Marek's disease virus unique genes pp38 and pp24 are essential for transactivating the bi-directional promoters for the 1.8 kb mRNA transcripts.

The pp38 and pp24 genes of Marek's diseases virus (MDV) share the same promoter, which controls the transcription of pp38 or pp24 and a 1.8-kb mRNA bi-directionally. To understand the trans-activating activity of pp38 and pp24 on the bi-directional promoter, both genes were cloned into pcDNA-3 or pBudCE4.1 vectors either singly or in combination. These plasmids were expressed in transfected chicken embryonic fibroblast (CEF) cells. Chloramphenicol acetyltransferase (CAT) activity expressed under the control of the promoter in CEF co-transfected with pP(1.8 kb)-CAT and pBud-pp38-pp24 was significantly higher than that following transfection with only pBud-pp38 or pBud-pp24. This indicates the combination of pp24 and pp38 together are essential for the activation of the promoter. In DNA mobility shift assays, the promoter binds to pp38 and pp24 together, but not to pp38 or pp24 alone. By competitive inhibition tests with a set of DNA fragments from the promoter region, the sequence 5'-CTGCTCATTT-3' was identified as the core sequence for binding by pp38-pp24 in up-regulation of the bi-directional promoter activity.