Effects of copy number of an octopine synthase enhancer element and its distance from the TATA box on heterologous expression in transgenic tobacco

The relationship among motif copy number, its distance from the TATA box and expression level was analyzed in transformed tobacco plants. Single or multiple copies of octopine synthase (ocs) enhancer elements from the ocs promoter were linked to the minimal ocs promoter and a beta-glucuronidase (GUS) reporter gene, then transformed stably into tobacco. Reporter gene assays revealed that mere repetition of the ocs enhancer sequence is not sufficient for promoter activity. Increasing the number of copies of the ocs element elevated the level of gene expression in an additive manner that was dependent of the element's distance from the TATA box. To our knowledge, this is the first report in which the regulation of transgene expression by interactions between these two factors has been documented.