results of the biological effects of the structural mutations are discussed in relation to their effects on the folding potential of the various stem loops.
[Show abstract][Hide abstract] ABSTRACT: Feline immunodeficiency virus (FIV)-based retroviral vector systems are being developed for human gene therapy. Consequently, it has become important to know the precise sequence requirements for the packaging of FIV genome so that such sequences can be eliminated from transfer vectors post-transduction for improved safety. Recently, we have shown that sequences both within the 5'-untranslated leader region (UTR) and the 5'-end of gag are required for efficient packaging and transduction of FIV-based vectors. However, the extent of gag sequence important in the encapsidation process is not clear as well as their relative contribution to packaging. In this study, using a biologically relevant packaging system, we demonstrate that at the most 100 bp of gag sequences are sufficient for efficient RNA packaging in conjunction with the 5'-UTR and no other sequences within the next 600 bp of gag exist that affect packaging. In addition, we show that sequences within gag do not simply act as spatial elements to stabilize other structural determinants of packaging located within the 5'-UTR, but are important in themselves for the encapsidation process.
[Show abstract][Hide abstract] ABSTRACT: It has previously been shown that the 5' untranslated leader region (UTR), including about 495 bp of the gag gene, is sufficient for the efficient encapsidation and propagation of Mason-Pfizer monkey virus (MPMV) based retroviral vectors. In addition, a deletion upstream of the major splice donor, SD, has been shown to adversely affect MPMV RNA packaging. However, the precise sequence requirement for the encapsidation of MPMV genomic RNA within the 5' UTR and gag remains largely unknown. In this study, we have used a systematic deletion analysis of the 5' UTR and gag gene to define the cis-acting sequences responsible for efficient MPMV RNA packaging. Using an in vivo packaging and transduction assay, our results reveal that the MPMV packaging signal is primarily found within the first 30 bp immediately downstream of the primer binding site. However, its function is dependent upon the presence of the last 23 bp of the 5' UTR and approximately the first 100 bp of the gag gene. Thus, sequences that affect MPMV RNA packaging seem to reside both upstream and downstream of the major splice donor with the downstream region responsible for the efficient functioning of the upstream primary packaging determinant.
[Show abstract][Hide abstract] ABSTRACT: We have used systematic deletion analysis of the 5' untranslated region (UTR) of the feline immunodeficiency virus (FIV) genome, both in the presence and absence of various amounts of gag, to define the cis-acting sequences responsible for efficient RNA packaging. Our analyses revealed that the primary FIV packaging signal consists of two essential core elements located within the first 90-120 bp of the 5'UTR and the first 90 bp of the gag gene. Interestingly, the region between the major splice donor (SD) and gag, including approximately 130-160 bp upstream of the SD, is dispensable for encapsidation. Finally, other determinants of packaging were found to be present in the viral LTR and/or within the 3' end of the viral genome. Taken together, our results suggest that the primary packaging determinants of FIV are multipartite and discontinuous, composed of two elements within the 5'UTR and gag gene.
No preview · Article · Apr 2003 · Journal of General Virology
[Show abstract][Hide abstract] ABSTRACT: Development of safe and effective gene transfer systems is critical to the success of gene therapy protocols for human diseases. Currently, several primate lentivirus-based gene transfer systems, such as those based on human and simian immunodeficiency viruses (HIV/SIV), are being tested; however, their use in humans raises safety concerns, such as the generation of replication-competent viruses through recombination with related endogenous retroviruses or retrovirus-like elements. Due to the greater phylogenetic distance from primate lentiviruses, feline immunodeficiency virus (FIV) is becoming the lentivirus of choice for human gene transfer systems. However, the safety of FIV-based vector systems has not been tested experimentally. Since lentiviruses such as HIV-1 and SIV have been shown to cross-package their RNA genomes, we tested the ability of FIV RNA to get cross-packaged into primate lentivirus particles such as HIV-1 and SIV, as well as a nonlentiviral retrovirus such as Mason-Pfizer monkey virus (MPMV), and vice versa. Our results reveal that FIV RNA can be cross-packaged by primate lentivirus particles such as HIV-1 and SIV and vice versa; however, a nonlentivirus particle such as MPMV is unable to package FIV RNA. Interestingly, FIV particles can package MPMV RNA but cannot propagate the vector RNA further for other steps of the retrovirus life cycle. These findings reveal that diverse retroviruses are functionally more similar than originally thought and suggest that upon coinfection of the same host, cross- or copackaging may allow distinct retroviruses to generate chimeric variants with unknown pathogenic potential.
Preview · Article · Jul 2001 · Journal of Virology
[Show abstract][Hide abstract] ABSTRACT: The Mason-Pfizer monkey virus (MPMV) constitutive transport element (CTE) is a cis-acting RNA element located in the 3' untranslated region (UTR) of the viral genome. The HIV-1 and SIV Rev/RRE regulatory system can be replaced with MPMV CTE (Bray et al., 1994; Zolotukhin et al., 1994; Rizvi et al., 1996a); similarly, CTE function can also be replaced by the HIV or SIV Rev/RRE regulatory system (Rizvi et al., 1996b; Ernst et al., 1997). In addition, we have shown that in the context of the SIV genome, position is important for CTE function (Rizvi et al., 1996a). To determine the importance of position for CTE function in the context of the MPMV genome, MPMV molecular clones were generated by deleting CTE or removing it from the 3' UTR and placing it in the approximately 40 bp of intervening sequences between the pol termination codon and env initiation codon. A test of these molecular clones in a single round of replication assay revealed that deletion or displacement of CTE in the intervening sequences between pol and env completely abrogated virus replication. Western blot analysis of cell lysates and pelleted culture supernatants revealed negligible amounts of Pr78 Gag/Pol precursor and the processed p27(gag) when CTE was deleted or displaced. Slot blot analysis of fractionated RNAs revealed entrapment of the viral Gag/Pol mRNA in the nucleus with CTE deletion or displacement. Upon reinsertion of CTE in the original genomic position of clones with the deleted or displaced CTE, virus replication, Gag/Pol protein production, and nucleocytoplasmic transport of viral mRNA were restored to normal levels. Displacement of CTE to the 5' UTR immediately upstream of the Gag initiation codon also resulted in aberrant Gag/Pol protein production and nucleocytoplasmic transport of viral RNA. Reinsertion of CTE at the original genomic position of the clone with CTE displacement at the 5' UTR restored normal Gag/Pol protein production and RNA transport, demonstrating that the 3' terminal position of CTE is important for its function. To explore why the 3' terminal location of CTE is important, heterologous DNA sequences of increasing lengths were inserted between CTE and the polyadenylation (poly(A)) signal of the virus to augment the distance between the two cis-acting elements. Test of these constructs revealed that CTE function was progressively lost with incremental increase in distance between CTE and poly(A). To explore this relationship further, CTE was displaced to the env region approximately 2000 bp upstream of the poly(A) signal which abrogated CTE function. However, cloning of poly(A) signal to approximately 200 bp downstream of CTE in the env region (the natural distance between CTE and poly(A)) restored CTE function. Together, these results demonstrate that the close proximity of CTE to the poly(A) signal is important for CTE function, suggesting a functional interaction between CTE and the polyadenylation machinery.
[Show abstract][Hide abstract] ABSTRACT: To study Mason-Pfizer monkey virus (MPMV) replication over a single round, virus particles were generated that contain a replication-defective vector encoding a dominant selectable marker, the hygromycin B phosphotransferase (hyg) gene. Genetic complementation with a homologous MPMV envelope glycoprotein (Env-gp) or pseudotyping by several heterologous Env-gps from a variety of viruses resulted in infectious MPMV particles containing the replication-defective RNA. Recently, it has been shown that human immunodeficiency virus type 1 (HIV-1) and simian immunodeficiency virus (SIV) Rev and Rev-responsive element (RRE) functions can be substituted in vitro by a cis-acting sequence, the constitutive transport element (CTE), from simian type D retroviruses like MPMV and simian retrovirus type 1 (SRV-1). To determine whether CTE of MPMV is necessary for MPMV nucleic acid propagation, an MPMV vector that lacked the terminally located CTE was generated. Propagation of this vector was completely abrogated in the absence of CTE, showing the importance of CTE in MPMV replication. Insertion of CTE back into the MPMV genome in the sense orientation rescued replication to wild-type levels. Slot-blot analysis of nuclear versus cytoplasmic RNA fractions revealed that most of the messages were sequestered in the nucleus of cells transfected with the CTE(-) vectors and very little was transported to the cytoplasm. To test whether HIV-1 or SIV RREs could complement CTE function, the HIV-1 or SIV RREs were inserted in the CTE(-) vectors, trans complementation of CTE(-)RRE(+) vectors with Env-and Rev-expression plasmids rescued propagation of the CTE(-) vectors. Computer analysis predicted an RNA secondary structure in MPMV CTE analogous to the HIV-1 and SIV RREs that could form three stable stem loops, the first of which contains a site similar to the Rev-binding domain in the HIV-1 RRE. The presence of a higher-order CTE structure was analyzed by mutational analysis. We conclude that CTE is important in the replication of MPMV and affects the nucleocytoplasmic transport and/or stability of viral messages similar to the Rev/RRE regulatory system of HIV-1 and SIV.
[Show abstract][Hide abstract] ABSTRACT: In a step toward creating live-attenuated or DNA subunit vaccines for AIDS, the replication of simian immunodeficiency virus (SIV) was studied independently of the Rev and RRE (Rev-responsive element) regulatory system, over a single round. To accomplish this, the env gene of an SIV vector was made defective by the insertion of a SV40 promoter/enhancer hygromycin B phosphotransferase gene cassette. Using this vector as the backbone, molecular clones of SIV were generated that contained a mutated Rev, Rev(-), a deleted RRE, RRE(-), or both, Rev(-)RRE(-). It has been shown recently that human immunodeficiency virus type 1 (HIV-1) Rev and RRE functions can be replaced in vitro by a cis-acting sequence, constitutive transport element (CTE), from simian type D retroviruses. To determine whether such a cis-acting element from Mason-Pfizer monkey virus (MPMV) would substitute for SIV Rev and RRE functions, the MPMV CTE was inserted either into the Nef ORF or at the junction of vpx and vpr of our Rev(-), RRE(-), and Rev(-)RRE(-) SIV molecular clones. Cell-free viral stocks harvested from Cos cells following transfections of these molecular clones revealed that these stocks were infectious over a single round of replication; however, their replication was attenuated 16-fold compared to that of wild-type virus. In addition, our experiments revealed that CTE functions in a position-dependent manner such that its insertion at the junction of vpx and vpr attenuated SIV replication 8- to 12-fold compared to the attenuation observed when it was inserted in the nef region. Our results demonstrate that MPMV CTE is capable of substituting for SIV Rev and RRE functions, resulting in an attenuated ability to produce infectious virus.