Screening of accurate clones for gene synthesis in yeast

Genome-wide screening reveals repression by nuclear exosome as a prerequisite for intron-mediated enhancement in Saccharomyces cerevisiae

Introns can enhance gene expression, a phenomenon called intron-mediated enhancement (IME). Previously proposed IME mechanisms do not sufficiently explain the variability in enhancement levels, suggesting that IME mechanism has not been fully understood. A comprehensive screening of genes involved in IME can provide valuable insights. Recently, using a luciferase coding sequence (yCLuc), we showed that IME functions by relieving repression rather than simply enhancing expression. The expression of yCLuc is repressed by the specific nucleotide sequence UCUU, and adding an intron relieves this repression in the yeast Saccharomyces cerevisiae. Herein, genome-wide screenings were conducted using S. cerevisiae knockout strain libraries to identify genes involved in IME. For screening, yCLuc was expressed with and without an intron in knockout strains. Consequently, CDC73, a regulator of RNA polymerase II (RNAPII), was identified as essential for enhancement. Additionally, 23 genes specifically involved in the repression were identified. These 23 genes are related to nuclear exosomes, RNA modification, RNAPII regulation, the nuclear pore complex, ribosomes, and chromatin modification. Among these, genes associated with nuclear exosomes, which degrade various RNAs in the nucleus, showed the largest impact on expression. The RNA sequence UCUU has been reported as a target for RNA degradation by nuclear exosomes. These findings suggested that UCUU-containing coding sequences are primarily repressed via RNA degradation by the nuclear exosome through UCUU recognition, with this repression being relieved by the presence of an intron.

The presence of an intron relieves gene repression caused by promoter-proximal four-bp specific sequences in yeast

Introns can enhance gene expression in eukaryotic cells in a process called intron-mediated enhancement (IME). The levels of enhancement are affected not only by the intron sequence but also by coding sequences (CDSs). However, the parts of CDSs responsible for mediating IME have not yet been identified. In this study, we identified an IME-mediating sequence by analyzing three pairs of IME-sensitive and -insensitive CDSs in Saccharomyces cerevisiae. Expression of the CDSs yCLuc, yRoGLU1, and KmBGA1 was enhanced by the presence of an intron (i.e., they were IME sensitive), but the expression of each corresponding codon-changed CDS, which encoded the identical amino acid sequence, was not enhanced (i.e., they were IME insensitive). Interestingly, the IME-insensitive CDSs showed higher expression levels that were like intron-enhanced expression of IME-sensitive CDSs, suggesting that expression of IME-sensitive CDSs was repressed. A four-nucleotide sequence (TCTT) located in the promoter-proximal position of either the untranslated or coding region was found to be responsible for repression in IME-sensitive CDSs, and repression caused by the TCTT sequence was relieved by the presence of an intron. Further, it was found that the expression of intron-containing yeast-native genes, UBC4 and MPT5, was repressed by TCTT in the CDS but relieved by the introns. These results indicate that TCTT sequences in promoter-proximal positions repress gene expression and that introns play a role in relieving gene repression caused by sequences such as TCTT.

Development of a comprehensive set of tools for genome engineering in a cold- and thermo-tolerant Kluyveromyces marxianus yeast strain

Kluyveromyces marxianus, a non-conventional thermotolerant yeast, is potentially useful for production of ethanol and other products. This species has a strong tendency to randomly integrate transforming DNA fragments, making necessary the development of more precise methods for gene targeting. In this study, we first demonstrated that K. marxianus NBRC1777 is cold-tolerant, and then established a highly efficient and precise technique for gene editing by introducing genes encoding deaminase-mediated targeted point mutagenesis (Target-AID) and clustered regularly interspaced short palindromic repeats (CRISPR) associated proteins (CRISPR-Cas9). We used Target-AID to introduce targeted point mutations that disrupted Nej1 or Dnl4, genes that are involved in non-homologous end-joining (NHEJ). Both of the resulting mutant strains showed enhanced proportions of homology-mediated integration compared to the wild-type parent. In combination with target cleavage by CRISPR-Cas9, markerless integration was performed using short (~50 bp) flanking homologous sequences. Together, these tools render this species fully tractable for gene manipulation, permitting targeted genetic changes in the cold- and thermo-tolerant yeast K. marxianus.

Selection of Error-Less Synthetic Genes in Yeast

Conventional gene synthesis is usually accompanied by sequence errors, which are often deletions derived from chemically synthesized oligonucleotides. Such deletions lead to frame shifts and mostly result in premature translational terminations. Therefore, in-frame fusion of a marker gene to the downstream of a synthetic gene is an effective strategy to select for frame-shift-free synthetic genes. Functional expression of fused marker genes indicates that synthetic genes are translated without premature termination, i.e., error-less synthetic genes. A recently developed nonhomologous end joining (NHEJ)-mediated DNA cloning method in the yeast Kluyveromyces marxianus is suitable for the selection of frame-shift-free synthetic genes. Transformation and NHEJ-mediated in-frame joining of a synthetic gene with a selection marker gene enables colony formation of only the yeast cells containing synthetic genes without premature termination. This method increased selection frequency of error-less synthetic genes by 3- to 12-fold.

Protein expression of preferred human codon-optimized Gaussia luciferase genes with an artificial open-reading frame in mammalian and bacterial cells

The protein expressions of three preferred human codon-optimized Gaussia luciferase genes (pGLuc, EpGLuc, and KpGLuc) were characterized in mammalian and bacterial cells by comparing them with those of wild-type Gaussia luciferase gene (wGLuc) and human codon-optimized Gaussia luciferase gene (hGLuc). Two synthetic genes of EpGLuc and KpGLuc containing the complete preferred human codons have an artificial open-reading frame; however, they had the similar protein expression levels to those of pGLuc and hGLuc in mammalian cells. In bacterial cells, the protein expressions of pGLuc, EpGLuc, and KpGLuc with approximately 65% GC content were the same and showed approximately 60% activities of wGLuc and hGLuc. The artificial open-reading frame in EpGLuc and KpGLuc did not affect the protein expression in mammalian and bacterial cells.

Synthetic signal sequences that enable efficient secretory protein production in the yeast Kluyveromyces marxianus

Background

Targeting of cellular proteins to the extracellular environment is directed by a secretory signal sequence located at the N-terminus of a secretory protein. These signal sequences usually contain an N-terminal basic amino acid followed by a stretch containing hydrophobic residues, although no consensus signal sequence has been identified. In this study, simple modeling of signal sequences was attempted using Gaussia princeps secretory luciferase (GLuc) in the yeast Kluyveromyces marxianus, which allowed comprehensive recombinant gene construction to substitute synthetic signal sequences.

Results

Mutational analysis of the GLuc signal sequence revealed that the GLuc hydrophobic peptide length was lower limit for effective secretion and that the N-terminal basic residue was indispensable. Deletion of the 16th Glu caused enhanced levels of secreted protein, suggesting that this hydrophilic residue defined the boundary of a hydrophobic peptide stretch. Consequently, we redesigned this domain as a repeat of a single hydrophobic amino acid between the N-terminal Lys and C-terminal Glu. Stretches consisting of Phe, Leu, Ile, or Met were effective for secretion but the number of residues affected secretory activity. A stretch containing sixteen consecutive methionine residues (M¹⁶) showed the highest activity; the M¹⁶ sequence was therefore utilized for the secretory production of human leukemia inhibitory factor protein in yeast, resulting in enhanced secreted protein yield.

Conclusions

We present a new concept for the provision of secretory signal sequence ability in the yeast K. marxianus, determined by the number of residues of a single hydrophobic residue located between N-terminal basic and C-terminal acidic amino acid boundaries.

Electronic supplementary material

The online version of this article (doi:10.1186/s12934-015-0203-y) contains supplementary material, which is available to authorized users.