Concerted integration of linear retroviral DNA by the avian sarcoma virus integrase in vitro: dependence on both long terminal repeat termini.

We have reconstituted integration reactions in vitro with specially designed donor DNAs, a supercoiled plasmid acceptor, purified bacterium-derived Rous sarcoma virus integrase (IN), and a host cell DNA-bending protein, HMG1. The duplex donor DNAs are approximately 300 deoxynucleotides in length and contain only 15 bp of the RSV U3 and U5 termini at the respective ends. The donor has blunt U3 and U5 termini which end with the sequence 5'CATT. Joining of the donor DNA to the acceptor DNA is detected by using a simple biochemical assay. Integration was found to be dependent on both U3 and U5 termini; mutations in either result in a significant decrease in the level of integration in vitro. Restriction digestion of the products is consistent with most integrants representing a concerted integration in which both long terminal repeat termini come from the same donor molecule. The U5 and U3 sequences in the substrate flank a supF tRNA gene, permitting biological selection of integrants. Many integrants have been sequenced, and have all of the hallmarks of authentic viral integration, including the removal of a terminal TT dinucleotide from each donor DNA end, and duplication of acceptor sequences at the integration site without introducing deletions into the acceptor. Target site selection in the acceptor plasmid was random except that the orientation of integrants selected was apparently influenced by supF transcription. Mutations which substituted the conserved CA dinucleotide with a GA pair led to a decreased rate of integration. In 2 of 14 mutant integrants sequenced, deoxynucleotides were deleted from either the U5 or U3 terminus. In one instance, an internal CA dinucleotide was used, which resulted in a 10-bp U5 donor deletion. In the other, an internal GA dinucleotide was used, which produced a 5-bp U3 donor deletion. Both of these integrants provide further evidence that concerted integration in this reconstituted system requires interactions between IN and the U3 and U5 termini from the same donor molecule.