THE MECHANISM OF GT ELEMENT-MEDIATED CELL-TYPE-SPECIFIC TRANSCRIPTIONAL CONTROL

Promoter studies have revealed that sequences related to the GT-1 bind ing site, known as GT elements, are conserved in plant nuclear genes o f diverse functions, In this work, we addressed the issue of whether G T elements are involved in cell type-specific transcriptional regulati on, We found that the inactivation of GT-1 site mediated transcription in roots is correlated with the absence of the GT-1 binding activity in root extracts, In addition, the mutation of the related GT-1 (from the pea rbcs-3A) and the S1F (from the spinach rps1) sites resulted in an increase of their transcriptional activity in roots that contain a distinct GT element-binding factor, referred to as RGTF, Although spe cific to GT elements, RGTF has a different sequence requirement and a lower sequence specificity than GT-1, Interestingly, RGTF has a higher binding affinity to the mutant GT-1 and S1F sites than to the wild-ty pe sequences, This cor relation suggests that RGTF may have some role in transcriptional regulation in roots, Furthermore, root cellular pro tein extracts contain an inhibitory activity that prevents GT-1 from b inding to DNA. This helps to explain the absence of the GT-1 binding a ctivity in roots in which the gene of GT-1 is expressed, Together, the se data suggest that the cell type-specific transcription modulation b y GT elements is achieved by using two different strategies.