Figure 1.

Generation of episomes from lentiviral vectors. The product of vector reverse transcription is a linear double-stranded DNA (dsDNA) with LTRs at both ends (deleted in the U3 region in self-inactivating vectors, here denoted as dLTR). This DNA is imported into the nucleus as part of the viral preintegration complex. (a) Conventional lentiviral vectors, harboring a functional IN, can be integrated into the host genome as proviruses. However, the linear DNA can also be circularized in several possible ways: nonhomologous end-joining produces 2-LTR circles, while intramolecular homologous recombination between the LTRs in linear DNA or 2-LTR episomes, or ligation of nicks (shown as arrow-bead structures) in intermediate products of reverse transcription, lead to 1-LTR circles. (b) When proviral integration is blocked through class I IN mutations, increased amounts of vector episomes are produced. att, IN attachment sites at the ends of viral DNA. LTR, long terminal repeat.