Molecular cloning of unintegrated visna viral DNA and characterization of frequent deletions in the 3′ terminus

Visna viral DNA, like other retroviral DNA, exists in two circular forms in infected cells. The larger probably contains two copies of the LTR, the smaller, one copy. Recombinant DNA techniques were used to clone unintegrated circular visna viral DNA in the λWES · λB vector. Circular visna viral DNA was digested with the restriction enzyme SstI, which yields a 9.2-kb viral DNA fragment containing 90% of the viral genome colinear with the restriction map of linear viral DNA. This fragment extends from a site about 900 bp from the left (5′) end of the viral DNA molecule, through the 3′ region, including U3 and R sequences at its right (3′) end. The recombinant clones isolated contain visna viral DNA inserts which range in size from 3.1 kb to 9.2 kb. All the clones contain the 5′ region intact, but most had sustained deletions of varying lengths in the 3′ terminal region of the cloned fragment.