The number of a U3 repeat box acting as an enhancer in long terminal repeats of polytropic replication-competent porcine endogenous retroviruses dynamically fluctuates during serial virus passages in human cells

The organization and transcriptional regulation of porcine endogenous retro virus (PERV) long terminal repeats (LTRs) are unknown. We have studied the activity of LTRs from replication-competent molecular clones by performing luciferase reporter assays. The LTRs differ in the presence and number of 3 9-bp repeats located in U3 that confer strong promoter activity in human, s imian, canine, feline, and porcine cell lines, whereas for LTRs devoid of t he repeats, the promoter strength was significantly reduced. As the activit y of a heterologous simian virus 40 promoter and a homologous repeat-defici ent LTR was elevated by four 39-bp repeats independently of its orientation and location, the repeat box complies with the definition of an enhancer, During serial virus passaging of molecular PERV clones on human 293 cells, proviral LTRs demonstrated adaptation of transcriptional activity by dynami c changes of the number of 39-bp repeats in the course of up to 12 passagin g cycles.