The 89,000-Mr murine cytomegalovirus immediate-early protein activates gene transcription

To study trans-activation of gene expression by murine
cytomegalovirus (MCMV) immediate-early (IE) proteins, the IE coding
region 1 (ie1), which encodes the 89,000-Mr IE phosphoprotein
(pp89), was stably introduced into L cells. A cell line was
selected and characterized that efficiently expressed the authentic
viral protein. The pp89 that was constitutively expressed in L
cells stimulated the expression of transfected recombinant
constructs containing the bacterial chloramphenicol
acetyltransferase (CAT) gene under the control of viral promoters.
The regulatory function of the ie1 product was confirmed by
transient expression assays in which MCMV IE genes were
cotransfected into L cells together with recombinant constructs of
the CAT gene. For CAT activation by the ie1 product, a promoter
region was required, but there was no preferential activation of a
herpes simplex virus type 1 delayed-early promoter. All plasmid
constructs that contained the intact coding sequences for pp89
induced gene expression in trans. The MCMV enhancer region was not
essential for the expression of a functional IE gene product, and
testing of the cis-regulatory activity of the MCMV enhancer
revealed a low activity in L cells. Another region transcribed at
IE times of infection, IE coding region 2, was unable to induce CAT
expression and also did not augment the functional activity of ie1
after cotransfection.