The gene encoding a Marek's disease virus (MDV) pp38 phosphoprotein has been identified, sequenced, and localized to the BamHI H fragment to the left of the putative MDV origin of replication. The open reading frame was defined by sequencing of a lacZ-pp38 fusion protein gene from a λgt11 expression library. The entire open reading frame is 290 amino acids long and codes for a protein with a calculated molecular weight of 31,169, compared with the size of 38 kDa of the phosphorylated form estimated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. S1 nuclease protection analysis showed that the pp38 gene is transcribed leftward as an unspliced mRNA. On the basis of transcriptional mapping studies, the pp38 transcript is predicted to be about 1.8 kb in length without a poly(A) sequence. Its promoter-enhancer region overlaps that of the major rightward BamHI H 1.8-kb transcript implicated in tumor induction. This region contains Oct-1, Sp1, and CCAAT motifs as well as the putative origin of replication. The pp38 protein is the only presently known antigen that is consistently associated with the transformation state. It may play a significant role in MDV transformation.
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