Promoter-proximal introns impact recombinant amylase expression in Saccharomyces cerevisiae

Consolidated bioprocessing (CBP) of starch requires recombinant Saccharomyces cerevisiae strains that produce raw starch-degrading enzymes and ferment the resultant sugars to ethanol in a single step. In this study, the native S. cerevisiae COX4 and RPS25A promoter-proximal introns were evaluated for enhanced expression of amylase genes (ateA, temA or temG_Opt) under the control of an S. cerevisiae promoter (ENO1(P), TEF1(P), TDH3(P), or HXT7(P)). The results showed that different promoters and promoter-intron combinations differentially affected recombinant amylase production: ENO1(P)-COX4(i) and TDH3(P)-RPS25A(i) were the best promoters for AteA, followed closely by HXT7(P). The latter was also the best promoter for TemA and TemG production, followed closely by TDH3(P)-RPS25A(i) for both these enzymes. Introducing promoter-proximal introns increased amylase activity up to 62% in Y294[ENO-COX-AteA] and Y294[TDH3-RPS-TemA], a significant improvement relative to the intron-less promoters. Strains co-expressing both an α-amylase and glucoamylase genes yielded up to 56 g/L ethanol from 20% w/v raw starch, with a higher carbon conversion observed with strains co-expressing TDH3(P)-RPS25A(i)-temG_Opt than HXT7(P)-temG_Opt. The study showed that promoter-proximal introns can enhance amylase activity in S. cerevisiae and suggest that these alternative cassettes may also be considered for expression in more efficient ethanol-producing industrial yeast strains for raw starch CBP.