![]() ![]() ![]() ![]() In order to further map the minimum viral sequences important for MMTV gRNA packaging and transfer vector RNA propagation, a systematic deletion approach was employed to delineate the extent of 5′ UTR and gag sequences important in these processes. Recently, we have shown that MMTV gRNA sequences from the first nucleotide (nt) in R up to 400 nts into gag are sufficient to allow efficient MTMV gRNA packaging and propagation, revealing the general significance of this region (R-U5-UTR-GAG) to the MMTV gRNA packaging process. However, early MMTV studies showed that MMTV genome containing deletion in the envelope gene was competent for gRNA packaging, while MMTV vectors in which the 5′ MMTV LTR was swapped by that from Rous sarcoma virus (RSV) LTR were defective for packaging, suggesting the importance of the 5′ region of the MMTV genomic RNA during the packaging process. Very little is known about the packaging determinants of MMTV that allow the virus to specifically incorporate its gRNA into the virus particle from a milieu of cellular and spliced RNAs. However, the recent observation of the cross-packaging abilities of MMTV with a non-human primate retrovirus, MPMV, highlights the need to further enhance our understanding of the primary and secondary gRNA packaging determinants among retroviruses to establish their key contributing nature during gRNA packaging and/or cross-packaging processes. Being a non-primate retrovirus, MMTV-based vectors are likely to obviate potential safety concerns (that might result from the use of primate retroviral vectors) such as cross- and co-packaging of the transfer vector RNA by related primate retroviruses as has been observed between many retroviruses ( – and references therein). This is because of the presence of hormone inducible promoters, making it an excellent candidate for tissue-specific and hormone-inducible gene therapy (reviewed in, ). Recently, there has been an increasing interest in studying mouse mammary virus (MMTV) replication with the hope of developing MMTV-based vectors for human gene therapy. For some retroviruses such as the human and simian immunodeficiency viruses (HIV and SIV), the packaging signal comprises of contiguous sequences within this region –, while for others, the packaging signal seems to have a bi-partite nature, as has been observed for feline immunodeficiency virus (FIV) and Mason-Pfizer monkey virus (MPMV) –. Studies over the past two decades have revealed the importance of the 5′ untranslated region (UTR) and beginning of the gag gene as crucial regions for augmenting retroviral gRNA encapsidation into the assembling particle (reviewed in – ). Successful and specific gRNA packaging/encapsidation determines the fidelity of viral genome incorporation into the progeny virions. In addition, for at least some retroviruses, it has been shown that the R-U5/U3-R regions at the 5′ and 3′ ends of the genomic RNA (gRNA) and their adjoining regions play important roles in RNA packaging (packaging signal- psi, ψ), dimerization (dimerization initiation site-DIS), and gRNA stability and transport (such as the constitutive transport element- CTE) –. The flanking long terminal repeats (LTRs) of retroviruses serve as important control regions that are responsible for regulating many aspects of retroviral replication, from gene expression (promoters, enhancers, negative regulatory elements, hormone-inducible elements, and polyadenylation sites) to reverse transcription (strand switching using “Repeat (R)” regions), and integration (attachment- att sites). ![]()
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