5A′-UTR introns enhance protein expression in the yeast Saccharomyces cerevisiae

Saccharomyces cerevisiae is one of the most suitable microorganisms for recombinant protein production. To enhance protein production, various expression systems have been intensively studied. However, the effect of introns on protein expression has not been examined deeply in S. cerevisiae. In this study, we analyzed the effect of some introns on protein expression. RPS25A, RPS26A, and RPS26B contain single introns within the 5A '-untranslated regions (5A '-UTRs), and RPS24A has an intron just downstream of the initiation codon. Expression activity of the promoter regions containing introns (intron promoters) were analyzed by luciferase reporter assays. These intron promoters showed higher expression than the TDH3 promoter (TDH3p), which is one of the strongest promoters in S. cerevisiae. Deletion of the introns from these promoters decreased luciferase expression, indicating that introns have a role in enhancing protein expression. To develop artificial strong intron promoters, several chimeric promoters were constructed using the TDH3p and the RPS25A intron promoter. A construct containing the entire TDH3p followed by the RPS25A intron showed about 50-fold higher expression than the TDH3p alone. Inducible expressions driven by the GAL10 promoter and the CUP1 promoter were also enhanced by the RPS25A intron. However, enhancement of mRNA accumulation by the TDH3p and the GAL10 promoter with the RPS25A intron was lower than the effect on luciferase activity, suggesting that the intron affects post-transcriptionally. The chimeric promoter, TDH3p-RPS25A-intron, enhanced expressions of some, but not all proteins examined, indicating that 5'-UTR introns increase production of a certain type of recombinant proteins in S. cerevisiae.