Effects of the position of the simian virus 40 enhancer on expression of multiple transcription units in a single plasmid.

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Abstract
We have examined the ability of the simian virus 40 72-base pair enhancer segment to simultaneously activate multiple transcription units with plasmids that contain one, two, or three simian virus 40-based transcription units in various arrangements. After transfection into CV1 cells, the expression of a marker gene, Ecogpt, was determined as a function of the position of that marker gene relative to the other transcription units and the position of the marker gene relative to enhancer elements on the plasmids. Two types of position effects were revealed by that analysis. The first, promoter occlusion, causes reduced transcription at a downstream promoter if transcription is initiated at a nearby upstream promoter. This effect does not involve enhancer elements directly, even though the effect is most pronounced when the downstream promoter lacks an enhancer element. The second effect stems from the ability of promoter sequences to reduce the effect of a single enhancer element on other promoters in the same plasmid. This latter effect is mediated by either promoters adjacent to the enhancer element or promoters interposed between the enhancer element and the other promoters on the plasmid.