Recombinant Yarrowia lipolytica Squalene synthase (SQS1)

How is SQS1 engineered in Y. lipolytica to enhance squalene production?

SQS1 overexpression is achieved through multi-copy integration or promoter replacement. Native squalene synthase (SQS) is often downregulated in non-triterpenoid strains by replacing its promoter (e.g., pERG9) with weaker promoters like pERG11 to redirect flux toward desired terpenoids . For triterpenoid production, SQS is upregulated alongside squalene epoxidase (SQE) to increase 2,3-oxidosqualene .

Key strategies:

What upstream/downstream pathways compete with SQS1 for precursors?

SQS1 competes with sterol synthesis (via lanosterol synthase, ERG7) and lipid metabolism (e.g., fatty acid biosynthesis). Downregulating ERG7 (via promoter truncation) or blocking sterol pathways enhances squalene accumulation . Additionally, NADPH recycling from the mannitol cycle mitigates redox imbalance caused by high SQS1 activity .

How to optimize Y. lipolytica strains for high squalene yields?

Strain optimization involves:

• Genetic engineering:

  • Overexpress HMG-CoA reductase (HMG1) to relieve MVA-pathway bottlenecks .

  • Delete po1 (pyruvate kinase) to redirect pyruvate toward acetyl-CoA .

• Media optimization:

  • Adjust C/N ratio to favor lipid accumulation (e.g., 40 g/L glucose + 2.5 g/L NH₄Cl) .

  • Maintain pH 6.0–6.5 to minimize acetyl-CoA loss to lipids .

Performance comparison:

How to resolve discrepancies in SQS1 expression levels across studies?

Discrepancies arise from:

• Promoter variability: Constitutive promoters (e.g., TEF) vs. inducible promoters (e.g., ICL1) .

• Integration sites: rDNA vs. LTR zeta of Ylt1 affects copy number and stability .

• Strain background: Oleaginous vs. non-oleaginous strains differ in lipid accumulation capacity .

Validation methods:

• Quantitative PCR: Measure SQS1 mRNA levels across promoters.

• Western blotting: Confirm protein expression (e.g., CYP11A1 detection in steroidogenic strains) .

• Southern blotting: Verify multi-copy integration .

What methods are effective for heterologous SQS1 expression in non-Y. lipolytica hosts?

SQS1 has been expressed in S. cerevisiae to enhance fatty acid production. Key approaches include:

• cDNA library screening: Identify SQS1 clones that suppress sterol synthesis, redirecting flux to free fatty acids .

• Promoter compatibility: Use S. cerevisiae PGK1 promoter for constitutive expression .

Performance metrics:

How to apply CRISPR-Cas9 for SQS1 gene editing?

CRISPR-Cas9 enables precise promoter modifications (e.g., truncating ERG7) or gene deletions (e.g., po1) to redirect metabolic flux. Key steps:

• Design guide RNAs: Target ERG7 or SQS promoter regions .

• Deliver Cas9 and gRNA: Use plasmid or ribonucleoprotein (RNP) delivery .

• Select edited strains: Screen via PCR or Southern blotting .

Case study: Truncating ERG7’s promoter to 50 bp reduced sterol synthesis by >80%, increasing squalene yields .

What metabolic flux analysis (MFA) tools are suitable for SQS1-engineered strains?

MFA integrates isotopic labeling (e.g., ¹³C-glucose) with metabolomics to quantify flux through MVA and shikimate pathways. Tools like:

• Flux balance analysis (FBA): Predict optimal gene deletions to maximize squalene flux .

• Isotopomer distribution analysis: Trace acetyl-CoA partitioning between SQS1 and lipogenesis .

Example application: MFA revealed that blocking pyruvate kinase (po1) increased acetyl-CoA availability for SQS1 by 40% .

How to integrate SQS1 with synthetic biology pathways?

Co-expression of SQS1 with:

• Geranylgeranyl diphosphate synthase (GGPPS): Enables diterpene production while maintaining squalene synthesis .

• Phosphoketolase: Diverts pyruvate to pentose phosphate pathway, reducing ethanol byproducts .

Synergy example: Overexpressing SQS1 and GGPPS variants (e.g., ERG20F88C) allows modular production of triterpenes and carotenoids .

What are the challenges in scaling SQS1-engineered strains to bioreactor levels?

Key challenges include:

• Oxygen transfer limitations: Squalene synthesis requires high aeration to sustain MVA flux .

• Acidic byproduct inhibition: Overaccumulation of organic acids (e.g., citrate) can inhibit growth .

• Strain stability: Multi-copy integrations may lose plasmids during extended fermentation .

Solutions:

• Fed-batch fermentation: Maintain glucose at 10–20 g/L to prevent overflow metabolism.

• pH control: Buffer media to pH 6.0–6.5 using phosphate or citrate buffers .