The in vivo detection and measurement of the unfolded protein response in recombinant cellulase producing Saccharomyces cerevisiae strains.

The yeast Saccharomyces cerevisiae possesses industrially desirable traits for ethanol production and has been engineered for consolidated bioprocessing (CBP) of lignocellulosic biomass through heterologous cellulase expression. However, S. cerevisiae produces low titers of cellulases and one suspected reason for this is that heterologous proteins induce the unfolded protein response (UPR). Current methods of measuring the UPR are RNA based and can be inconsistent and cumbersome. We developed vector-based biosensors that will detect and quantify UPR activation. The vector consisted of either the Trichoderma reesei xylanase 2 or codon optimized green fluorescent protein (eGFP) reporter genes under the control of the S. cerevisiae PHAC1 or PKAR2 promoters. The eGFP reporter under control of PKAR2 was identified as the preferred combination due to its superior dynamic range and its greater sensitivity when measuring UPR induction in cellulase producing strains. To our knowledge, we show for the first time that significant UPR activation differences could consistently be observed for different cellulase candidate genes unlike previous RNA-based tests, which were unable to detect these differences. The ability to quantify UPR induction will assist in identifying candidate cellulase genes that do not greatly induce the UPR, making them favorable for use in CBP yeasts.