Circles with two tandem LTRs are precursors to integrated retrovirus DNA.

Infection of susceptible cells by retroviruses results in the synthesis of linear DNA with two long terminal repeats (LTRs), circular DNA with a single LTR, and circular DNA with two tandem LTRs. To determine which of these unintegrated molecules serves as the precursor to the provirus, we inserted into a retrovirus vector a 49 bp fragment containing the junction formed by in vivo blunt-end ligation of two LTRs. Infection of chicken embryo fibroblasts with virus recovered from this vector and subsequent characterization of the proviral DNA revealed that efficient integration can occur from this introduced junction sequence. Therefore, circular DNA with two tandem LTRs is a precursor to the provirus.