In cell lines established from Marek's disease tumors, several viral transcripts 
are expressed and among them the products of pp38/pp24 mRNA and 1.8 kb-mRNA have 
been suggested to be involved in viral oncogenicity. The long inverted repeats 
of Marek's Disease virus serotype 1 (MDV1) genome contain closely located 
transcriptional promoters for phosphorylated protein pp38/pp24 and 1.8 kb-mRNA. 
These promoters initiate transcription in opposite directions and are separated 
only by a short enhancer region, which is likely to regulate both promoters 
simultaneously. We have analyzed the transcription activity of these promoters 
in MDV1 (Md5 strain) infected CEF by transient expression of CAT reporter genes 
and found that the promoters were in fact active in infected cells and the 
promoter for 1.8 kb-mRNA was more active than the pp38/pp24 promoter. Deletion 
analysis of the short enhancer region revealed that the 30 bp region overlapping 
the enhancer elements for 1.8 kb-mRNA was important for promoter activity for 
pp38/pp24. The gel shift analysis revealed that nuclear factor(s) actually bound 
to the overlapping 30 bp region. In addition, the activity of these promoters in 
infected cells varied with MDV strains. These results suggest that pp38/pp24 and 
1.8 kb-mRNA promoters share a common regulatory sequence but a viral or a 
cellular factor(s) induced by viral infection regulates the promoter by distinct 
mechanisms.