THE PLANT TRANSCRIPTION FACTOR TGA1 STIMULATES EXPRESSION OF THE CAMV35S PROMOTER IN SACCHAROMYCES-CEREVISIAE

We have previously shown that two CRE elements situated on a 31 bp reg ion of the cauliflower mosaic virus (CaMV) 35S promoter activate gene expression in the yeast Saccharomyces cerevisiae and are regulated by cAMP. Studies with the yeast transcription factors GCN4, SKO1 and YAP1 , which bind CRE-like sequences, showed no influence on expression of the 35S promoter indicating that a yet unknown factor is involved in a ctivation. Band shift experiments with the 31 bp promoter region revea led binding of similar factors in yeast and plant protein extracts. In a previous study this promoter region was shown to confer tissue-spec ific expression in plants and to interact with the transcription facto r TGA1. To test whether expression of TGA1 in yeast also stimulates tr anscription of the 35S promoter, we co-transformed yeast cells with a cDNA clone of this transcription factor and a 35S promoter/ reporter g ene construct. Promoter activity studies revealed that TGA1 confers en hanced expression of a reporter gene under the control of the 35S prom oter in yeast cells. Yeast cells that were transformed with a 35S prom oter construct that containing a mutated TGA1-binding site showed that both TGA1 and the intact binding site are necessary for this activati on. These results suggest that stimulation of the 35S promoter by TGA1 is mediated by competition with an endogenous down-regulating yeast f actor that is modulated by the nutritional state of the cells.