Bidirectionalization of polar promoters in plants

A typical eukaryotic promoter consists of a minimal promoter and other upstream cis elements(1-3). The minimal promoter is essentially a TATA box region where RNA polymerase II, TATA-binding protein (TBP), and TBP-associated factors (TAFs) bind to initiate transcription(4), but minimal promoters alone have no transcriptional activity(1). The cis elements, to which tissue-specific or development-specific transcription factors bind, individually or in combination, determine the spatio-temporal expression pattern of a promoter at the transcriptional level(1). The arrangement of upstream cis elements followed by a minimal promoter sets the polarity of the promoter. Promoters in plants that have been cloned and widely used for both basic research and biotechnological application are generally unidirectional, directing only one gene that has been fused at its 3' end (downstream). It is often necessary to introduce multiple genes into plants for metabolic engineering and trait stacking(5). It is also desirable to minimize or avoid repeated use of a single promoter that may cause transcriptional gene silencing(6). Here we describe a strategy to make polar promoters bidirectional so that one promoter can direct the expression of two genes, one on each end of the promoter.