AG4sequence withinPHR1promoter acts as a gate for cross‐talks between damage‐signaling pathway and multi‐stress response

Abstract

Rph1 and Gis1 are damage‐responsive repressors involved in PHR1 expression. They have two C2H2 zinc finger motifs as putative DNA binding domains and N‐terminal conserved domain with unknown function. They are also found in the human retinoblastoma binding protein 2 and the mouse jumonji‐ encoded protein. The repressors are able to bind to AG4 sequence within a 39‐bp sequence called upstream repressing sequence of PHR1 promoter (URSPHR1 ) responsible for the damage‐response of PHR1. We report here that Rph1 is predominantly localized in the nucleus as examined by fluorescence microscopic analysis with GFP‐Rph1 fusion protein. On the basis of the fact that the AG4 sequence that is recognized by Rph1 and Gis1 is also recognized by Msn2 and Msn4 in a process of stress response, we also tried to examine the in vivo function of AG4 and the role of Msn2 and Msn4 in PHR1 expression. Our results demonstrate that Msn2 and Msn4 are actually required for the basal transcription of PHR1 expression but not for its damage induction. When AG4 sequence was inserted into the minimal promoter of the cyc1‐LacZ reporter, the increased LacZ expression was observed, indicating its involvement in transcriptional activation. The data suggest that the AG4 is primarily required for basal trans criptional activation of PHR1 or CYC1 promoter through the possible involvement of Msn2 and Msn4. However, since the AG4 is also involved in the repression of PHR1 via Rph1 and Gis1, it is proposed that AG4 functions as either URS or upstream activating sequence (UAS) depending on the promoter context.