Interactions between Marek's disease virus encoded or induced factors and the Rous sarcoma virus long terminal repeat promoter.

Marek's disease virus transactivates promoters of avian leukemia and sarcoma viruses. In this study, a series of RSV-LTR promoter deletion mutants were used to map sites within the LTR important for MDV-mediated transactivation. MDV-responsive elements within RSV-LTR promoters were localized to a 28-bp segment (nucleotides -109 to -137) which contains a GGTGG pentanucleotide repeat element (PRE). Nuclear extract proteins from uninfected cells bound to the RSV PRE in a sequence-specific manner. Extracts from cells infected with MDV produced novel, sequence-specific complexes with RSV PRE probes. Transactivation in other herpesvirus-retrovirus systems has been shown to depend, at least in part, on expression of herpesvirus immediate-early genes. In this report, we demonstrate that MDV ICP4 is capable of transactivating RSV-LTR promoters containing an intact PRE region. Transactivation with an isolated MDV ICP4 gene expressed from its cognate promoter was less efficient than with intact MDV, suggesting that other MDV-encoded factors are likely to play a role in MDV-mediated transactivation of RSV-LTR promoters. RSV-LTR promoters lacking a PRE region were not efficiently transactivated by MDV ICP4. We conclude that MDV ICP4 may be at least partially responsible for transactivation of RSV-LTR promoters and that this transactivation is likely to be dependent upon presence of a PRE region within RSV-LTR promoters.