Heterologous iron-sulfur cluster biogenesis and delivery for cytosolic isobutanol and isopentanol production in Saccharomyces cerevisiae

Saccharomyces cerevisiae is an excellent microbial platform for sustainable production of next generation biofuels such as the branched chain higher alcohols (BCHAs) isobutanol and isopentanol. A cytosolic pathway for BCHA production is generated from expression of prokaryotic orthologs of branched-chain amino acid (BCAA) enzymes acetolactate synthase (ALS), mutant NADH-dependent ketol-acid reductoisomerase (KARIP2D1-A1), and dihydroxy-acid dehydratase (DHAD). The potential for this pathway has been hindered by the availability of iron-sulfur clusters, particularly the 2Fe-2S cluster, required for DHAD to function in the cytosol. ILV3, the endogenous yeast DHAD located in the mitochondria, can be deleted to create a valine auxotroph. In this study we use bioinformatics, heterologous gene library synthesis, and a valine complementation assay to find prokaryotic iron-sulfur cluster biosynthetic gene clusters (BGC) and accessory genes that aid DHAD function in the yeast cytosol. This work presents, to our knowledge, the first functional BGC that enhances the cytosolic activity of prokaryotic DHADs in S. cerevisiae. The SUF BGC from Bacillus subtilis combined with a ferritin-like protein (FTNB) from Escherichia coli and the Lactococcus lactis DHAD enhanced the production of BCHAs. Combined expression gave an average isobutanol titer of 412mg/L, 1.8-fold greater than L. lactis DHAD expressed alone. This work establishes a blueprint for better biofuel production by improving iron-sulfur cluster dependent enzyme activity in the yeast cytosol.