Sequence determination of cDNA clones of transcripts from the tumor-associated region of the Marek's disease virus genome.

The number of 132-bp tandem direct repeats within the long inverted repeat region of the Marek's disease virus type 1 (MDV1) genome increases concomitantly with the loss of oncogenicity during serial passages in cultured cells. Twelve clones carrying the 132-bp sequence were isolated from a cDNA library constructed from chicken embryo fibroblasts infected with the MDV1 Md5 strain. Through sequence analysis of a cDNA clone and primer extension analysis, the corresponding mRNA was found to be a linear transcript which included the two 132-bp tandem direct repeats. Two open reading frames were found in this transcript. One had a week homology with v-fms. The other should increase its size concomitantly with expansion of the 132-bp tandem direct repeat. PCR analysis of both cDNA clones and RNA gave amplified products which were as large as that produced from the genomic clone, indicating that a majority of mRNA from this region is composed of unspliced transcripts.