Mutual cross talk between the regulators Hac1 of the unfolded protein response and Gcn4 of the general amino acid control of Saccharomyces cerevisiae.

Hac1 is the activator of the cellular response to the accumulation of unfolded proteins in the endoplasmic reticulum. Hac1 function requires the activity of Gcn4, which mainly acts as a regulator of the general amino acid control network providing Saccharomyces cerevisiae cells with amino acids. Here, we demonstrate novel functions of Hac1 and describe a mutual connection between Hac1 and Gcn4. Hac1 is required for induction of Gcn4-responsive promoter elements in haploid as well as diploid cells and therefore participates in the cellular amino acid supply. Furthermore, Hac1 and Gcn4 mutually influence their mRNA expression levels. Hac1 is also involved in FLO11 expression and adhesion upon amino acid starvation. Hac1 and Gcn4 act through the same promoter regions of the FLO11 flocculin. The results indicate an indirect effect of both transcription factors on FLO11 expression. Our data suggest a complex mutual cross talk between the Hac1- and Gcn4-controlled networks.