Retrovirus1/4/2023 ![]() ![]() Note that the (more.)īecause retroviral replication involves an RNA intermediate, all strategies for inserting genes into a retroviral vector must permit synthesis of full-length copies of the vector genome. (U3) Unique 3′LTR RNA (U5) unique 5′LTR RNA (R) repeat at both ends of the viral RNA (poly) polyadenylation (LTR) long terminal repeat. ![]() Ordinarily, R sequences should arise primarily from the 5′plasmid LTR, but they may also include 3′plasmid LTR sequences.Ĭritical cis-acting elements in retroviral vectors and vector replication cycle. 1), the U3 regions in both LTRs are derived from the U3 region originally present in the 3′LTR in the plasmid form of the vector, and both U5 regions are derived from the U5 region originally present in the 5′LTR in the plasmid. After one round of viral replication ( Fig. An important general consideration in the design of retroviral vectors is the effect of viral replication on vector structure. For a discussion of these elements, and their roles in viral replication, see Chapters 4, 5, and 6. These include (1) a promoter and polyadenylation signal in the viral genome (2) a viral packaging signal (ψ or E) to direct incorporation of vector RNA into virions (3) signals required for reverse transcription, including a transfer RNA-binding site (PBS) and polypurine tract (PPT) for initiation of first- and second-strand DNA synthesis, and a repeated (R) region at both ends of the viral RNA required for transfer of DNA synthesis between templates and (4) short, partially inverted repeats located at the termini of the viral LTRs required for integration. Other strategies include the use of multiple promoters, insertion of genes in the reverse orientation, and the use of internal ribosome entry sites (IRESs).Įfficient gene transduction and integration depend on the inclusion in the retroviral vector of a number of cis-acting viral elements ( Fig. However, vector design is not limited to the use of the single retroviral promoter with alternative splicing. ![]() Expression of retroviral proteins in most of the naturally occurring oncogenic retroviruses is driven by a single promoter in the 5′long terminal repeat (LTR), and the expression of multiple viral coding regions is achieved by alternative splicing. In addition, the amount of foreign DNA that can be accommodated in replication-competent vectors is much smaller than can be accommodated in replication-defective vectors. ![]() It is possible to make replication-competent retroviral vectors by adding sequences to existing viruses, but a more common design involves the replacement of retroviral sequences to create replication-defective vectors. ![]()
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