The plant transcription factor TGA1 stimulates expression of the CaMV 35S promoter in Saccharomyces cerevisiae

We have previously shown that two CRE elements situated on a 31 bp region 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 activation. Band shift experiments with the 31 bp promoter region revealed binding of similar factors in yeast and plant protein extracts. In a previous study this promoter region was shown to confer tissue-specific expression in plants and to interact with the transcription factor TGA1. To test whether expression of TGA1 in yeast also stimulates transcription of the 35S promoter, we co-transformed yeast cells with a cDNA clone of this transcription factor and a 35S promoter/reporter gene construct. Promoter activity studies revealed that TGA1 confers enhanced expression of a reporter gene under the control of the 35S promoter in yeast cells. Yeast cells that were transformed with a 35S promoter construct that containing a mutated TGA1-binding site showed that both TGA1 and the intact binding site are necessary for this activation. These results suggest that stimulation of the 35S promoter by TGA1 is mediated by competition with an endogenous down-regulating yeast factor that is modulated by the nutritional state of the cells.