Human cytomegalovirus (HCMV) transforming region II (mtrll), a 980-bp sequence, can transform rodent fibroblasts. This region contains three small potential open reading frames (ORFs) of 79, 83, and 34 amino acid residues. To identify the specific role of mtrll sequences in transformation, we investigated (1) the presence of mtrll DNA and mtrll-specific RNA transcripts in NIH 3T3 transformants and their tumor derivatives, and (2) the ability of mtrll DNA and its subregions to cis-activate chloramphenicol acetyl transferase (CAT) gene expression. By hybridization analysis, mtrll-specific DNA and RNA were detected in both transformed and tumor cells. A 285-bp sequence upstream of the ORFs in mtrll exhibited weak promoter activity when linked to CAT gene in the sense orientation with respect to the ORFs. Promoter activity of this region was stronger than the enhancer activity in the sense orientation. No promoter or enhancer activity was detected in the antisense orientation. The mtrll 980-bp sequence did not exhibit any promoter activity in either orientation. These results suggested that HCMV mtrll may contain a small transforming gene that may be transcribed at low levels from its own promoter.
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