Properties of an isolated transcription stimulating sequence derived from the cauliflower mosaic virus 35S promoter

As a highly active plant viral promoter that is able to function in a wide variety of cell types, the cauliflower mosaic virus (CaMV) 35S promoter has the potential for harboring a plant enhancer element. We tested this possibility and demonstrated that a 338 base pair fragment isolated from the region upstream of the 35S TATA box can increase the expression of a low-activity heterologous promoter up to the level observed for the intact 35S promoter. This fragment is fully active in both orientations when placed 150 base pairs upstream of the transcription start site. However, the activity of this fragment is sensitive to location, demonstrating a reduction in activity and loss of orientation-independent function when the distance from the transcription start site is increased. By assaying fragments of different sizes, we have also characterized regions that are functional in directing the stimulation of the heterologous promoter.