Expression of Varied GFPs in Saccharomyces cerevisiae: Comparison of Codon-Optimized and non-Codon-Optimized GFPs

Green fluorescent protein (GFP), which was originally isolated from jellyfish, is a widely used tool in biological research, and homologs from other organisms are available. However, researchers must determine which GFP is the most suitable for a specific host. Here, we expressed GFPs from several sources in the yeast Saccharomyces cerevisiae, which represents an ideal eukaryotic model. Five non-codon-optimized GFPs (EGFP, AcGFP1, TagGFP2, mUkG1 and ZsGreen) and seven codon-optimized GFPs (yEGFP, yAcGFP1, yTagGFP2, ymUkG1, yZsGreen, ymWasabi and ymNeonGreen) were used to compare the protein expression levels and the green fluorescence intensities. The commercially available GFPs that have been optimized for mammalian codon usage (EGFP, AcGFP1 and TagGFP2) unexpectedly exhibited extremely low expression levels and low fluorescence intensities in S. cerevisiae. The mean fluorescence intensities of these codon-optimized GFPs (yEGFP, yAcGFP1 and yTagGFP2) were markedly 22-, 67- and 101-fold higher than those of corresponding non-codon-optimized GFPs. The codon-optimization of other GFPs also increased both the expression levels and the fluorescence intensity of the cells. Among the codon-optimized GFPs tested, monomeric ymUkG1 and tetrameric yZsGreen exhibited the highest fluorescence intensities in S. cerevisiae. Furthermore, we applied the codon-optimized ymUkG1 to the Gγ recruitment system, which enables to detect protein-protein interactions and to screen for protein variants presenting desirable affinities. The expression of ymUkG1 as a fusion-tagged protein successfully improved the reporting ability of the Gγ recruitment system.

Keywords:
GFP, codon-optimization, Saccharomyces cerevisiae