DNA Extraction from Yeast: Protocols for Genomic and Plasmid DNA

DNA extraction from Saccharomyces cerevisiae is a fundamental laboratory procedure that enables downstream applications including PCR, sequencing, cloning, and genomic analysis. This article provides validated protocols for extracting both genomic DNA (gDNA) and plasmid DNA from yeast using enzymatic and mechanical lysis methods, with emphasis on the EASY-C (Extraction and Analysis of Small Yeast Chromosomes) workflow for circular DNA recovery and standard phenol-chloroform methods for genomic DNA. These protocols are designed for BSL-1 teaching and research laboratory settings and are applicable to wild-type and synthetic yeast strains.

At a Glance

Scientific Principle

Yeast DNA extraction relies on breaking the robust cell wall of S. cerevisiae to release intracellular DNA while protecting nucleic acids from degradation. The cell wall, composed primarily of β-glucan and mannoproteins, requires enzymatic digestion with zymolyase (a β-1,3-glucanase preparation) or lyticase to create spheroplasts. Mechanical disruption using glass beads provides an alternative or complementary approach for cells that resist enzymatic lysis.

For genomic DNA extraction, the goal is to obtain high-molecular-weight DNA representative of the entire genome. This requires gentle handling to avoid shearing and effective removal of proteins, RNA, and polysaccharides. The classic phenol-chloroform extraction exploits differential solubility: proteins partition into the organic phase, while DNA remains in the aqueous phase. Ethanol precipitation then concentrates the DNA.

For plasmid DNA extraction, the challenge is selectively recovering circular extrachromosomal DNA while eliminating genomic DNA. The EASY-C protocol addresses this by first extracting total DNA from yeast, then transforming it into E. coli, where only circular DNA (plasmids and mini-chromosomes) replicates and can be purified using standard bacterial plasmid kits [1]. This approach elegantly separates plasmid from genomic DNA without requiring cesium chloride gradients or alkaline lysis optimization for yeast.

The EASY-C methodology has been validated for recovering low-copy centromeric vectors (CEN/ARS), high-copy plasmids (pan/ARS), and artificial mini-chromosomes (42–52 kb) from S. cerevisiae wild-type and synthetic Sc2.0 strains [1]. It operates at small volumes (~1 mL) and requires less than 2.5 hours, making it compatible with commercial plasmid purification kits.

Materials and Instrumentation

Core Reagents

• Zymolyase (20T or 100T): Lyophilized enzyme from Arthrobacter luteus, reconstituted in 1 M sorbitol, 50 mM Tris-HCl (pH 7.5), 10 mM β-mercaptoethanol. Store at -20°C for up to 6 months.
• Lyticase: Alternative enzyme preparation; use at 200–500 U/mL.
• Glass beads: Acid-washed, 0.4–0.6 mm diameter. Sterilize by baking at 180°C for 2 hours.
• Phenol:chloroform:isoamyl alcohol (25:24:1): Saturated with TE buffer (pH 8.0). Store at 4°C in amber glass.
• Chloroform:isoamyl alcohol (24:1): For post-phenol extraction cleanup.
• Ethanol: 100% and 70% (v/v), ice-cold.
• Sodium acetate (3 M, pH 5.2): For ethanol precipitation.
• RNase A: DNase-free, 10 mg/mL stock.
• TE buffer: 10 mM Tris-HCl, 1 mM EDTA, pH 8.0.
• SDS: 10% (w/v) solution.
• Proteinase K: 20 mg/mL stock.

Buffers for Genomic DNA Extraction

• Lysis buffer: 50 mM Tris-HCl (pH 8.0), 50 mM EDTA, 1% SDS. Add 2% β-mercaptoethanol fresh.
• Spheroplast buffer: 1 M sorbitol, 50 mM Tris-HCl (pH 7.5), 10 mM β-mercaptoethanol.
• Wash buffer: 50 mM Tris-HCl (pH 8.0), 10 mM EDTA.

Buffers for EASY-C Protocol

• STES buffer: 0.2 M Tris-HCl (pH 7.5), 0.5 M NaCl, 0.01 M EDTA, 1% SDS.
• Neutralization buffer: 3 M potassium acetate (pH 5.5).
• TE-RNase: TE buffer containing 20 µg/mL RNase A.

Equipment

• Microcentrifuge (refrigerated, 14,000–21,000 × g)
• Vortex mixer with adapter for microcentrifuge tubes
• Heat block (37°C, 42°C, 65°C, 95°C)
• Water bath (30°C for enzymatic lysis)
• Spectrophotometer (NanoDrop or equivalent)
• Agarose gel electrophoresis apparatus
• UV transilluminator or gel documentation system

Critical Decision Points

Enzyme choice: Zymolyase 20T is suitable for most S. cerevisiae strains. For strains with thicker cell walls (e.g., industrial strains, diploids), use Zymolyase 100T or increase incubation time. Lyticase is an acceptable alternative but may require optimization.

Glass bead size: 0.4–0.6 mm beads provide optimal disruption for yeast. Smaller beads (0.1 mm) are less effective; larger beads (1 mm) may cause excessive shearing of genomic DNA.

Phenol quality: Use molecular biology-grade phenol saturated with TE buffer. Oxidized phenol (pink or brown) degrades DNA and should be discarded.

Controls

Positive Controls

• Genomic DNA extraction: Include a control sample of S. cerevisiae BY4741 or S288C (well-characterized laboratory strains). Process alongside experimental samples.
• Plasmid extraction: Include a control sample transformed with a known plasmid (e.g., pRS416, a CEN/ARS vector at ~7 kb). Verify recovery by transformation into E. coli.

Negative Controls

• No-cell control: Process a tube containing only lysis buffer through the entire protocol. This detects reagent contamination.
• No-enzyme control: For enzymatic lysis, include a sample processed without zymolyase to assess mechanical lysis efficiency.

Process Controls

• Lysis efficiency check: After enzymatic treatment, examine cells under a microscope. Spheroplasts appear rounder and more refractile than intact cells. >90% spheroplast formation indicates adequate lysis.
• DNA integrity check: Run 1 µL of extracted DNA on a 0.8% agarose gel. Genomic DNA should appear as a single high-molecular-weight band (>10 kb). Plasmid DNA (after bacterial recovery) should show distinct bands corresponding to supercoiled, nicked circular, and linear forms.

Conceptual Workflow

Protocol A: Genomic DNA Extraction from Yeast

This protocol yields high-molecular-weight genomic DNA suitable for PCR, Southern blotting, and whole-genome sequencing.

Step 1: Cell Harvesting

1. Grow S. cerevisiae in 5 mL YPD medium at 30°C with shaking (200 rpm) to late-log phase (OD₆₀₀ = 1.5–2.0, approximately 2–3 × 10⁷ cells/mL).
2. Pellet cells by centrifugation at 3,000 × g for 5 minutes at 4°C.
3. Wash pellet with 1 mL sterile water, transfer to a 1.5 mL microcentrifuge tube, and centrifuge at 5,000 × g for 2 minutes.
4. Resuspend in 1 mL spheroplast buffer.

Step 2: Cell Wall Digestion

1. Add 20 µL zymolyase (20T, 5 mg/mL stock) and 5 µL β-mercaptoethanol.
2. Incubate at 30°C for 30–60 minutes with gentle agitation.
3. Check spheroplast formation: mix 10 µL cell suspension with 10 µL 1% SDS on a microscope slide. Spheroplasts lyse immediately, appearing as debris; intact cells remain refractile.
4. Pellet spheroplasts at 500 × g for 5 minutes at 4°C. Aspirate supernatant carefully.

Step 3: Lysis

1. Resuspend spheroplast pellet in 500 µL lysis buffer.
2. Add 5 µL proteinase K (20 mg/mL) and incubate at 55°C for 30 minutes.
3. Add 5 µL RNase A (10 mg/mL) and incubate at 37°C for 15 minutes.

Step 4: Phenol-Chloroform Extraction

1. Add 500 µL phenol:chloroform:isoamyl alcohol (25:24:1).
2. Vortex vigorously for 30 seconds.
3. Centrifuge at 14,000 × g for 10 minutes at 4°C.
4. Transfer upper aqueous phase (approximately 450 µL) to a fresh tube. Avoid the white interphase.
5. Repeat extraction with 500 µL chloroform:isoamyl alcohol (24:1).

Step 5: Precipitation

1. Add 0.1 volume (45 µL) 3 M sodium acetate (pH 5.2) and 2.5 volumes (1.125 mL) ice-cold 100% ethanol.
2. Mix by inversion. Incubate at -20°C for 30 minutes (or -80°C for 15 minutes).
3. Centrifuge at 14,000 × g for 15 minutes at 4°C.
4. Wash pellet with 500 µL 70% ethanol. Centrifuge at 14,000 × g for 5 minutes.
5. Air-dry pellet for 5–10 minutes (do not over-dry).
6. Resuspend in 50 µL TE buffer. Store at 4°C (short-term) or -20°C (long-term).

Protocol B: Plasmid DNA Extraction via EASY-C

This protocol recovers circular DNA (plasmids and mini-chromosomes) from yeast by transferring it into E. coli for amplification and purification [1].

Step 1: Yeast Cell Lysis

1. Harvest yeast cells from 1 mL culture (OD₆₀₀ = 1.0–2.0) by centrifugation at 5,000 × g for 2 minutes.
2. Wash with 1 mL sterile water.
3. Resuspend in 200 µL STES buffer.
4. Add 200 µL glass beads and 200 µL phenol:chloroform:isoamyl alcohol (25:24:1).
5. Vortex at maximum speed for 5 minutes.
6. Add 200 µL TE buffer. Centrifuge at 14,000 × g for 10 minutes.
7. Transfer aqueous phase to a fresh tube.

Step 2: DNA Precipitation

1. Add 0.1 volume 3 M sodium acetate (pH 5.2) and 2.5 volumes 100% ethanol.
2. Incubate at -20°C for 30 minutes.
3. Centrifuge at 14,000 × g for 15 minutes.
4. Wash with 70% ethanol, air-dry, and resuspend in 20 µL TE-RNase.

Step 3: Bacterial Transformation

1. Use 2–5 µL of yeast DNA extract to transform competent E. coli (e.g., DH5α, TOP10) via heat shock or electroporation.
2. Plate on selective medium (e.g., LB + ampicillin for pRS vectors).
3. Incubate at 37°C overnight.

Step 4: Plasmid Purification from Bacteria

1. Pick single colonies and inoculate 2–5 mL selective LB medium.
2. Grow overnight at 37°C with shaking.
3. Purify plasmid DNA using a commercial miniprep kit following manufacturer instructions.
4. Elute in 30–50 µL elution buffer or water.

Step 5: Verification

1. Digest 5 µL purified plasmid with appropriate restriction enzymes.
2. Analyze by agarose gel electrophoresis (0.8–1.2% gel).
3. For mini-chromosomes (>40 kb), use pulsed-field gel electrophoresis or long-read sequencing [1].

Quality Checks

Spectrophotometric Assessment

• A₂₆₀/A₂₈₀ ratio: 1.8–2.0 indicates pure DNA. Lower ratios suggest protein or phenol contamination. Higher ratios may indicate RNA contamination (if RNase treatment was insufficient).
• A₂₆₀/A₂₃₀ ratio: 2.0–2.2 is ideal. Lower ratios indicate carbohydrate, guanidine, or EDTA carryover.
• Concentration: Genomic DNA typically yields 100–400 ng/µL from 5 mL culture. Plasmid DNA yields 50–200 ng/µL from 2 mL bacterial culture.

Gel Electrophoresis

• Genomic DNA: Run 200–500 ng on a 0.8% agarose gel at 5 V/cm for 45 minutes. A single high-molecular-weight band (>10 kb) indicates intact DNA. Smearing below 10 kb suggests shearing.
• Plasmid DNA: Run 200–500 ng on a 1% agarose gel. Three bands are typical: supercoiled (fastest), nicked circular (slowest), and linear (intermediate). Compare with uncut and digested samples.

Functional Validation

• PCR: Amplify a single-copy gene (e.g., ACT1, PDR1) using 10–50 ng genomic DNA. Successful amplification confirms DNA is PCR-compatible.
• Restriction digestion: Digest 1 µg genomic DNA with a 6-base cutter (e.g., EcoRI, HindIII) for 2 hours. Complete digestion produces a smear on agarose gel; partial digestion indicates inhibitors.
• Transformation efficiency (EASY-C): Count colony-forming units (CFU) per µg of yeast DNA extract. Expect 10²–10⁴ CFU/µg for CEN/ARS plasmids [1].

Result Interpretation

Genomic DNA

Plasmid DNA (EASY-C)

Troubleshooting

Limitations

Genomic DNA Extraction

• Shearing sensitivity: High-molecular-weight DNA (>50 kb) is difficult to obtain from yeast due to the vigorous lysis required. For long-read sequencing (PacBio, Oxford Nanopore), use gentle protocols with minimal vortexing and wide-bore pipette tips.
• Polysaccharide contamination: Some yeast strains (particularly industrial and wild isolates) produce high levels of polysaccharides that co-precipitate with DNA and inhibit downstream enzymes. Additional purification steps (CTAB extraction, silica column cleanup) may be necessary.
• Mitochondrial DNA co-extraction: Standard protocols extract both nuclear and mitochondrial DNA. For applications requiring pure nuclear DNA, mitochondrial isolation prior to extraction is needed.

Plasmid DNA Extraction (EASY-C)

• Size limitation: The EASY-C protocol has been validated for plasmids up to ~12 kb and mini-chromosomes up to ~52 kb [1]. Larger constructs may transform E. coli inefficiently.
• Bacterial host requirement: The method depends on transformation into E. coli, which may not replicate all yeast plasmids (e.g., those lacking bacterial origins of replication). For such cases, direct yeast plasmid extraction using alkaline lysis is required.
• Copy number bias: Low-copy plasmids (CEN/ARS, 1–2 copies per cell) yield fewer transformants than high-copy plasmids (2µ-based, 50–100 copies per cell). This may affect recovery efficiency.
• Not for linear plasmids: The EASY-C protocol recovers only circular DNA. Linear plasmids (e.g., killer plasmids in Kluyveromyces lactis) require alternative methods.

General Limitations

• Species specificity: These protocols are optimized for S. cerevisiae. Nonconventional yeast species (e.g., Starmerella sp., Maudiozyma sp., Kazachstania sp.) may require modified lysis conditions [1].
• Scale: Protocols are designed for small volumes (1–5 mL culture). Scaling up requires proportional increases in reagent volumes and may need protocol optimization.
• Reagent stability: Zymolyase and proteinase K lose activity over time. Always verify enzyme activity with positive controls.

Documentation

Required Records

• Sample metadata: Strain name, genotype, growth conditions (medium, temperature, time), OD₆₀₀ at harvest.
• Protocol details: Enzyme lot numbers and concentrations, incubation times and temperatures, buffer compositions.
• Quality control data: A₂₆₀/A₂₈₀ and A₂₆₀/A₂₃₀ ratios, gel images with molecular weight markers, concentration measurements.
• Downstream results: PCR amplification success, restriction digestion patterns, sequencing quality metrics.

Template for Laboratory Notebook

Date: [DD/MM/YYYY]
Project: [Name/Number]
Sample: [Strain name, source]
Culture conditions: [Medium, temperature, OD600]
Extraction method: [Genomic DNA / EASY-C]

Reagents:
- Zymolyase: [Lot #, concentration, incubation time]
- Proteinase K: [Lot #, incubation time]
- Phenol: [Lot #, pH]

Results:
- Concentration: [ng/µL]
- A260/A280: [ratio]
- A260/A230: [ratio]
- Gel image: [attached or referenced]

Notes:
[Observations, deviations, troubleshooting steps]

Storage location: [Freezer box, position]

Biosafety Considerations

Risk Assessment

Saccharomyces cerevisiae is classified as a Biosafety Level 1 (BSL-1) organism by the CDC and NIH [4]. It is not known to cause disease in healthy humans. However, standard microbiological practices must be followed:

• Personal protective equipment (PPE): Lab coat, gloves, and safety glasses when handling cultures and reagents.
• Work surface: Decontaminate with 70% ethanol or 10% bleach before and after procedures.
• Waste disposal: Yeast cultures and contaminated materials should be autoclaved before disposal. Phenol-containing waste must be collected separately and disposed according to institutional hazardous waste protocols.

Chemical Hazards

• Phenol: Toxic, corrosive, and absorbable through skin. Use in chemical fume hood. Double-glove with nitrile gloves. Have polyethylene glycol (PEG 300) available for decontamination of skin spills.
• Chloroform: Carcinogenic and hepatotoxic. Use in fume hood. Store in amber glass away from light.
• β-mercaptoethanol: Toxic, strong odor. Use in fume hood. Dispose as hazardous waste.
• Ethanol: Flammable. Keep away from open flames.

Recombinant DNA Considerations

If working with recombinant plasmids or synthetic yeast strains (e.g., Sc2.0), follow NIH Guidelines for Research Involving Recombinant or Synthetic Nucleic Acid Molecules [5]. Most experiments with S. cerevisiae carrying standard vectors fall under NIH Guidelines Exempt status, but institutional biosafety committee (IBC) approval may be required for certain constructs (e.g., those containing toxin genes, mammalian virus sequences).

Emergency Procedures

• Skin contact with phenol: Immediately remove contaminated clothing, rinse skin with copious water for 15 minutes, then apply PEG 300. Seek medical attention.
• Eye exposure: Flush eyes with water for 15 minutes. Seek medical attention.
• Spill containment: For small spills (<100 mL), absorb with inert material (vermiculite, paper towels), place in hazardous waste container. For large spills, evacuate area and contact institutional biosafety officer.

Frequently Asked Questions

Q1: Can I use the same protocol for both haploid and diploid yeast strains?

Yes, the protocols work for both haploid and diploid S. cerevisiae strains. However, diploid strains may have thicker cell walls and require longer zymolyase treatment (45–60 minutes instead of 30 minutes). Additionally, diploid strains contain twice the DNA content per cell, so expect higher yields. For industrial polyploid strains, consider using Zymolyase 100T and increasing the incubation time to 60–90 minutes.

Q2: Why do I get no colonies after EASY-C transformation even though I know my yeast strain contains a plasmid?

Several factors could cause this. First, verify that your plasmid has a bacterial origin of replication (e.g., pUC or ColE1) and a selectable marker functional in E. coli. Second, check that your competent cells are sufficiently competent (>10⁶ CFU/µg for control plasmid). Third, the plasmid copy number in yeast may be low (especially for CEN/ARS vectors); try increasing the yeast culture volume to 5–10 mL. Finally, ensure complete removal of phenol from the yeast DNA extract, as residual phenol kills E. coli.

Q3: How can I improve the purity of my genomic DNA for sensitive downstream applications like long-read sequencing?

For long-read sequencing (PacBio, Oxford Nanopore), DNA integrity and purity are critical. Use the following modifications: (1) Replace vortexing with gentle inversion during lysis; (2) Use wide-bore pipette tips throughout; (3) Add a second RNase treatment step; (4) Perform a SPRI bead cleanup (e.g., AMPure XP beads) at 0.45× bead-to-sample ratio to remove small fragments; (5) Elute in low-EDTA TE (0.1 mM EDTA) to avoid inhibiting sequencing enzymes. Expect yields of 1–5 µg from 50 mL culture.

Q4: Can I store extracted yeast DNA for long periods, and what conditions are best?

Genomic DNA can be stored at -20°C for up to 1 year in TE buffer (pH 8.0). For longer storage (>1 year), store at -80°C. Avoid repeated freeze-thaw cycles; aliquot into working volumes (10–20 µL). Plasmid DNA (purified from E. coli) is stable at -20°C for several years. DNA stored in water is more susceptible to degradation than DNA stored in TE buffer, as EDTA chelates magnesium ions required for nuclease activity. Always verify DNA integrity by gel electrophoresis before use after long-term storage.

References and Further Reading

1. Swidah R, Monti M, Delneri D. EASY-C: Extraction and Analysis of Small Yeast Chromosomes-A rapid and universal platform for recovering artificial mini-chromosomes from synthetic Sc2.0 yeast and large plasmids from Saccharomyces cerevisiae and nonconventional yeast species. (2026). PubMed ID: 41726160. Link

2. Hemani D, Grissom JH, Chi RJ. Protocol for marker-free genome editing in Saccharomyces cerevisiae using universal donor templates and multiplexed CRISPR-Cas9. (2026). PubMed ID: 41433155. Link

3. Jann J, Gagnon-Arsenault I, Pageau A, et al. A cost-effective and scalable barcoded library construction method for deep mutational scanning studies. (2026). PubMed ID: 41671286. Link

4. CDC and NIH. Biosafety in Microbiological and Biomedical Laboratories (BMBL), 6th Edition. U.S. Department of Health and Human Services (2020). Link

5. National Institutes of Health. NIH Guidelines for Research Involving Recombinant or Synthetic Nucleic Acid Molecules. Link

6. National Center for Biotechnology Information. NCBI Bookshelf: Molecular Biology and Laboratory Methods. Link

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