Characterization of molecular clones of porcine endogenous retrovirus-A containing different numbers of U3 repeat boxes in the long terminal repeat region.

When full-length molecular clones of porcine endogenous retrovirus (PERV)-A and porcine endogenous retrovirus (PERV)-B were passaged on human cells, an increase in the length of the long terminal repeat (LTR) was reported. A 39-bp repeat box in the LTR U3 region was multimerized dynamically upon replication, acting as a viral enhancer element that contains binding sites for nuclear transcription factor NF-Y. To analyze the optimum number of 39-bp repeats for viral replication, molecular clones of PERV-A with one, two, three, and four 39-bp repeats were constructed. Each full-length PERV-A molecular clone contained a different number of 39-bp repeat boxes and was used to transfect human 293 cells, the relative transcriptional activity of the LTRs 48 h posttransfection was determined. PERV LTRs containing 3 copies of a 39-bp repeat showed the strongest promoter activity by real-time reverse transcription PCR in human 293 cell lines. Virions generated by the transfection of a provirus with 3 enhancer repeats replicated efficiently in human cells and 2.5×10(4)virion copies/μL were released. Although the transcriptional activity of all PERV-A LTRs was lower than 293 cells (293-PERV-PK-CIRCE) infected productively with PERV-A and PERV-B. It was found that 3 was the optimal number of 39-bp repeats for viral replication. This molecular clone with a higher replication capacity could be used to study the biology of porcine endogenous retrovirus by genetic approaches or in vivo infection experiments.