Structural organization, expression, and functional characterization of the murine cytomegalovirus immediate-early gene 3.

We have previously defined ie3 as a coding region located
downstream of the ie1 gene which gives rise to a 2.75-kb
immediate-early (IE) transcript. Here we describe the structural
organization of the ie3 gene, the amino acid sequence of the gene
product, and some of the functional properties of the protein. The
2.75-kb ie3 mRNA is generated by splicing and is composed of four
exons. The first three exons, of 300, 111, and 191 nucleotides
(nt), are shared with the ie1 mRNA and are spliced to exon 5, which
is located downstream of the fourth exon used by the ie1 mRNA. Exon
5 starts 28 nt downstream of the 3' end of the ie1 mRNA and has a
length of 1,701 nt. The IE3 protein contains 611 amino acids, the
first 99 of which are shared with the ie1 product pp89. The IE3
protein expressed at IE times has a relative mobility of 88 kDa in
gels, and a mobility shift to 90 kDa during the early phase is
indicative of posttranslational modification. Sequence comparison
reveals significant homology of the exon 5-encoded amino acid
sequence with the respective sequence of UL 122, a component of the
IE1-IE2 complex of human cytomegalovirus (HCMV). This homology is
also apparent at the functional level. The IE3 protein is a strong
transcriptional activator of the murine cytomegalovirus (MCMV) e1
promoter and shows an autoregulatory function by repression of the
MCMV ie1/ie3 promoter. The high degree of conservation between the
MCMV ie3 and HCMV IE2 genes and their products with regard to gene
structure, amino acid sequence, and protein functions suggests that
these genes play a comparable role in the transcriptional control
of the two cytomegaloviruses.