The IE1/2 transcriptional control region of human cytomegalovirus (HCMV) drives the expression of the HCMV major immediate-early genes (UL123-122), which encode proteins crucial for initiation of the virus replicative cycle. Nucleotide sequence polymorphism in this region of the viral genome could account for variations in the replication of HCMV wild-type strains. In order to test this hypothesis, the constitutive transcription-enhancing activity of the IE1/2 transcriptional control region derived from 12 clinical isolates of HCMV was compared. This was done by PCR amplification of the respective elements followed by cloning up-stream of a beta-globin reporter gene. After transient expression in various cell types, including human teratocarcinoma cell lines, and quantification of RNA levels, the activating function of this complex cis-element was shown to be strongly conserved. This was mirrored by high nucleotide sequence conservation, even within the so-called modulator region. This strong evolutionary conservation of sequence and of transcription-enhancing function strengthens the assumption that the IE1/2 transcriptional control region plays an essential role in initiation of the HCMV replicative cycle.


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