Cell-specific effects of human cytomegalovirus unique region on recombinant protein expression

The major immediate-early (MIE) promoter of human cytomegalovirus (CMV) is widely used to express recombinant proteins in mammalian cells. CMV MIE promoter contains a strong enhancer and an AT-rich unique region (UR). The UR can function as an insulator or a negative element of CMV MIE promoter, depending on the cellular proteins associated with it. To examine the effects of UR on recombinant protein expression in mammalian cells, we constructed two CMV MIE promoter-based expression plasmids for comparison to the conventional CMV MIE promoter by removing or adding UR. Addition of UR enhances transgene expression in HEK293 stable cells while removal of UR increases both transient and stably integrated expression in HeLa cells. Our results further demonstrate that the cell-specific effect of UR depends on the protein levels of UR-binding proteins, pancreatic-duodenal homeobox factor-1, special AT-rich sequence binding protein 1, and CCAAT displacement protein, in these cells. Collectively, these modified CMV expression plasmids can be utilized to improve recombinant protein production in specific mammalian cell lines.