Localization and DNA sequence analysis of the transforming domain (mtrII) of human cytomegalovirus

To define the morphological transforming region II (mtrII) of human cytomegalovirus (HCMV), a series of subclones of the Xba I/BamHI fragment EM was constructed in vitro and tested for focus-forming activity and tumorigenicity. A 980-base-pair subclone of fragment EM was identified, and its nucleotide sequence revealed three small open reading frames (ORFs), encoding 79, 83, and 34 amino acid residues. S1 nuclease analysis of HCMV-infected cells identified several distinct early RNA species within mtrII, two of which (P1 and P2) were of particular interest, since the length of the protected DNA fragments would position the 5' end of the RNAs upstream of the open reading frames. In addition, the 980-base-pair transforming sequence revealed DNA elements capable of forming stem-loop structures. Thus the transforming mtrII domain of HCMV strain Towne contains both small open reading frames that are expressed in lytically infected cells and sequences resembling insertion-like structures that may be involved in transformation.