The expression of integrated plasmid DNA depends on copy number

The effect of copy number, integration site, and enhancers on the expression of stably integrated exogenous DNA was examined in Chinese hamster cells. Three similar plasmids were constructed with the mouse beta maj-globin promoter fused to the galK gene either with no enhancer or with the SV40 or Harvey sarcoma virus (HaSV) enhancer. Eighteen stable cell lines were obtained and characterized with respect to plasmid copy number and galactokinase activity. At copy numbers of four or less, the enhancers showed detectable activity and a DNase I hypersensitive site was present. Above four copies, gene activity decreased as the copy number increased, the enhancer sequences were apparently inactive, and the DNase I hypersensitive site disappeared. These data suggest that, at least in this model system, when exogenous DNA is integrated as multiple head-to-tail copies, the entire multigene unit expresses poorly and inappropriately. When the same exogenous DNA integrates as a single (or low number) copy, expression appears to be relatively normal as judged by enhancer stimulation and DNase I hypersensitivity.