Characterizing the effect of long‐range basepairing interactions on translational control of the unfolded protein response transcription factor Hac1

Abstract

Under conditions of ER stress, the unfolded protein response (UPR) induces expression of the transcription factor Hac1. In contrast, under non‐stress conditions the HAC1 mRNA is constitutively expressed, but the protein is not detected. It has been proposed that HAC1 expression is regulated by a basepairing interaction between nucleotides in the mRNA 5′ UTR and intron. The implicit mechanism is that secondary structure created by the interaction blocks scanning ribosomes and that this block is alleviated by removal of the HAC1 intron through a non‐conventional splicing reaction. The goal of the current study is to define the mechanism of HAC1 translational control.A series of HAC1 expression plasmids was created with mutations that disrupt 5′ UTR/intron basepairing. We have found that disruption of a single basepair within the 21 basepairs reported to block HAC1 mRNA translation is sufficient to allow Hac1 synthesis. Whereas the location and identity of the mutation within the 5′ UTR element resulted in varying levels of Hac1 expression, any mutation within the basepairing element in the intron derepressed Hac1 production. Additional studies are examining the association of HAC1 mRNA with ribosomes. The results indicate that Hac1 expression may be regulated through multiple mechanisms in addition to long‐range basepairing interactions.