Figure 5-1.4.4(c): An oncoretrovirus genome comprising long terminal repeats (LTRs) enclosing the three open reading frames gag, pol and env. PB represents primer binding sites in the viral replication cycle, ψ is the packaging signal and small circles represent splice sites.

Figure 5-1.4.4(d): Structure of a packaged RNA genome having a poly (A) tail but lacking the LTRs.

Construction of a retroviral vector and propagation in helper cell

The retroviral construct involved in gene delivery comprises two constructs-

•  A vector consisting of all cis -acting elements required for gene expression and replication (Figure 5-1.4.4(f).)

•  A helper cell expressing all the viral proteins ( gag, pol, env ) lacking in vector and support the replication of vector. Helper cell lacks RNA containing packaging signal which is required for formation and release of infectious particles but not for non-infectious viral particles.

When the vector DNA is introduced into a helper cell, helper cell produces the viral proteins which help in the assembly of viral particles containing RNA transcribed from the viral vector. These viral particles on infecting the target cell, reverse transcribe the vector RNA into ds-DNA which gets integrated into the host genome forming a provirus which encodes the gene of interest. Target cells do not express viral proteins and cannot generate infectious viral particles containing the vector RNA and thus cannot infect other target cells ( Figure 5-1.4.4(e).).