Characterization of the murine cytomegalovirus early transcription unit e1 that is induced by immediate-early proteins

The regulation of murine cytomegalovirus early (E) gene expression
was studied in the cell line B25, which is stably transfected with
the immediate-early ie1/ie3 gene complex. Infection of B25 cells in
the presence of the protein synthesis inhibitor cycloheximide
resulted in the expression of some E genes, whereas for the
expression of other E genes prior protein synthesis was still
mandatory, thus showing differences in the expression requirements
of individual E genes. Transcription unit e1, a member of the E
genes induced by immediate-early products of the ie1/ie3 gene
complex, was characterized. It is located between map units 0.709
and 0.721 of the genome of murine cytomegalovirus strain Smith. A
2.6-kilobase RNA specified in this region is spliced from three
exons of 912, 177, and 1,007 or 1,020 nucleotides, which are
separated by introns of 93 and 326 nucleotides. The second AUG
located in the first exon 119 nucleotides downstream of the 5' cap
site is followed by an open reading frame of 990 nucleotides. The
predicted polypeptide of 330 amino acids has a calculated molecular
mass of 36.4 kilodaltons. Transfection with e1 revealed three
antigenically related proteins of 36, 37, and 38 kilodaltons; these
proteins probably represent differently modified forms of the
predicted protein. These three proteins are phosphorylated and are
associated with intranuclear inclusion bodies. A 33-kilodalton
protein also derived from e1 was identified as a product of
nonspliced transcripts. Comparison of amino acid sequences revealed
homology between the murine cytomegalovirus transcription unit e1
and a human cytomegalovirus E transcription unit.