Part:BBa_I766555

pCyc (Medium) Promoter

Mid-expression level constitutive promoter in yeast

Team Estonia_TUIT 2023 characterization of BBa_I766555 (pCYC1)

The pCYC1 promoter drives the expression of CYC1, which yields the iso-1 variant of cytochrome c. The protein, Cyc1, plays a pivotal role in the mitochondrial respiratory chain by aiding electron transport. With its heme group, it facilitates electron transfer across respiratory complexes, ending with oxygen. Cyc1 is predominantly located in the inner mitochondrial membrane. In oxygen-rich cell environments, the iso-1 type accounts for 95% of total cytochrome c (Sherman, 2005; Sherman et al., 1966). pCYC1 is a weaker promoter compared to pADH1.

Plasmid formation

The promoters were PCR-amplified from the yeast genome using primers that contained SacI (forward primer) and BamHI (reverse primer) restriction sites in their 5’-overhangs. After PCR and restriction digestion, the DNA fragments containing the promoters were ligated into SacI/BamHI-restricted pRS304-based vector carrying EGFP coding sequence and tCYC1 terminator.

Promoter	Reporter	Assembly methods
pCYC1	sfGFP	Restriction-ligation

Yeast strain construction

Prior to yeast transformation, the integration plasmids were restricted with HindIII to linearise the plasmids for homologous recombination into the yeast genome TRP1 locus. The restricted plasmids were used to transform the S. cerevisiae DOM90 strain. Transformants were selected for Trp+ phenotype on tryptophan-dropout synthetic media (CSM-TRP) agar plates containing 2% glucose. All yeast strains generated and used for promoter characterization are listed in the table:

Strain name	Genotype	Description
DOM90	MATa {leu2-3,112 trp1-1 can1-100 ura3-1 ade2-1 his3-11,15 bar1::hisG} [phi+]	Background strain used for transformation and as a negative control
I85	DOM90 trp1::pRS304-pCYC1-sfGFP-tCYC1	Strain with sfGFP under pCYC1 promoter, integrated into Trp1-1 locus

EGFP fluorescence measurements

Before conducting fluorescence measurements, yeast seed cultures were cultivated in complete synthetic media (CSM) containing 2% (m/v ratio) raffinose until the cultures reached an optical density (OD⁶⁰⁰) ranging from 1 to 2. Subsequently, the yeast cultures were diluted to an OD⁶⁰⁰ of 0.3, and various carbon sources, including glucose, raffinose, galactose, or glycerol, were added into the cultures to achieve a 2% (m/v) concentration of the respective carbon source. After 6 hours of growth, 200 μl of the cell suspension was carefully transferred into designated wells on 96-well plates for subsequent fluorescence measurements. To measure EGFP fluorescence, a BioTek Synergy Mx Microplate Reader equipped with a 458 nm wavelength LED for GFP excitation was utilized. The emitted fluorescence was measured at a wavelength of 528 nm.

Results

In this study, we assessed the level of gene expression driven by the promoter pCYC1 by employing a fluorescent protein as a reporter. The promoter-containing constructs were integrated into the yeast genome, and the resulting reporter protein fluorescence was quantified in a 96-well plate. To establish a baseline of background fluorescence in the culture, we measured the fluorescence in a control strain, DOM90, which does not express any fluorescent proteins. Compared to the background fluorescence of DOM90, yeast strain with sfGFP under the control of pCYC1 promoter displayed a 1.13-fold increase in sfGFP fluorescence intensity.

Bars indicate the mean fluorescence intensity (expressed in arbitrary units, AU) measured in pCYC1-sfGFP strain or in DOM90 negative control strain. Error bars show standard deviation.

In our study, we found that the pCYC1 promoter is preferable when a weaker level of gene expression is needed. Expanding the quantitative information on yeast promoter activities facilitates the engineering of fine-tuned synthetic biology applications.

References:

Sherman, F. (2005). The importance of mutation, then and now: studies with yeast cytochrome c. Mutation Research, 589(1), 1–16. https://doi.org/10.1016/j.mrrev.2004.07.001

Sherman, F., Stewart, J. W., Margoliash, E., Parker, J., & Campbell, W. (1966). The structural gene for yeast cytochrome C. Proceedings of the National Academy of Sciences of the United States of America, 55(6), 1498–1504. https://doi.org/10.1073/pnas.55.6.1498

Sequence and Features

Assembly Compatibility:

• 10
  COMPATIBLE WITH RFC[10]
• 12
  COMPATIBLE WITH RFC[12]
• 21
  COMPATIBLE WITH RFC[21]
• 23
  COMPATIBLE WITH RFC[23]
• 25
  COMPATIBLE WITH RFC[25]
• 1000
  COMPATIBLE WITH RFC[1000]