Characterization of (+) Strand Initiation and Termination Sequences Located at the Center of the Equine Infectious Anemia Virus Genome

Permeabilized preparations of equine infectious anemia virus (EIAV) are shown here to support efficient and accurate synthesis of full-length double-stranded proviral DNA. When (-) and (+) strand products were analyzed by Southern blotting, a discontinuity, mapping approximately to the center of the EIAV genome, could be demonstrated for the (+) strand, predicting a second site for initiation of DNA synthesis and a specific mechanism of (+) strand termination. Precise localization of this (+) strand origin within the integrase (IN) coding region was achieved through its in vitro selection and extension into, and excision from, nascent DNA by purified recombinant p66/p51 EIAV reverse transcriptase (RT), suggesting that the EIAV genome harbors a central polypurine tract (cPPT). In addition, a model system was developed for evaluating whether sequences immediately downstream of the cPPT would terminate (+) strand synthesis in the context of strand displacement. Such a sequence was indeed discovered which functions in a manner analogous to that of the central termination sequence (CTS) of HIV, where A-tract-induced minor groove compression has been suggested to induce localized distortion of the nucleic acid duplex and termination of (+) strand synthesis. This interpretation is reinforced by experiments indicating that read-through of the CTS can be efficiently promoted by substituting 2,6-diaminopurine for adenine, thereby relieving minor groove compression. The nucleotide substitution can also shift the site of termination in strand displacement (+) strand synthesis. Collectively, our data support proposals that lentiviruses may have evolved specialized mechanisms for initiating and terminating (+) strand DNA synthesis at the center of their genomes.