THE SAGA COMPLEX IS ESSENTIAL FOR THE REGULATION OF GENES INVOLVED IN YEAST TREHALOSE METABOLISM

Saccharomyces cerevisiae accumulates trehalose as a stress metabolite in adverse environmental conditions. The trehalose synthesis and breakdown are important for the regulation of trehalose levels within the yeast cell. Therefore, TPS1 and NTH1 gene expressions are tightly regulated during transcription and also translation. Since both genes contain Stress Response Elements (STRE) in the promoter regions, they are co-activated under stress conditions. However, the presence of similar regulatory elements in the promoter of both genes shows that these genes undergo a different regulation at the transcriptional level. In our study, the role of the Spt-Ada-Gcn5 Acetyltransferase (SAGA) complex in the transcriptional regulation of TPS1 and NTH1 genes was determined in nutrient-poor environment. For that purpose, the wild type and Delta ada] mutant yeast cells, where Ada1p is a member of the SAGA complex, were grown in normal and nitrogen starvation conditions. In addition, trehalose level was detected enzymatically in both wild type and mutant yeast cells. In silico promoter analysis of TPS1 and NTH1 promoters revealed that the STRE sequences required for binding of Msn2/4 transcription factors are closed by nucleosomes at the NTH1 promoter, but open at the TPS1 promoter. In the absence of Ada1p, stress-induced promoter activation in the TPS1 gene was observed, while NTH1 gene expression was not activated. According to these results, the nucleosomes spanning the STRE sequences could not be mobilized in the absence of Ada1 protein, and therefore the Msn2/4 transcription factors cannot bind to the promoter and activate the NTH1 gene expression under stress conditions. It was also observed that in the absence of Ada1p, trehalose accumulation was reduced regardless of stress conditions.