Functional analysis of the true late human cytomegalovirus pp28 upstream promoter: cis-acting elements and viral trans-acting proteins necessary for promoter activation.

As a model for analyzing the regulation of human cytomegalovirus late genes, we investigated the 28-kDa phosphoprotein (pp28) gene region. Transcripts of 1.6 and 1.3 kb were expressed in wild-type human cytomegalovirus-infected cells but not in cells infected with a DNA-negative temperature-sensitive mutant (ts66), indicating that DNA replication is absolutely required for pp28 gene expression. Transient promoter activation studies revealed that the pp28 gene region upstream promoter (pp28US) functioned early when expressed independently of the viral genome. However, the promoter was not efficiently activated by immediate-early (IE) proteins but was activated equally well by both wild-type virus and ts66. Deletion analysis of the pp28US promoter indicated that sequences upstream of the CAP site between -107 and -32 were required for activation of the pp28 promoter. Within that region exist a 10-bp sequence at -90 (AGTGAT CGTG) and its inverted repeat at -32 which positively influence pp28 promoter function. Therefore, in the case of the pp28US promoter, viral proteins interact through discrete sequences to facilitate late gene expression.