Characterization of a Serum Response Factor-Like Protein in Saccharomyces cerevisiae, Rlm1, Which Has Transcriptional Activity Regulated by the Mpk1 (Slt2) Mitogen-Activated Protein Kinase Pathway

The Mpk1 (Slt2) mitogen-activated protein (MAP) kinase has been implicated in several biological processes in Saccharomyces cerevisiae. The Rlm1 protein, a member of the MADS box family of transcription factors, functions downstream of Mpk1 in the pathway. To characterize the role of Rlm1 in mediating the transcriptional activation by the Mpk1 pathway, we constructed a LexA-Rlm1 deltaN chimera in which sequences, including the MADS box domain of the Rlm1 protein, were replaced by the LexA DNA binding domain and tested the ability of this chimera to activate a LexA operator-controlled reporter gene. In this assay, the Rlm1 protein was found to activate transcription in a manner regulated by the Mpk1 pathway. The Mpk1 protein kinase phosphorylated Rlm1 deltaN in vitro and the LexA-Rlm1 deltaN chimera protein was phosphorylated in vivo in a Mpk1-dependent manner. These results suggest that Mpk1 regulates the transcriptional activity of Rlm1 by directly phosphorylating it. We identified a Mpk1-like protein kinase, Mlp1, as an Rlm1-associated protein by using the yeast two-hybrid system. Overexpression of MLP1 suppresses the caffeine-sensitive phenotype of the bck1 delta mutation. The additivity of the mlp1 delta defect with the Mpk1 delta defect with regard to the caffeine sensitivity, combined with the results of genetic epistasis experiments, suggested that the activity of Rlm1 is regulated independently by Mpk1 MAP kinase and the Mlp1 MAP kinase-like kinase.