Bidirectional regulation of two DNA-damage-inducible genes, MAG1 and DDI1, from Saccharomyces cerevisiae

MAG1 encodes a Saccharomyces cerevisiae 3-methyladenine DNA glycosylase that initiates a base-excision-repair pathway and protects yeast cells from killing by methylating agents such as methyl methanesulphonate (MMS). In the promoter region of the MAG1 gene, there is an 8bp GC-rich direct repeat (DR). Here we report that the DR sequence functions as an upstream activating site (UAS) that upregulates the expression of MAG1 as well as another DNA-damage-inducible gene, DDI1, which is transcribed divergently from MAG1. Deletions, or point mutations, within this repeat completely abolished DNA-damage induction and resulted in a reduced basal-level expression for both MAG1 and DDI1 genes. Furthermore, yeast cells carrying the MAG1 gene with the DR deletion displayed an increased sensitivity to MMS compared with wildtype cells. The DR sequence alone can activate transcription of a CYC1 minimal promoter and confer a partial DNA-damage responsiveness. Electrophoretic mobility-shift assays indicate that the DR function is not due to interaction with the yeast RPA. Like MAG1, the DDI1 gene is also controlled by an upstream repressing site (URS) located 5' to the direct repeat. Based on this and previous studies, a model is proposed whereby the constitutive expression of MAG1 and DDI1 is controlled by two functionally opposite regulatory elements, UAS and URS, probably through an antagonistic mechanism, whereas the damage-induced expression appears to be regulated by mechanisms of derepression at the URS as well as activation at the UAS.