The development of strong and tunable promoter elements is necessary to enable metabolic and pathway engineering applications for any host organism. Here, we have expanded and generalized a hybrid promoter approach to produce libraries of high-expressing, tunable promoters in the nonconventional yeast Yarrowia lipolytica. These synthetic promoters are comprised of two modular components: the enhancer element and the core promoter element. By exploiting this basic promoter architecture, we have overcome native expression limitations and provided a strategy for both increasing the native promoter capacity and producing libraries for tunable gene expression in a cellular system with ill-defined genetic tools. In doing so, this work has created the strongest promoters ever reported for Y. lipolytica. Furthermore, we have characterized these promoters at the single-cell level through the use of a developed fluorescence-based assay as well as at the transcriptional and whole-cell levels. The resulting promoter libraries exhibited a range of more than 400-fold in terms of mRNA levels, and the strongest promoters in this set had 8-fold-higher fluorescence levels than those of typically used endogenous promoters. These results suggest that promoters in Y. lipolytica are enhancer limited and that this limitation can be partially or fully alleviated through the addition of tandem copies of upstream activation sequences (UASs). Finally, this work illustrates that tandem copies of UAS regions can serve as synthetic transcriptional amplifiers that may be generically used to increase the expression levels of promoters.
