Unit 2: RNA isolation

RNA isolation extracts intact ribonucleic acid from cells while excluding DNA and protein, and preserving it against the ubiquitous ribonucleases (RNases) that degrade it within seconds of cell lysis. The TRIzol reagent (introduced by Chomczynski and Sacchi, 1987, as the acid guanidinium thiocyanate–phenol–chloroform (AGPC) method; commercialised by Molecular Research Center / Invitrogen) performs a single-step monophasic lysis that simultaneously denatures RNases and partitions macromolecules.

Defining properties and assumptions the rest of the unit relies on:

• RNA chemistry: Single-stranded, bearing a 2′-OH ribose group that makes it more labile than DNA; readily cleaved by base and by RNases.
• RNase problem: RNases are heat-stable, need no cofactors, and coat skin and glassware; every downstream step is designed to inhibit or exclude them.
• Chaotrope + phenol principle: Guanidinium thiocyanate denatures proteins (including RNases); acidic phenol keeps RNA in the aqueous phase while DNA and protein move to the interphase/organic phase.
• pH dependence: At acidic pH (~4.5) DNA becomes protonated and hydrophobic, partitioning away from RNA; at neutral pH DNA co-purifies. This is the core of AGPC selectivity.
• Yeast-specific obstacle: Saccharomyces cerevisiae has a rigid cell wall of β-1,3-glucan, mannoprotein and chitin that TRIzol alone cannot breach, requiring mechanical or enzymatic disruption.

II. The TRIzol Method for Yeast RNA — Single-Step Phenol Extraction

A. Purpose and principle

The method recovers total RNA (rRNA, mRNA, tRNA) from yeast in one monophasic lysis followed by phase separation.

• Purpose: Obtain intact, protein-free, DNA-free total RNA suitable for RT-PCR, northern blotting, cDNA synthesis and RNA-seq.
• Reagent composition (TRIzol): A monophasic solution of phenol and guanidinium isothiocyanate buffered to acidic pH, with ancillary chaotropes.
• Working principle:
  • Denaturation: Guanidinium isothiocyanate disrupts membranes, unfolds proteins and inactivates RNases instantly on contact.
  • Solubilisation: Phenol dissolves lipids and denatured protein while keeping nucleic acids in solution during initial lysis.
  • Phase separation: Addition of chloroform and centrifugation splits the mix into three layers, with RNA confined to the top aqueous layer.

B. Isolation of RNA from yeast cell using Trizol method

The protocol proceeds through cell harvest, wall disruption, phase separation, precipitation, washing and resuspension; each step maps onto a specific chemical rationale.

1. Precautions establishing an RNase-free workspace

• Gloves and surfaces: Change gloves frequently; wipe benches and pipettes with RNaseZap or 0.1% DEPC solution — skin is a major RNase source.
• DEPC-treated water: Diethyl pyrocarbonate (0.1% v/v) covalently modifies histidine residues in RNases; the water is then autoclaved to destroy residual DEPC before use.
• Consumables: Use certified RNase-free, filtered tips and tubes; bake glassware at 180 °C for several hours.

2. Culturing and harvesting yeast cells

• Growth: Grow S. cerevisiae in YPD medium to mid-log phase (OD₆₀₀ ≈ 0.5–0.8), where RNA content per cell is highest and metabolism is active.
• Harvest: Pellet ~1–5 × 10⁷ cells by centrifugation at ~3,000 × g for 5 min at 4 °C; keeping everything cold slows RNase action.
• Wash: Resuspend the pellet in ice-cold DEPC-water or PBS and re-pellet to remove residual medium.

3. Cell-wall disruption (yeast-specific step)

• Why needed: TRIzol lyses membranes but not the glucan wall, so RNA stays trapped without mechanical/enzymatic breach.
  • Bead beating: Add ~1 mL TRIzol plus acid-washed glass beads (~0.5 mm) and vortex vigorously in short pulses (e.g., 30 s on, 30 s on ice) to shear the wall without heating the sample.
  • Enzymatic option: Pre-treat with zymolyase or lyticase (β-1,3-glucanase) to generate spheroplasts, then add TRIzol — gentler, giving higher-integrity RNA.
• TRIzol volume: Add 1 mL TRIzol per ~10⁷ cells and incubate 5 min at room temperature so nucleoprotein complexes dissociate fully.

4. Phase separation

• Chloroform addition: Add 0.2 mL chloroform per 1 mL TRIzol, cap, shake vigorously ~15 s, incubate 2–3 min at room temperature.
• Centrifugation: Spin at 12,000 × g for 15 min at 4 °C; three phases form:

Top      → colourless aqueous phase  = RNA
Middle   → white interphase          = DNA
Bottom   → red-pink organic phase    = proteins, lipids

• Rationale of layering: Acidic phenol protonates DNA into the organic/interphase; RNA, still charged, stays aqueous. The aqueous phase is ~50% of total volume.
• Recovery: Pipette off only the upper aqueous layer, avoiding the interphase, which would carry DNA contamination.

5. RNA precipitation

• Isopropanol: Add 0.5 mL isopropanol per 1 mL of original TRIzol; incubate 10 min at room temperature (or −20 °C for dilute samples).
• Mechanism: Isopropanol lowers RNA solubility and neutralises phosphate-backbone charge, causing RNA to aggregate.
• Pellet: Centrifuge at 12,000 × g, 10 min, 4 °C; RNA forms a gel-like, often invisible pellet on the tube side.

6. Washing the pellet

• 75% ethanol: Wash with ≥1 mL 75% ethanol (in DEPC-water) per 1 mL TRIzol; vortex briefly, then centrifuge at 7,500 × g, 5 min, 4 °C.
• Purpose: Removes co-precipitated salts (guanidinium, phosphate) while ethanol keeps RNA insoluble so it is not lost. A 75% concentration balances salt removal against pellet retention.

7. Drying and resuspension

• Air-dry: Discard ethanol and air-dry the pellet 5–10 min; do not over-dry — a fully desiccated RNA pellet becomes hard to redissolve.
• Dissolve: Resuspend in RNase-free / DEPC-treated water (or TE buffer); gentle warming at 55–60 °C for 10 min aids dissolution.
• Storage: Keep at −80 °C; avoid repeated freeze–thaw cycles that fragment RNA.

8. Quality and yield assessment

• Spectrophotometry: Measure at A₂₆₀; 1 A₂₆₀ unit ≈ 40 µg/mL RNA.

Concentration (µg/mL) = A260 × 40 × dilution factor

• Purity ratios:
  • A₂₆₀/A₂₈₀ ≈ 2.0 indicates protein-free RNA (lower values signal protein/phenol carry-over).
  • A₂₆₀/A₂₃₀ ≈ 2.0–2.2 indicates freedom from guanidinium and phenol.
• Integrity: Denaturing agarose gel should show sharp 25S and 18S rRNA bands with the 25S roughly twice the intensity of the 18S; smearing indicates degradation.

C. Applications and limitations

The method's speed and single-tube format make it the default for yeast total RNA, but it carries reagent hazards and yield trade-offs.

• Applications:
  • Downstream uses: cDNA synthesis, RT-qPCR gene-expression studies, northern blotting, microarray and RNA-seq library prep.
  • Scalability: Works from single tubes to many samples; also recovers DNA and protein from the lower phases if needed.
• Limitations:
  • Toxicity: Phenol and guanidinium isothiocyanate are corrosive and toxic — handle in a fume hood with gloves.
  • Phenol carry-over: Traces inhibit reverse transcriptase and depress A₂₆₀/A₂₃₀; careful aqueous-phase pipetting is essential.
  • DNA contamination: Small genomic DNA carry-over may require a follow-up DNase I digestion before RT-PCR.
  • Small-RNA loss: Standard isopropanol precipitation can under-recover miRNA and tRNA; glycogen carrier or column hybrids improve small-RNA yield.

D. Significance for yeast work

Yeast-specific adaptation of TRIzol integrates mechanical lysis into a chemical extraction, which is why it dominates fungal RNA protocols.

• Wall handling: Bead beating in TRIzol denatures RNases at the exact moment the wall breaks, minimising the window in which endogenous RNases act.
• Model-organism relevance: S. cerevisiae is a eukaryotic model for transcription, stress response and cell-cycle studies, all demanding intact mRNA — TRIzol delivers this reliably.
• Reproducibility: The fixed reagent ratios (1 mL : 0.2 mL chloroform : 0.5 mL isopropanol) make yields comparable across experiments and laboratories.